Passive Solar Heating in Built Environment

Glossary Solar architecture The deliberate use of solar energy by means of the building architecture, thereby reducing purchased energy dependence while enhancing the quality of enclosed space. - Passive Not requiring actions to achieve a desired goal. In the case of passive solar energy use, solar energy is captured and distributed in a building without machinery by using the physics of conduction, free convection, and radiation. - Direct gain The direct gain of heat within a building by sunlight entering through glazed openings in the enclosure, which then traps and stores the heat. - Indirect gain Solar energy absorbed in some fashion on or in walls or roofs and converted to heat. This heat either remains entrapped in the building envelope to reduce building heat losses, or it is transferred into the building by conduction or convection. There may be a delay between the time when sunlight is absorbed and when heat penetrates into the enclosed volume. - Isolated gain Solar energy absorbed outside the insulated building envelope and then transported by free convection to the enclosed volume. - Solar air system Type of isolated gain system where heat from the collector transported to the point of use or storage by air (verses water in active thermal systems). - Hybrid solar system A passive system assisted by a small fan to increase system efficiency, possibly PV-powered. The energy ratio of heat output to electrical input can easily exceed 20:1.

Definition and Importance of Passive Solar Heating

Passive solar heating is the use of solar energy to heat a building without mechanical or electrical energy. The architecture and construction capture, store, and distribute the sun's energy. Every building with windows exposed to the sun is passively heated, but heat losses may exceed the solar gains. Accordingly, if the passive heat gain is to reduce heating costs, the system heat losses must be minimized. Ideally, the concept includes mass to store daytime solar heat for nights, increasing the usability of the gains. Finally, the heating system must shut off when solar heating achieves the desired room temperature. Two constraints on passive solar use are glare control and shading during non-heating months.

Maximizing usable passive solar gains is an important design aspect, but often designers focus only on minimizing heat losses. Taking the finance world as an analogy, no one will accumulate wealth through savings alone, income must be maximized and wisely invested. So, not only reducing heat loss is essential to low energy architecture, maximizing solar gains is important, as it has been over the history of building.

An often cited early example of solar design awareness is the "Megaron House" described by Socrates in the year 400 B.C. Numerous other examples can be found, i.e., the New England "salt box" of the seventeenth century or Swiss farm houses of the eighteenth century. In the twentieth century, the term "solar house " became popular and following the first oil shock of 1973, the term "passive solar buildings " was coined. In all these examples, the basic principles are the same; maximize the south exposure of a building to capture as much solar heat as possible and insulate the enclosure well to keep the heat in.

Passive solar building design is an important means for slowing climate change by reducing the burning of fossil fuels. It is not, however, a least first-cost way to build. Larger, better insulating windows or opaque collector constructions cost more than conventional constructions. Three arguments justifying this investment are:
  • Long-term (>10 years) good return on the investment
  • Economy and security as future fossil fuel prices increase and supply subject to interruptions
  • Living qualities of passive solar buildings flooded with light and natural warmth (Fig. 1)
Passive Solar Heating in Built Environment. Figure 1 A living room flooded with sunlight from large south-facing windows (photo source:



Buildings that consume less fossil fuel are "nice to have" today, but will be essential in the future. Since buildings are long-term investments, they must be built or rebuilt looking to the future.

Heating is a major use of energy in buildings and dependence on fossil fuels for heating can be dramatically reduced through two strategies: reducing heat loses and increasing the use of solar heat. Logically, a combination of these two strategies is desirable.

Solar energy can be used by passive or active means:
  • Passive solar use does not rely on mechanical components to capture, store, and distribute the heat, the building construction fulfills these functions.
  • Active solar use typically involves a remote solar collector and a pump or fan to transfer the heat to storage and from storage to point of use.
A low energy building must lose as little heat as possible, hence the importance given to insulation, air tightness, and heat recovery. An example design standard promoting extreme energy conservation is the "Passive House Standard" [1]. To meet this standard in Europe, three requirements must be fulfilled:
  • The annual heating requirement must be less than 15 kWh/(m2a) or maximum heating power 10 W/m2a based on the net heated floor area.
  • The combined primary energy consumption for heating, hot water and household electricity may not exceed 120 kWh/(m2a).
  • The air leakage of the enclosure tested under a pressured difference of 50 Pa (n50) may not exceed 0.6 house air volumes per hour.

A passive solar building is not defined to this extent; it simply describes a structure in which the designer deliberately maximized using solar energy passively. So, in fact, a passive house can also be a passive solar house and indeed, in the planning recommendation for a passive house, using passive solar energy is encouraged and credited.

Three passive solar heating concepts were defined after the first oil shock of 1973 and are still useful today: MediaObjects/978-1-4419-0851-3_15_Part_Fig27-372_HTML.jpg

Direct gain : Windows capture the sun in a well-insulated building; interior construction mass stores the potentially excess daytime gains into the night; and some form of shading provides comfort during non-heating seasons. Direct gain is the oldest and still most cost-effective concept, given its potential to also enhancing the quality of life in buildings. MediaObjects/978-1-4419-0851-3_15_Part_Fig28-372_HTML.jpg

Indirect gain : The building envelope captures solar heat, which is then conducted and/or convected to the building interior, possibly with a time delay of up to 8 h. Alternatively, the goal may be simply to capture enough solar heat in the envelope construction to eliminate heat losses from the building much of the time, i.e., a dynamic U-value over the heating season approaching zero. Indirect gain systems nicely compliment direct gain systems. MediaObjects/978-1-4419-0851-3_15_Part_Fig29-372_HTML.jpg

Isolated gain passive : Solar energy is converted to heat outside the insulated building envelope and then deliverd to the building interior or storage. This can be by gravity-driven convection, or with the help of a small fan (a "hybrid" system). While not purely passive, hybrid systems are reported here because the proportion of delivered heat to electrical energy is so small. MediaObjects/978-1-4419-0851-3_15_Part_Fig30-372_HTML.jpg

A sunspace or attached greenhouse with controlled opening to the building is also an isolated gain system. Isolated gain systems are the most complex and expensive, but offer the most control of when and how much solar heat is delivered into the building.

It can be useful to consider passive solar heating opportunities by building types and climates. Note, that in this section, locations north of the equator are assumed. South of the equator, north orientations have priority.

Building Types

Buildings where heating loads dominate over cooling loads are the obvious candidates for passive solar design, i.e., residential buildings and small commercial or institutional buildings. Three factors are decisive here:
  • As the ratio of enclosing surface to enclosed volume increases, heat loss increases, so a solar heating can be more beneficial.
  • As the density of heat production from people or appliances increases, the usefulness of solar heat decreases.
  • Direct solar gains in the form of heat and light are a combined asset, i.e., for hospitals, old age homes, and schools as well as residences. (Examples and Design insights for passive solar use in commercial and institutional buildings were researched and documented in an IEA project [2].

The energy optimization of a building must balance passive solar and daylight benefits against mechanical cooling and electric lighting energy demands, both of which have very high primary energy factors.


Northern climates such as Scandinavia would seem to pose a problem for passive solar use. Winter days are short, the sun is weak, and the sun path is at a very low angle. This means, however, that windows or vertical collection surfaces intercept the sun at a more direct angle. Furthermore, the heating season is very long, extending from early autumn to late spring. Before 21 September or after 21 March, heating is still needed, when days are longer than in southern latitudes. Passive solar concepts must maximize the usefulness of spring and autumn solar heating, while minimizing mid-winter heat losses.

Temperate climates are the ideal situation for passive solar buildings. Not just sunny temperate, but also overcast temperate climates. This has become possible with the development of very high-performance glass (U-value < 1.0 W/m2K). Consider the example of diffuse solar radiation at only 100 W/m2 for 6 h and an ambient temperature of 5°C. The solar gains through a glass with a g-value of 0.5 (admitting 50% of the solar radiation) will offset the 24 h heat losses of a glass with a U glass of 0.8 W/m2K. If the sun shines with more intensity or more hours, it is a passive solar winner. Because temperate climates often have hot summers, the concept must also include shading.

Mild climates offer a challenge: to achieve zero-heating energy buildings by combining passive solar design and conservation without degrading summer comfort. This is at least as challenging as achieving net-zero-energy buildings . The latter achieve a net zero balance by taking a credit from the summer electrical output of a large PV-roof (multiplied by a high primary energy factor) against the energy deficit in winter, which must somehow be met. Passive solar heating of a highly insulated building can answer part of the "somehow" question.

Strengths and Weaknesses


Living quality: daylight and naturally warmth from the sun's warmth.


Security: in the event of energy supply interruptions.


Costs: only the marginal costs of added aperture area, be it collector or window area and mass, must be amortized by energy savings.


Return on investment: As energy prices rise, return on the investment in passive solar measures increasingly attractive.


Low maintenance: There are no maintenance costs for pumps or fans.


All of these factors can positively affect resale value of the property.

Operation: often passive solar use requires active occupants adjusting sun-shading or opening windows or vents.

Poorly designed or incorrectly used passive solar buildings may use more energy than conventional buildings. Informed design, strict quality control, and intelligent operation are essential.

Road Map to This Section

A historic review of passive solar design shows how this approach has developed in parallel with technological developments of building components. It is instructive to examine which concepts came into existence, evolved, flourished, or died out. This may save reinventing a broken wheel, or ideas might arise for new variations or concepts.

Principles and applications review different approaches to passively capturing, storing, and using solar energy to heat buildings.

Direct, indirect, and isolated gain concepts are reported in detail.

Finally, the past, present, and future of passive solar heating are discussed in the context of expected energy supply developments, demographics, and increasingly well-insulated and automated buildings.


Concepts for passive solar heating date back millennia. Materials and components were very primitive by today's standards, but comfort expectations were also much lower. A net solar gain is possible even with single glazing if the required room temperature is only 16°C. The twentieth century saw dramatic developments in material science and production techniques, e.g., in glass production. The evolution in the last century has been equally spectacular. Single glazing at the beginning of the twentieth century (U = 5.8 W/m2K), evolved to fused double glazing in the 1950s (U = 2.8 W/m2K). Insulating glazing (U = 1.2 W/m2K) in the 1990s is now available in triple glazing (U = 0.5 W/m2K), or more than a factor 10 better than window glazing a century ago.

As a result, some concepts, which earlier proved ineffective for a given climate or building type, may indeed be effective today and should be "rediscovered." Following is a short-time journey through the evolution of passive solar heating.

Ancient times

The most often cited example of awareness of passive solar use is a concept house, the "Megaron House " (Fig. 2) described by Socrates (469-397 B.C.). He expressed the following thoughts: "Doesn't the sun shine into houses facing south in winter, whereas in summer the sun wanders over us and the roof so that we have shade? Because this is comfortable, then south-oriented rooms should be built higher in order not to shut out the sun, whereas the north rooms should be lower because of the cold north wind." This was the logic for this funnel-shaped house concept, opening in plan and section to the south. A roofed porch admitted sunlight into the main room in winter but shaded it in summer. A room to the north served as both storage and as a buffer from the north exposure.
Passive Solar Heating in Built Environment. Figure 2 The Megaron House concept described by Socrates


New England Salt Box : A classic passive solar house form appeared between 1650 and 1830 in New England, the "Salt Box" (Fig. 3). Its name is derived from the shape of boxes used to store salt at that time. Initially, the house form came about when an addition was made to the rear and the roof slope carried down from the two-story main house. Typically, the addition incorporated a kitchen with its own fireplace, a pantry, and a room for child birth or nursing the ill [3]. The main chimney rose inside the house to keep its heat inside. Also, very practical are the double-hung windows. The sashes were hung on ropes with counterweights of iron or bricks in a cavity of the window frame. The upper sash could be lowered and/or the lower sash raised independently. The height difference between the upper and lower openings induced air circulation.
Passive Solar Heating in Built Environment. Figure 3 A New England "salt box" house with large south façade and long protecting roof to the north (photo source:
Swiss Appenzell House : The Swiss Appenzell houses from the eighteenth to early nineteenth century had facades with many window bands protected by a projecting roof at each storey (Fig. 4). This afforded summer shading and weather protection for the windows. The curved white under surfaces captured and deflected additional daylight down to the windows.
Passive Solar Heating in Built Environment. Figure 4 An Appenzell house with large window area protected by multiple roof projections (photo:


In The year 1927 saw a breakthrough in glass production. Using the Penn vernon Drawing Machine, glass was pulled through rollers in a new process implemented by PPG Industries. For the first time, large sheets of glass could be produced. This opened exciting new architectural possibilities, but with large heat losses and comfort problems. With the introduction of insulating glass by LOF, it was possible to have large window areas and net solar heat gains. Architects played with the design opportunities this new technology offered. Researchers quantified how long a room could be kept warm by what outside conditions. The press publicized what was then possible with new solar houses. Solar buildings were a mainstream topic. An example of such architecture is the work of the Architect George Fred Keck. Figure 5 shows the living room with a stone floor and fireplace to absorb the sunlight flooding in from the full southwest front of windows [4]. This house, built for Dr. and Mrs. Hugh Duncan of Flossmore, IL, United States, was monitored by two researchers. The performance was surprisingly good. One winter day in 1941, when the ambient temperature was −20°C, the heating system shut off by 08:30 h and stayed off until 20:30 h [5].
Passive Solar Heating in Built Environment. Figure 5 Direct gain maximized in the Duncan House (picture by permission of Pilkington, North America, Inc.)


Oil was plentiful and cheap, everyone was happy, renewable energy was not a topic of any popular importance and very little happened.


In 1973, an oil embargo imposed on the United States led to a crisis of historic proportions. Americans can react astonishingly effectively and quickly to a crisis and this was the case then: "overnight," a national program to reduce foreign oil dependency was initiated. The Energy Research and Development Administration (ERDA) was activated on 19 January 1975 and the Solar Energy Research Institute (SERI) in Golden CO was founded. The department of Housing and Urban Development (HUD) held a national competition with grants for building the solar houses . Many built projects were monitored by national laboratories and published [6], as, for example, the Balcomb house shown in Fig. 6. In 1977, the first National Passive Solar conference was held [7] and in subsequent years, each conference drew over a 1,000 enthusiasts.
Passive Solar Heating in Built Environment. Figure 6 The Balcomb house in New Mexico (photo source:

Passive solar use was a major topic of the American Solar Energy Society (ASES), a national organization linked with the International Solar Energy Society (ISES). These were the boom years for passive solar buildings. Research and demonstration projects were well funded at the federal and state levels. Atomic physicists "saw the light" and became solar building physicists at renowned national laboratories, including Los Alamos, Lawrence Berkeley, Brookhaven, and the National Bureau of Standards. Exemplary demonstration projects were sponsored by the Tennessee Valley Authority (TVA), an enormous interstate electrical utility. Regional solar energy centers oversaw the evaluation and publicizing of countless solar buildings.

To help energy consultants, researchers, and academics analyze concepts, complex computer models were developed. These could quantify the dynamics of solar and heating input, heat storage, and building heat losses. Auxiliary heat demand and comfort performance were reported on an hourly basis. These tools were, however, difficult to use. Input was cumbersome and errors easily made. Computers in the 1970s still had to be "spoken to" via punched cards. The input was in rows of numbers, separated by spaces or commas punched into cards. Examples of programs include DEROB, NBSLD and BLAST, and later, DOE2. To provide design consultants (designers couldn't compute) with calculation tools, two approaches were followed:
  • Gigantic data bases were computed using research computer models for all thinkable design variations, and then clever nomographs generated. The Passive Solar Handbook by Doug Balcomb and R. Jones is a classic example [8].
  • Simplified calculation tools were programmed, such as SERIRES (later called SUNREL) and CALPAS. These were small enough to run on the first versions of portable computers ("mini" or "midi" computers).

The goal was to learn how sensitive performance was to a given parameter. To demonstrate how terrific a design was, it was useful to compare it to a conventional builder house of the time. For this purpose three reference houses were defined, based on statistics from the national home builders association (NHBA). The reference designs were published by the National Bureau of Standards (NBS, today NIST) [9].

To be sure, the computer models were telling the truth, measurement data from components and even whole buildings were essential. Test cabins for monitoring systems became a common sight at many national research facilities. Figure 7 shows a test house with an interchangeable modular south façade and clerestory windows sun lighting the north rooms.
Passive Solar Heating in Built Environment. Figure 7 NBS (NIST) test house 1980, Gaithersburg, MD (architect and photo source:

Meanwhile at universities, architecture schools continued to teach Le Corbusier as the model for good design. Energy and solar use were not significant design issues, with the exceptions of a small but growing number of architecture and engineering professors. They were the authors of some superb text books, which clearly presented passive solar design principles. Some examples are a passive solar textbook for architects [10], a guide for adapting solar concepts to regional climates and constructions across the whole continent [11], and guidelines for window design strategies to conserve energy [12].

During this period, there were a few good examples of passive solar innovation in Europe as well. The Michelle-Trombe Wall concept was a notable example (Fig. 8). The original pilot building was constructed at the Centre National de la Rescherche Scientifique (CNRS) in 1967 in the south of France and further developed with a vented version of the wall in 1974 [13].
Passive Solar Heating in Built Environment. Figure 8 The Michel-Trombe wall house in Odello, FR (photo:


Europeans began to take interest in the American passive solar movement. Many architects and building researchers travelled to the United States to personally visit passive solar houses . Passive houses began to appear across Europe, from Scandinavia to Italy. National research programs investigated how to optimize passive solar concepts to local European climates and constructions. This was essential. Several passive solar buildings did not function as hoped. Europe gets less sun than New Mexico!

During this time, windows were still mostly double glazed or at best triple glazed (U glass = 3.0 or 2.2 W/m2K). Glazing with selective coatings and noble gas fillings were just beginning to enter the market. Accordingly, only windows facing south achieved a net passive solar gain. In northern climates, night insulation of windows was needed for the long dark winters. Several innovative, but expensive roll-down insulating blankets were developed for windows. These largely disappeared from the market as high-performance glazings appeared.

By the end of the 1990s, the growing pains of adapting passive solar architecture to European climates and constructions were over and countless exemplary projects had been built and published. An IEA SHC program searched out and documented exemplary projects [14].


During this period, many conventional passive solar-heated houses were built across Europe. Sunspaces were a favored architectural element. Many houses included active solar systems to heat domestic hot water, with Austria leading in the number of such houses. European architects often succeeded in adapting passive solar house designs into good architecture. An example project from 1992 by a Norwegian architect practicing in Austria, Sture Larsen is shown in Fig. 9 [15]. The exterior of the house is in light, wooden framing, the interior is in massive construction. Solar heated air is circulated through the walls and floors to radiate into the rooms. A sunspace also helps heat the house.
Passive Solar Heating in Built Environment. Figure 9 An Austrian passive solar house with solar air radiant heating and a sunspace in Nüziders, Vorarlberg (architect and photo source: Sture Larsen,

By the year 2000, a new concept, The Passivhaus (Passive House ) had become well established and on the way to becoming the new mark of excellence in low-energy design. It came out of the PHD work of Wolfgang Feist under his Professor, Bo Adamson in Sweden. To reach this standard requires a highly insulated, thermal bridge-free, and airtight building enclosure. Mechanical ventilation is needed to assure good air quality by such tight construction. Heat from exhaust air is then recovered to preheat incoming air. The ventilation air can be used to deliver the small amount of heating needed (maximum 15 kWh/m2 heated floor area). Obviously, passive solar heating of such houses is also desired, but challenging to dimension because of the small heating load and short heating season.


Today, in the second decade of the new millennium, the term "passive solar heating" is less common. This is paradoxical because with new window frame and glazing systems, highly insulated building envelopes, and sophisticated heating control systems, passive solar gains can cover all heating needs for an extended part of the year in temperate climates. However, the interaction between passive solar gains, internal gains and envelope heat loss needs to be carefully studied to assure comfort and the hope for energy savings.

The former research computer models to study passive solar building concepts, requiring several hours on a main frame computer, today can run in seconds on a laptop. However, today's tools do not consider many of the phenomena the former models did, such as when mass is directly sunlit or only indirectly warmed by room air, or how passive solar heat in south rooms convects to other rooms. This can strongly affect passive solar usability and comfort.

Principles, Applications, and Integration


Passive solar heating requires glass, frames, seasonal sun shading, mass, and extra planning effort. The economics are clear: energy won is more expensive than the energy saved by adding insulation or eliminating air leakage up to a certain point. However, as the insulation thickness is increased, the marginal energy and economic benefits of the next increment decrease. Further, when conventional thicknesses are exceeded, the costs of anchoring the insulation and detailing jump. In contrast to this, the cost of a larger window is not proportional to the window area increase. Larger windows loose less heat per unit area. There is less perimeter for the glass area, so edge losses are smaller. Also, there are increased benefits such as more daylight, the view outside and sense of well being from being sun-warmed (especially for cats). The challenging questions are therefore, which passive solar heating concept is most effective for a given building type and climate, and how big should the system be?

Applications by Building Types

Well-Suited Building Types

Residences are the most common passive solar application. Detached single-family houses with four outside walls, a roof, and earth contact can best benefit from passive solar gains. Row houses and apartment buildings have many units with only two exposures. If they face east and west, passive solar heating is difficult. One idea would be to install an indirect gain system on the south-facing end walls to compliment morning and afternoon direct gains.

Large buildings suitable for passive solar indirect heating include warehouses, gyms (American), or athletic halls (i.e., tennis halls) [2]. Because a lower air temperature is acceptable or even desired, passive solar gains can make a greater contribution to meeting the heating demand. Heat losses of glazed areas decrease proportionately with less inside to outside temperature difference. Lower required space temperatures increase system efficiency and number of hours when useful passive solar heat can be delivered.

Swimming pool halls are a potentially good building type for passive solar heating because a high air temperature is needed, so there is a very long heating season, well into long day spring and fall seasons. Also, there is a great appeal for the space being sunlit. Direct gain, indirect gain for radiant comfort, and isolated solar air systems for humidity control are possibilities.

Limited Cases

School class rooms have high internal gains and high ventilation requirements. The benefit of passive solar heating occurs primarily during heating season weekends and holidays. At that time, typically there is a temperature set back and temperature swings are tolerated, maximizing the usability of passive solar gains. The obvious choice is direct gain with daylighting within the constraints of glare control, thermal comfort near the windows, and the view out being more interesting than the view to the front. Isolated passive solar heating is another alternative. Figure 10 shows a Swiss school with glazed-in balconies off the classrooms. Sunspace heated air preheats incoming ventilation air over a heat exchanger for the classrooms [16].
Passive Solar Heating in Built Environment. Figure 10 A Swiss school in Gumpenwiese ZH with sunspaces tied into the ventilation concept [16] (photo:

Old-age homes, nursing homes, and hospitals can benefit especially from direct gain with daylighting. Indirect gains systems to supply heat at night or isolated gain systems to preheat ventilation supply air are further possibilities. The occupancy tends to be "24/7" and demand somewhat higher room temperatures, extending the heating season.

Hotels have a transient occupancy. Guest rooms may be vacant with no internal loads for heating, but must be kept at room temperature or, if the thermostat is set back, very quickly warmed up. Accordingly, passive solar gains to maintain a minimal room temperature can be very useful, but mass can slow the heat-up. Daylight and view out may be assets, but overheating is totally unacceptable. Hotels in cities often may have to be isolated from traffic or airport noise. This can be solved with acoustical glazing or by using an indirect or isolated passive solar concept.

New Construction Verses Renovation

New construction should have very low energy demand as a result of very good insulation, mechanical ventilation with heat recovery. They likely will have a sophisticated heat production, delivery, and control system. Therefore, internal heat from occupancy and appliances will maintain the desired room temperature later in autumn and earlier in spring. The design of passive solar heating must consider this shorter heating season. Storing solar heat is very important because the gains can quickly exceed demand and result in overheating. Ideally, the thermal mass should be sunlit directly. Indirect passive solar systems can contribute to helping shorten the heating season.

Renovation is an excellent opportunity to increase passive solar heating. Older buildings have a greater and longer heating demand than well-done new structures. The subject of renovating existing housing was studied in a 4 year project of the Solar Heating and Cooling Program (SHC) of the International Energy Agency (IEA). As part of this work, 60 exemplary projects and an overview with insights were documented in brochures and are available on the Internet (Fig. 11) [17]. Included are apartment buildings, row houses, and single-family houses as well as the special case of historic buildings. The examples come from 10 countries: AT, BE, CA, CH, DE, DK, I, NL, NO, and SE.
Passive Solar Heating in Built Environment. Figure 11 Renovation with solar and conservation, 60 examples of projects across Europe and Canada [17]

Inappropriate Building Types

Since the sun shines during the daytime, all buildings that do not need heat during the day are not good candidates. Large office buildings, shopping centers, airport terminals are examples of inappropriate building types. Such buildings must cope with energy-intensive cooling problems.

Applications by Climate

The best climates for passive solar heating buildings are climates that are sunny (so there is energy available) and cold (so there is heat demand). Buildings located at high elevations often have both sun and long heating seasons, which is ideal for passive solar heating. Not surprisingly, there are many good examples of buildings in alpine regions in Europe or the Rocky Mountains in the United States. The absolute best climate for passive solar heating is the southwest of the United States, which is why the passive solar renaissance after the oil shock began there. A very good tool for generating climate data to analyze systems is Meteonorm [18].

Cold climates in northern regions have short days in mid-winter. This disadvantage is somewhat offset by the heating season beginning before the autumn equinox and extending past the spring equinox. At those times, days are longer than in more southern latitudes. A further help is that, due to the low sun path, windows or wall collectors intercept sunrays more directly.

Temperate climates are well suited for all types of passive solar heating, as is evidenced by the many examples in temperate regions of North America and Europe. In overcast, temperate climates, direct gains systems can profit from even diffuse solar gains, given the highly insulating glass available today.

Mild climates pose the challenge to achieve zero heating energy performance through conservation and passive solar measures. Paradoxically, people in mild and sunny climates have the least interest in passive solar design. This is perhaps due to priority being given to passive cooling.

Dry climates generally have very good solar availability and large day-night temperature swings. Passive solar heating with mass for storage can be very effective.

Humid climates are a problem for both passive solar heating and natural cooling. Humidity reduces solar intensity, day-night ambient temperature swings and blocks night sky radiation for natural cooling. Humid climates are difficult.

Architectural Integration

Direct gain, indirect gain, and isolated gain are simple concepts; the challenge is to translate a diagram into architecture. The aesthetics of solar design is interesting to observe historically.

Before the twentieth century, building glass was expensive. In some cities, windows were even taxed. Because of their value, they were carefully and artistically integrated into a façade design. Baroque facades are a beautiful example of the celebration of windows, in contrast to the austere holes punched in "contemporary" buildings, as can be seen in a street photo taken in Pilsen CR (Fig. 12).
Passive Solar Heating in Built Environment. Figure 12 Classical and "modern" fenestration of facades, two contrasting buildings in Pilsen CR (photo source:

Early twentieth century architects could play with large glass formats for the first time, but the resulting architecture, while making history books, often resulted in buildings that were an energy disaster.

After the oil shock of 1973 passive solar design (post oil shock), an epoch of innovation and experimentation, both technically and also aesthetically, began. The aesthetics varied greatly, from "California hippy" and New Mexico oil drum to developer Colonial Style. Inventors developed movable reflectors to concentrate sunlight on passive solar elements and movable shading systems for overheat protection. Also, thermal mass became a design opportunity, with glass blocks filled with colored water, glazing filled with phase change material, and rock bins as standing wall elements. Many components were used before they were technically mature. As a result, some components disappeared as quickly as they had appeared, and a design aesthetic never really matured.

By the end of the twentieth century, only technically and economically viable passive solar heating concepts remained. In Europe, the aesthetics of passive solar architecture profited from the attention given to detailing and superb craftsmanship. Design also suffered, however, from the box form architecture which became a craze and results in sterile, boring cubes. Name architects began to apply such concepts, wanting to profit from the growing environmental awareness. North American architects shifted to green architecture, with an emphasis on use of natural material and renewable energy in environmentally benign designs. The best examples are corporate offices and local institutional buildings from schools to park headquarters.

In the twenty-first century, the focus is on conservation. Manufacturers have responded to demand from Passive House planners, so there are now very good components on the market. An example is the windows, now available with a combined U-value (frame and glazing) of 0.8 W/m2K.

The aesthetic integration of passive solar, active solar, and photovoltaic (PV) systems is still evolving. Too often, the engineering may be excellent but the resulting appearance not, or vice versa. First semester architectural design principles are also valid for solar systems integration. The resulting "design" should please laymen and not just editors of high-end architecture journals. These concerns were discussed in a session of the Passivhaus conference in Krems AT [19].

Following is a presentation of the three passive solar heating concepts and their variations with example built projects, hopefully which appeal also as "designs."

Direct Gain



Windows transmit sunlight into the building interior where it is absorbed, and becomes heat. The windows trap the heat in the room and interior construction mass stores some of the heat for the night.

How well the glass transmits solar energy is characterized by its g value. A value of 100% would mean all the solar energy gets through the window, i.e., when the window is open. Otherwise, the glass absorbs some of the radiation and is warmed. The warmth is radiated to the ambient and into the room. Since the ambient in winter is colder than the room, it receives more of that heat. Still, some of the heat absorbed in the glass is radiated into the room. That heat plus solar energy transmitted through the glass comprise the total solar gain. This sum divided by the amount of solar radiation striking the window is the g-value. Multiple pane glazing systems with selective coating drastically reduce heat loss, but also let less solar radiation into the room. However, the benefit of the lesser heat loss overweighs the reduced solar transmission.

The usefulness of passive solar gains depends strongly on the match or mismatch of solar intensity, occupancy heat gains, and heating demand over the course of a day. Table 1 summarizes characteristics of different window orientations.
Passive Solar Heating in Built Environment. Table 1 Window orientations and characteristics




Maximum usable winter solar gains. Easiest summer shading


Poor solar usability (solar gains follow all day occupancy gains). Overheating risk in summer. Shading more difficult (adjustable vertical elements)


Limited solar gains in winter, especially by morning fog. Good solar usability (solar gains after night set-back). Less overheating risk (no direct sunlight after mid-morning)


Least solar gains. Greater heat loss (colder microclimate of north side of bld.). Best, daylight orientation, least glare problem Ideal for offices, school class rooms. Good insulation glass required for comfort near windows


Construction complicated, expensive. More difficult to keep weather and water-tight. Greater summer overheating risk (except tilted north). Mounting movable shading elements more difficult


Maximal daylight by overcast skies. Highest overheating risk (max. solar gains in summer). Difficult to shade. Greatest heat losses in winter accentuated by clear night sky radiation, minimal solar gains on flat roofs

Advantages and disadvantages of direct gain:



Window construction is highly developed with a long history of passive solar heating experience.



Mid-winter solar usability can approach 100%.



People need daylight; buildings need windows, so only marginal cost for better, larger windows must be amortized by energy savings.



Light and warmth from the sun are assets. Fenestration strongly defines the "personality" of a building, hopefully linked to functionality.


Risk greater than a windowless, mechanically cooled, and ventilated insulated cube. This risk, however, can be calculated and minimized and such a cube is no alternative for providing living quality.


Sunlight on a work surface, computer screen, or poster from Klimt is highly detrimental. Variable, occupant-adjusted shading is essential.


Glazing: Table 2 compares daylight transmission (t-value), solar transmission (g-value), and heat loss (U-values) for a sample of glazings [20]. Exact values are readily available from glass manufacturers' catalogs. The first three glass types are seldom used today and serve here a reference for comparing modern glass.
Passive Solar Heating in Built Environment. Table 2 Glass properties

Glass type



U-Value W/m2K

Single 3 mm












Double, low e, Argon




Triple, low e, noble gas




Double, low e, vacuum




Vacuum glazing : In multiple pane glazings, heat is transported by radiation between the panes and convection of the gas in the cavity between the glass panes. To improve performance, coatings are applied to the cavity side of the glass. The coating selectively lets more solar radiation through than heat back out. To reduce the convection heat loss, a noble gas, like Argon or Krypton, can be used. Their higher viscosity slows the convection loop. If there is no gas in the cavity, there can be no convection heat transfer. The only problems are to keep the atmospheric pressure from collapse, the glass panes together, and to maintain the vacuum. Small plastic pillars spaced evenly across the glazing area can keep the panes separated. Maintaining the vacuum is addressed in several patented edge sealing technologies. An important benefit of vacuum glazing is its slimness, with a total thickness of 6.5-11 mm depending on the needed glass strength. The gap for the vacuum is only about 0.25 mm. A vacuum between 4 and 10 Torr is used (a pressure unit equal to 1/760 of an atmosphere). This is a relatively weak vacuum; a thermos bottle has 6-10 Torr [21].

Glazing spacers in multiple-pane glazing are thermal bridges. Earlier insulating glazing used aluminum spacers. Unfortunately, aluminum is a good heat conductor, so edge losses were high. A next generation used stainless steel spacers, with a lower conductivity. Modern insulating glass units use spacers with a plastic thermal break. The improvement is substantial. Aluminum spacers have linear heat loss (Ψ) of 0.07-0.8 W/mK. The Ψ of a thermally separated spacer (i.e., stainless steel separated with plastic) can be as low as 0.04 W/mK. Table 3 illustrates how strongly the linear thermal bridging of the edge spacer affects the overall U-value of the glazing, depending on glass area [22].


U frame = 1.6 W/(m2K)

U glass = 1.1 W/(m2K)

Ψ = 0.070 W/(mK)

Passive Solar Heating in Built Environment. Table 3 U window value of different windows sizes, including the effect of the edge spacer

w × h (mm × mm)

A window (m2)

Perimeter (m)

A window/Perimeter

U w (W/(m2 K)

400 × 800





1,300 × 1,300





1,230 × 1,480





2,750 × 2,500





Window frames are the weak thermal component of windows with highly insulating glass. The frame has a worse U-value and of course it blocks the sunlight. So, frames with a small profile are desirable. Frames with some form of thermal break to interrupt the heat path are desirable. Even the U-value of a solid wooden window must today be judged as optimal for very low energy buildings, as can be compared in Table 4. Note that, with the exception of the aluminum frames, good U-values can be obtained for all materials. Exact U-values should be obtained from manufacturers because the values given here can vary relative to specific products. Also, of course, insulation value is only one selection criteria among many, i.e., strength to resist wind forces, life span, and maintenance costs.
Passive Solar Heating in Built Environment. Table 4 Example window frame constructions and thermal properties

Frame construction

U f frame (W/m2K)

Solid wood1




Wood with air cavities2






Aluminum with break3


1EgoKiefer, CH-9450 Altstätten SG,
2Tischlerei Sigg GmbH, AT-6912 Hörbranz,
3Schüco/Jansen AG, CH-9463 Oberriet,

Fixed shading by roof overhangs is promoted as a solution for south facades. This must be questioned for climates with overcast winters. By an overcast sky, the most daylight comes from the zenith. Therefore, fixed overhangs block daylight during long gray periods when daylight is most desired. For such climates, moveable shading is superior.

To estimate the adequacy of a south-facing overhang, the highest and lowest noon sun angles (21st June and December) are calculated as follows:

21 June

90° - latitude + 23.45°

21 December

90° - latitude − 23.45°

Taking Zurich (latitude approx. 47°N) as an example, the highest and lowest sun angles are 66.45°and 19.55°, respectively.

While this is a good first estimation for designing a shading geometry , the problem is that the sun has a lower angle before and after solar noon. An overhang should extend horizontally beyond either side of the window to give diagonal shading as the sun rises, falls, and moves laterally before and after noon.

East- and west-facing windows need vertical shading since at sunrise and sunset the sun will get under any overhang. Vertical shading elements that can be rotated away from the lateral movement of the sun are best, to allow shading and some view concurrently.

Mass increases the effectiveness of passive solar gains and is especially effective if directly sunlit (primary mass). It is up 150% more effective than secondary mass heated indirectly by the room air [10]. A recommendation for middle European-like climates is to provide 2,800 kg of mass per m2 of window area [23]. Another recommendation is that for each m2 of south-facing glass above 7% of the floor area, there should be between 6 and 8 m2 of exposed thermal mass. An example would be a 200 m2 house with 20 m2 of south facing glazing. 6 m2 of that glazing will require 36-48 m2 of solar-exposed thermal mass [24]. Also, note for day-night heat storage, thickness greater than approximately 10 cm will not increase the sola usability.

If the primary mass, i.e., a stone floor or brick wall, is a dark color, it will absorb the solar radiation better, but the impact on daylight distribution must be considered. Light-colored sunlit surfaces, especially floors or side walls, are essential to diffuse daylight deeper into a room. Such surfaces should have a mat color to avoid glare.

How well a materials stores heat is indicated by its capacity. Table 5 gives the physical properties of some construction materials to compare their effectiveness as thermal storage [25].
Passive Solar Heating in Built Environment. Table 5 Heat storage properties of common construction materials [25]


Density ρ (kg/m3)

Conductivity λ (W/mK)

Thermal capacity c (Wh/kgK)

Volumetric heat capacity Wh/m3

Metamorphic stone





Sedimentary stone










Sand, gravel





Concrete reinforced





Concrete aerated





Interior plaster





Gypsum board





Wood (pine, fir)





Wood (oak)





Wood (fiber board)





An Example Building: A very impressive example of passive solar heating with windows is a single family house built at 900 m above sea level in Trin, CH. It has no auxiliary heating, not even a wood stove. The architect, Andrea G. Ruedi, matched a very large, south-facing window area (46 m2), very plentiful mass inside the insulated envelope (Fig. 13). The envelope is wooden frame construction to simplify achieving a high insulation value (0.14 W/m2K); the interior incorporates limestone bricks and concrete (see Table 6). The room temperatures were measured in a research project over the heating season, with no auxiliary heating. They varied between 18 and 23°C. Only very rarely did the temperature fall below 19°C. The theoretical annual heating energy , were 20°C maintained, was calculated to be less than 30 L of heating oil equivalent (1.1 kWh/m2a) [26].
Passive Solar Heating in Built Environment. Figure 13 Extreme example of a direct-gain house in Trin/CH with no auxiliary heating (architect Andrea Ruedi, CH-7000 Chur)
Passive Solar Heating in Built Environment. Table 6 Properties of the solar House in Trin CH


Trin solar house

A reference house

South window area

48 m2, (8% frame)

28 m2, (20% frame)

South-facing window to façade proportion



Interior storage mass

277 t

190 t

Wall construction

Exterior: insulated wooden frame, Interior: limestone masonry

15 cm limestone

30 cm cellulose insulation

Design Advice

Maximize solar gains
  • Orient direct-gain window gains between ±45° from south
  • South window to façade ratio: 30-50%, not more
  • Large, uninterrupted glass areas (to minimize frame and glass edge losses)
  • Window U window < 1.0 W/m2K (including frame) and a good g-value (>50%)
  • Account for winter sun blockage by neighboring buildings, trees, terrain
Maximize usefulness of passive solar gains
  • Interior construction with adequate sunlit (primary) mass
  • Room interior finishes light color to maximize light distribution
  • Open floor plan. Largest rooms on south-side (small rooms overheat faster)
  • Auxiliary heat control responsive to passive solar gains
  • Shading elements: horizontal for south, vertical for east/west
  • Adjustable shading to allow concurrent view and ventilation
  • Exterior sun shading to keep absorbed heat outside
  • Generous operable window area with max. height difference to induce natural ventilation (diurnal cooling where possible)

Indirect Gain



Sun warms building walls and roofs but normally the heat is radiated and convected back to the ambient. By protecting the surface behind glass, heat can be trapped within the construction, stored or transported into the building to reduce auxiliary heating demand.

Several variations of this concept have been built including: the mass wall, mass roofs, transparent insulation, and solar insulation. Of the many innovative concepts, only a few have survived into the present, but with rising energy prices and availability of new high-performance components, these concepts can be promising.

How much passive solar heat gains can reduce purchased heat demand depends on the intensity and timing of the sunlight, occupancy heat gains, and room temperatures desired. Table 7 summarizes characteristics of different window orientations.
Passive Solar Heating in Built Environment. Table 7 Indirect gain system orientations and characteristics




Maximum usable winter solar gains. Fixed overhang possible


Less solar gains compared to south-facing facades. Heat delivered to space at time of day when least needed, so storage important. Greatest risk of summer discomfort


Limited solar gains compared to south-facing facade. Less overheating risk (no direct sunlight after mid-morning). If storage included, heat delivered at mid-day when least needed


Least solar gains, questionable cost-benefit


Maximal night-sky cooling in summer, least benefit in winter. Steeper roofs intercept winter sun better

Advantages and Disadvantages:



The concept is simple, some variations do not have any moving parts.



Most systems include large glass areas, which can be integrated with window areas into an attractive transparent façade concept.


Heat losses from the solar energy captured in the collector are radiated both to the inside and the ambient, reducing system efficiency.


Natural cooling

The chimney effect of an indirect gain system can draw cooler air from a ground channel or the north facade through the building for summer comfort.


Many systems need seasonal shading to avoid summer overheating. Some require dampers to regulate and direct air flows.


Complex systems proved, in most cases, to be too expensive for the energy benefit.

Following are four variations of indirect gain systems, mass walls, mass roofs, transparent insulation, and solar insulation.

Mass Walls

A stone, concrete, brick, or adobe wall will absorb and store solar heat, but the heat is rapidly radiated and convected back to the ambient with little or no benefit to the heated building space. If the wall is protected behind glass, the heat is better retained. With a time delay, much of the heat can then penetrate and be radiated and convected to the room behind the wall (Fig. 14). This is the principle of the solar wall , patented in 1881 by Edward Morse, an American Botanist. In 1964, a French engineer, Felix Trombe and architect Jacques Michel built such a wall to demonstrate this principle. Since then, the mass wall or Michell-Trombe Wall has become popularly known as the Trombe Wall [27].
Passive Solar Heating in Built Environment. Figure 14 Concept of the Michel-Trombe mass wall

The wall has been built in two variations. In the unvented version, the wall delivers heat to the room only by conduction and then radiation from the wall surface, with up to an 8-10 h time delay, depending on how massive the wall is. In the vented version (Fig. 14), the vents open when the air in the cavity is sun-warmed. The air circulates into the room at the top of the wall and returns to the cavity through slots at the bottom. This variation delivers heat sooner so is better for east-facing walls, and would not be good for a west-facing wall. To prevent back circulation of cold air in the gap into the room at night, dampers are needed. One solution was a Mylar film damper, which simply flapped open or was pressed closed against a wire mesh by the air pressure. In summer, dampers could be opened at the top and bottom of the air gap to vent it to the outside. Alternatively, only the top damper could be opened and a north window of the house opened. The chimney effect of the mass wall draws cooler air from the north side of the house, across the room and exhausts it out the top of the mass wall to the ambient.

The first project in Odeillo France (Fig. 15) was subsequently copied at Los Alamos, NM, United States, where it was instrumented and a computer model of its physics calibrated. Versions were then built in the 1980s in middle Europe. The performance during long, overcast winter periods was disappointing, while in summer the rooms behind the wall were too warm. Another solution was needed for this climate.
Passive Solar Heating in Built Environment. Figure 15 The prototype Michel-Trombe wall house in Odeillo, FR (source:

Mass Roof

An innovative alternative to a solar mass wall is a solar mass roof. It can provide both winter heating and summer cooling. One innovative concept uses a series of roof water bags (like a water bed) and moveable insulation panels. During the winter days, the insulation is slid back on its tracks, so that the water is sun-warmed. Nights, the insulation is rolled back over the water bags, which then conduct heat through the steel deck ceiling to be radiated down to the rooms beneath. In summer, the process is reversed. Nights the cover is removed and the water is cooled by radiation to the sky, days the insulation is slid in place and the rooms below are cooled by the cold water bags.

A prototype house was built in 1973 by the inventor of the concept, Harold R. Hay (Fig. 16). The three-bedroom, two-bath structure in Atascadero California was constructed and monitored with funding from the US department of HUD. It was the first documented 100% passive solar heated and cooled building and the only instrumented passive solar house in operation during the 1973 energy crisis. To engineer the system, the then new generation of computer simulation tools was used (simulations for this project done by Phil Niles) [28]. It could be worthwhile to reexamine this concept, given the materials and insulation systems available today.
Passive Solar Heating in Built Environment. Figure 16 A mass roof: the Skytherm house in Atascadero California by Harold Hay (photo source: Evelyn and Harold Hay Fund at Cal Poly, San Luis Obispo, USA)

Transparent Insulation

The transparent insulation wall (TWD) improved the performance of the mass wall concept by addressing one of its weaknesses. In the cavity, the air warmed by the black surface of the solar wall rises, while cooler air against the surface of the glass falls. The resulting circulation loop transports heat from the wall to the glass where it is then lost by conduction to the ambient. In the transparent insulation system, the air gap is filled with some form of transparent cellular structure, inhibiting the convective loop (Fig. 17). The infill material is typically a cylindrical, rectangular, or honeycomb geometry, which directs the light in multiple reflections to the mass wall at the back. A small vertical gap between the TWD and glass should be maintained, to allow moisture to diffuse and prevent the TWD from being in direct contact with the hot absorber surface.
Passive Solar Heating in Built Environment. Figure 17 Transparent insulated wall concept

In non-heating months, vents to the ambient can be opened at the top and bottom to the ambient to cool the wall. These proved to be difficult to keep air tight in winter and added cost. Some form of shading for the wall was needed. Window roller blinds are effective, but make the whole system prohibitively expensive. An innovative solution was to use fixed, metal micro louvers with the fins set at an angle to block high summer sun angles. They, unfortunately, also reduced winter solar performance and were expensive. A third variation uses fused transparent spheres 2-3 mm in diameter as the glazing, applied like transparent stucco. These let less solar energy through, but with the benefit of much better summer comfort behind the wall. The area of the glazing patches is for physical reasons limited [29].

Several transparent materials and geometries have been used to fill the air gap, including extruded PMMA-Capillaries or Polymethlymethacrylat (Plexiglas), extruded PC Polycarbonate (Makrolon) capillaries or honeycomb forms, and extruded polycarbonate multi-cell panels. Critical for the selection are the upper temperature tolerance and UV-stability. Some materials are stable up to 120°C, other to only 90°C. A good overview of products and properties is available from the association of TWD manufacturers [29].

A well-publicized example of a TWD-building under extreme conditions is a Swiss alpine hostel at Hundwiler Höhe at an elevation of 1′306 m above sea level (Fig. 18). It was built in 1995 and 42 m2 of prefab TWD-Modules 130 × 90 × 18.5 cm (h × w × d) were used. No summer solar protection was needed at this altitude. The 185 cm thick TWD wall construction (outside to inside) is as follows: 8 mm framing projection beyond the glass, 4 mm glass, 30 mm gap, 120 mm transparent insulation, 8 mm absorber, 15 mm air gap between the absorber and wall. This air gap was needed to provide the needed tolerance for mounting the prefab TWD modules. The gap was estimated to cause a 10% reduction in efficiency, which was considered acceptable. Simulations indicated that the temperature in the TWD construction should not exceed 80-90°C, well within the 110°C tolerance of the TWD material used [30].
Passive Solar Heating in Built Environment. Figure 18 Swiss TWD house at Hundwiler Höhe (architect and photo source: P. Dransfeld,

Solar Insulation

The solar wall concept is the simplest of the indirect gain systems and perhaps, therefore, most economical.

The goal of this concept is to provide dynamic wall insulation. During the day, air chambers in the cavity protected by glass are warmed by the sun. Nights, the cavity slowly cools down. The air chambers together with the glazing to the outside and insulation to the inside help reduce heat loss from the building.

Two construction variations exist for creating the insulating air chambers: a type of treated, corrugated cardboard and wood routed with horizontal slits.

The cellulose system (GAP), shown in Fig. 19, achieves on south facades a dynamic U-value of 0.08 W/m²K in middle Europe [31].
Passive Solar Heating in Built Environment. Figure 19 Wall section of a cellulose solar wall insulation system (Redrawn based on a figure in the report: Domenig-Meisinger et al. [31])
A well-publicized example project using this concept is an Austrian Apartment building on Makartstraße, Linz (Fig. 20). To minimize disturbing the tenants, prefabricated wall panels including the solar walls, windows, sun-shading systems, and ventilation channels were mounted. The south facades achieve a dynamic U-value of 0.08 W/m²K averaged over the heating season. As a result of a combination of this wall system and other measures, the heating demand could be reduced by 92% to 13.4 kWh/m2K [31] and [32].
Passive Solar Heating in Built Environment. Figure 20 Apartment building with the GAP solar insulation on Makartstraße, Linz AT (photo source: S. Grünewald and S. Rottensteiner)
The routed wooden system (Lucido) entraps air in inward-sloping slots (Fig. 21). Important in this and also the cellulose board system is that the wall behind the solar wall be well insulated. A benefit of the wooden absorber is that, being weather protected, it can be left natural and hence conveys the character of a wooden façade.
Passive Solar Heating in Built Environment. Figure 21 A solar insulated wall detail of the routed wooden Lucido system (source: Lucido Solar AG Solares Bauen,

Design Advice

Following is design advice for temperate climates. In mild climates, these systems might make it possible to reduce auxiliary heating demand to zero, but summer comfort strategies must be well done. In a humid, hot climate, this system make no sense, nor is performance likely to be good in northern, very cold and weak-sun winter climates. Many projects were built in temperate climates but a market breakthrough has not yet occurred. The energy they save for the investment is high compared to energy from cheap fossil fuel.
  • Because indirect solar gain is less efficient than direct gain, a large collection area is needed. South-facing orientations are most sensible. Depending on the desired timing of heat release, east- or west-facing solar mass walls are possible, but the absolute amount of delivered heat will be smaller.
  • Overheating is a risk in mass wall systems, so summer sun shading and venting are important. The solar insulation concept has the comfort advantage of having an insulated wall separating it from the building interior.
  • Durability was a problem for early prototypes, including untight vent dampers and degradation of sun-exposed wooden framing. A typical greenhouse construction with a metal cap to protect exterior wood is one solution. Transparent insulation can deform at high temperatures, so the right material must be chosen for the design, or reliable shading provided. Freeze protection UV-durable materials are obvious requirement for the roof-mass system using water.
  • The thickness and density of a mass wall and hourly solar radiation should be calculated to dimension a mass wall to deliver its heat to the room when desired.
  • System performance might be improved by very good insulating glass. This is a trade-off of g-value and U-value in the context of the economics. Single glazing in low-iron glass could still be the best solution (maximizing the g-value).
  • The mass roof system is only plausible in clear-sky climates with both a heating and cooling demand.
  • The room side of the mass wall or transparent insulation wall should not be blocked by furniture. For the solar insulation system, this is not an issue. The room surface of all the wall concepts can be any color desired.
  • Prefabrication can provide cost savings for a second project, not necessarily the first project. The benefits are shorter on-site erection time, less disturbance of occupants, and better quality control, which can lead to better durability.
  • Indirect gain systems are well suited for building renovations.
  • These systems have gone through development pains; valuable experience is available from the project designers, research institutes, and the manufacturers of system solutions. Homework to assure a new project starts from the state-of-the-art is essential!

Isolated Gain/Hybrid



Solar energy is collected outside the insulated envelope of the building, then transported as heat by convection into the building or into storage. Two variations are considered here: solar air systems (with or without mass) and sunspaces.

The orientation of a sunspace, like any room, depends on view and when sunlight is wanted. For solar air systems, design issues are similar to those of indirect gain systems. Buildings uninhabited and kept at a minimum temperature much of the year (i.e., vacation homes) are an ideal application.

Advantages and disadvantages:



These systems are simple and reliable.



The gravity-driven solar air systems work without moving parts. However, a small fan would improve the efficiency and can easily be PV-powered, making the system immune to grid power interruptions.



Reduced purchased energy costs and reduced wear from less running time of the auxiliary heating system, extending its lifetime.



Sunspaces are built for the space they provide, energy savings are only a fringe benefit, so in effect a bonus.



Isolated solar gains systems, because they are outside the insulated building, are advantageous regarding summer comfort. Sunspaces need large, low, and high ventilation openings, and effective shading and glare protection.


Natural cooling

The chimney effect of an isolated solar collector or sunspace can draw cooler air from a ground channel or the north facade through the building.

Solar Air Systems


A solar air system is in effect a sunspace with the depth reduced to a few centimeters. The principle is the same as an active solar water system, except that heat is transported from the collector to the point of need or storage by air. Two variations are reviewed here: systems that are directly coupled to the building and systems in which the sun-warmed air passes through mass before entering the building. All together, six system types were researched in a project of the International Energy Agency. Out of this work, a design handbook for solar air systems [33] and a book of example built projects [34] were published. The other four system types typically require an electrical fan, dampers, and more complex control systems to function, so are not included here under passive systems.

No-Mass Solar Air Systems

These systems operate on the principle that sun-warmed air in a vertical or upward sloped volume behind glass will rise. This warm air can then be channeled through the insulated wall of a building to provide solar heating (Fig. 22). Two variations are possible for the air supply at the bottom of the collector:
  • If the opening is to the ambient, the collector can deliver sun-warmed fresh air to the building.
  • If the opening is to the building, the collector delivers recirculated, higher temperature air than the first variation. In either configuration, in summer an outlet at the top can be opened to the ambient to exhaust the hot air.
Passive Solar Heating in Built Environment. Figure 22 Diagram of a free convecting façade solar air collector
Example: Figure 23 shows a solar air system to keep a vacation home in Koroni GR heated to a low level, ventilated and dry during periods of vacancy. Two collectors, each 6 m2, circulate up to 200 m3/h of fresh air into the house. A small PV panel (50 wp) integrated into a corner of the air collector powers a small fan to increase the efficiency of the system, which has been in operation since 2004.
Passive Solar Heating in Built Environment. Figure 23 A solar air heater for a vacation home on a Greek island (photo and system information:

Mass Solar Air Systems

This concept is like the no-mass solar air system described above, except that the solar-heated air circulates through the building structural mass before entering the room (Fig. 24). In this way, the air enters the room at not as high a temperature, and the mass continues to radiate the stored heat after sunset. The mass may be a concrete ceiling or floor (hypocaust) or walls (murocaust) with air channels. The system can function with only free convection of air movement [33].
Passive Solar Heating in Built Environment. Figure 24 A solar air collector linked to air channels in the building structure
Example: Figure 25 shows an apartment building in Marostica, Italy (20 km from Vicenza) with a façade integrated passive solar air system. The sun-warmed air in the collector rises naturally and circulates though channels in the concrete ceiling/floor structure before entering the apartments in the north-facing rooms. The concept was developed by Barra-Costantini. Each 84 m2apartment is heated by 16 m2 of collector. Each m2 of collector is estimated to contribute about 100 kWh/a [34].
Passive Solar Heating in Built Environment. Figure 25 Solar apartment buildings in Marostica by Barra Costantini (photo: Gianni Scudo)

Design Advice

  • The collector can be mounted below floor level, i.e., in the case of a building with an above-grade basement. The height difference strengthens the free convection.
  • No-mass solar air systems are well suited for buildings often vacant, which need to be tempered and supplied fresh air. In permanently occupied buildings, mass is essential to maximize the usefulness of the collector gains and avoid overheating.
  • Dampers are essential to prevent reverse-flow and cooling of room air into the collector at night.
  • A small fan can increase system efficiency. Commercial solar air systems with PV-powered fans are available.
  • The collector and solar-heated air channeling require good engineering. Consult the literature to not have to reinvent the wheel [33, 34].



Sunspaces became popular element of passive solar architecture. They were designed as an architectural feature which, in addition, reduced purchased energy consumption in several ways:
  • Passively by creating a warm buffer zone on the south side of the house, reducing wall and window heat losses.
  • Passively by occupants simply opening house windows and doors into the sunspace when its temperature exceeded the house temperature. Alternatively, a small thermostat could open a damper and switch on a fan to automate this.
  • Actively, when sunspace supplied sun-warmed air to a mechanically ventilated building. Alternatively, the sunspace air could be ducted to a heat exchanger to warm incoming ventilation air for the building.

Today, in highly insulated buildings, a sunspace's buffering effect is no longer a significant energy saving. In mechanically ventilated buildings with heat recovery, the benefit of heating the incoming air is also less significant, but still a benefit. Sunspace heated air can exceed room temperature, thus supplying useful heat. A sunspace can increase purchased energy consumption if occupants heat it to near room temperatures. Comfort expectations of a sunspace must be less than for rooms.

Example: The Wydacker row houses in Zollikofen (Bern) CH are earth sheltered to the north and protected behind a sunspace to the south (Fig. 26). This construction provides energy benefits and protection from nearby street noise.
Passive Solar Heating in Built Environment. Figure 26 Attached sunspaces as part of the concept of low energy row housing in Bern (Architects, AARPLAN; Bern, CH)

Each house has a 108 m2 sunspace with a 57 m2 of insulated glass (U = 2.9 W/m2K) at a 60° slope oriented 20° west of south. Being slightly west of south is beneficial. Frequent morning fog reduces solar radiation mornings compared to afternoons. Sun shading is provided by a roller shade beneath the glazing. The concrete block wall of the house and concrete pavers over gravel provide thermal mass for the sunspace. The measured heating energy consumption of the houses was 37 kWh/m2a, which for the year 1995 was excellent performance [35].

Design Advice

  • Insulating glazing for both the sunspace (minimize freeze risk for plants) Insulating glazing for the house to minimize heat loss to the sunspace.
  • Sunspace frame out of laminated wood to be dimensionally stable and metal exterior cap to reduce weathering. Alternatively, aluminum framing with a thermal break.
  • Two or more story sunspaces offer d more collection area for the enclosed volume. Comfort is better because stack effect ventilation improves with stack height).
  • Large operable sash at base of the sunspace and at its top, ideally with rain sensor-activated closers. Rule of thumb: Minimum1/6 glass area operable.
  • Sun shading on exterior most effective, on interior it is less subject to wind damage and weathering. Interior sunshade installed min. 10 cm below glass so gap acts as thermal chimney between operable low inlet and high outlet sashes.
  • As in direct gain systems, thermal mass helps reduce temperature swings (i.e., minimize hours below freezing.
  • A freeze-protecting heater with thermostat activation when the sunspace temperature falls below 4°C can help protect plants. If the sunspace is designed, the purchased energy for this is well worth the plants.

Future Directions

The future directions to using passive solar energy can best be forecast by reviewing technical and political events in the past. In the early twentieth century, when production of glass in large formats became possible, architects began experimenting with large glazed areas. The resulting buildings were uncomfortable to occupy in winter and expensive to heat. First, with the introduction of insulating glass, a net passive solar heat gain in winter became possible. After World War II, oil became plentiful and cheap and interest in solar dwindled. Then, the oil crisis of 1973 renewed interest in finding alternatives to fossil fuels. New solar building concepts evolved under the collective term "passive solar heating ." The first of annual "National Passive Solar Conference" was held in 1977 in Sante Fe, NM (United States) A US federal department (HUD) held a landmark national competition for innovative passive and active solar building concepts. Interest and built projects spread from the sunny southwestern United States across the entire continent. By the next decade, passive solar heating concepts were being applied by architects in Europe as well. Passive solar concepts, originating from inventive individuals, became a topic for national research institutions. Test cells and buildings were monitored, computer models developed, and engineering handbooks written.

Today, the term, "passive solar buildings " is less commonly heard. The similar sounding concept "Passive House" now enjoys international attention. The new, future-oriented trend is sustainable buildings [36, 37], net-zero-energy buildings , carbon neutral buildings, and even energy-plus buildings. Well-designed new buildings constructed to such high standards need very little heat, and so passive solar gains are less beneficial than before, but still an asset. Such solar gains help delay when the heating season finally begins and end the heating season earlier. Passive solar gains are also a major heat source when a building is unoccupied during the day or for extended periods. The daylighting aspect of direct solar gain concepts will continue in the future, being a major appealing factor.

Future buildings must address new requirements. There will be
  • More elderly people (demographics) with greater comfort expectations.
  • High energy prices, regardless whether the source is increasingly scarce fossil fuels, electricity, or renewable energy.
  • Less disposable income because salary increases will not match the inflation of energy costs affecting prices of all goods and services.
New technological developments will offer new possibilities for meeting these requirements. Likely developments may include:
  • Nanotechnology selective coatings for glazings to allow larger passive solar collection areas in winter and no overheating in summer. Similarly, material science will deliver high-performance coatings for absorber surfaces.
  • Vacuum technologies for both window glazing and as very compact insulation for indirect or isolated gain systems.
  • Intelligent, self-learning control systems for switchable property components, responding to solar intensity, ambient temperatures, and programmable occupant comfort profiles.
  • Chemical thermal storage for heat or "cold" to provide very compact and high density, compact storage with no losses during storage.
  • Building skins, which produce both electricity and low-temperature heat amplified by high-efficiency heat pumps for space and water heating.

Finally, in the future as in the past, political developments may result in supply interruptions. In any event, as the world-known reserves diminish, prices will not steadily and gradually increase. Large price swings amplified by speculation can be expected.

These changing occupant requirements, technical developments, and possible political and oil market events will change how new buildings are constructed and existing buildings renovated. They will more effectively draw energy from the environment, providing heat, light, and "cool" with less external energy input. Buildings and climate will no longer be combatants, but allies working together. The needed investment and maintenance costs will have to be low, simply because there will be less disposable income. Passive solar heating was, is, and will be an important strategy for achieving low-energy buildings offering excellent living quality.

Acknowledgments The author thanks in particular three institutions in his career, whose support made possible the personal experiences to write this section.
The Swiss Federal Office of Energy, Buildings Program (in particular, Gerhard Schriber).
The International Energy Agency, Solar Heating and Cooling Program and, in particular, all the researchers and architects who, with such dedication, worked together in research tasks over the decades and the founder of the Program, Fred Morse.
The Donau University-Krems, Department of Buildings and Environment (in particular: Peter Holzer, who convinced me to become a professor so people would believe my stories).


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