Passive solar homes range from those heated almost entirely by the sun to those with south-facing windows that provide some fraction of the heating load. The difference between a passive solar home and a conventional home is design. The key is designing a passive solar home to best take advantage of your local climate. For more information, see how a passive solar home design works.

To understand how a passive solar home design works, you need to understand how heat moves and how it can be stored.

As a fundamental law, heat moves from warmer materials to cooler ones until there is no longer a temperature difference between the two. To distribute heat throughout the living space, a passive solar home design makes use of this law through the following heat-movement and heat-storage mechanisms:

• Conduction

  Conduction is the way heat moves through materials, traveling from molecule to molecule. Heat causes molecules close to the heat source to vibrate vigorously, and these vibrations spread to neighboring molecules, thus transferring heat energy. For example, a spoon placed into a hot cup of coffee conducts heat through its handle and into the hand that grasps it.

• Convection

  Convection is the way heat circulates through liquids and gases. Lighter, warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises because it is lighter than cold air, which sinks. This is why warmer air accumulates on the second floor of a house, while the basement stays cool. Some passive solar homes use air convection to carry solar heat from a south wall into the building's interior.

• Radiation

  Radiant heat moves through the air from warmer objects to cooler ones. There are two types of radiation important to passive solar design: solar radiation and infrared radiation. When radiation strikes an object, it is absorbed, reflected, or transmitted, depending on certain properties of that object.

  Opaque objects absorb 40%–95% of incoming solar radiation from the sun, depending on their color—darker colors typically absorb a greater percentage than lighter colors. This is why solar-absorber surfaces tend to be dark colored. Bright-white materials or objects reflect 80%–98% of incoming solar energy.

  Inside a home, infrared radiation occurs when warmed surfaces radiate heat towards cooler surfaces. For example, your body can radiate infrared heat to a cold surface, possibly causing you discomfort. These surfaces can include walls, windows, or ceilings in the home.

  Clear glass transmits 80%–90% of solar radiation, absorbing or reflecting only 10%–20%. After solar radiation is transmitted through the glass and absorbed by the home, it is radiated again from the interior surfaces as infrared radiation. Although glass allows solar radiation to pass through, it absorbs the infrared radiation. The glass then radiates part of that heat back to the home's interior. In this way, glass traps solar heat entering the home.

• Thermal capacitance

  Thermal capacitance refers to the ability of materials to store heat. Thermal mass refers to the materials that store heat. Thermal mass stores heat by changing its temperature, which can be done by storing heat from a warm room or by converting direct solar radiation into heat. The more thermal mass, the more heat can be stored for each degree rise in temperature. Masonry materials, like concrete, stones, brick, and tile, are commonly used as thermal mass in passive solar homes. Water also has been successfully used.

You can apply passive solar design techniques most easily when designing a new home. However, existing buildings can be adapted or "retrofitted" to passively collect and store solar heat.

To design a completely passive solar home, you need to incorporate what are considered the five elements of passive solar design.

Five Elements of Passive Solar Home Design

The following five elements constitute a complete passive solar home design. Each performs a separate function, but all five must work together for the design to be successful.

Aperture (Collector)

The large glass (window) area through which sunlight enters the building. Typically, the aperture(s) should face within 30 degrees of true south and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the heating season.

Absorber

The hard, darkened surface of the storage element. This surface—which could be that of a masonry wall, floor, or partition (phase change material), or that of a water container—sits in the direct path of sunlight. Sunlight hits the surface and is absorbed as heat.

Thermal mass

The materials that retain or store the heat produced by sunlight. The difference between the absorber and thermal mass, although they often form the same wall or floor, is that the absorber is an exposed surface whereas thermal mass is the material below or behind that surface.

Distribution

The method by which solar heat circulates from the collection and storage points to different areas of the house. A strictly passive design will use the three natural heat transfer modes—conduction, convection, and radiation—exclusively. In some applications, however, fans, ducts, and blowers may help with the distribution of heat through the house.

Control

Roof overhangs can be used to shade the aperture area during summer months. Other elements that control under- and/or overheating include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; and awnings.

Other design elements include:

• Window location and glazing type
• Insulation and air sealing
• Auxiliary heating and cooling systems, if needed.

These design elements can be applied using one or more of the following passive solar design techniques:

• Direct gain
• Indirect gain (Trombe wall)
• Isolated gain (Sunspace).

When incorporating these design elements and techniques, you want to design for summer comfort, not just for winter heating.

Your home's landscaping can also be incorporated into your passive solar design.

Direct Gain

Some builders and homeowners have used water-filled containers located inside the living space to absorb and store solar heat. Water stores twice as much heat as masonry materials per cubic foot of volume. Unlike masonry, water doesn't support itself. Water thermal storage, therefore, requires carefully designed structural support. Also, water tanks require some minimal maintenance, including periodic (yearly) water treatment to prevent microbial growth.

The amount of passive solar (sometimes called the passive solar fraction) depends on the area of glazing and the amount of thermal mass. The glazing area determines how much solar heat can be collected. And the amount of thermal mass determines how much of that heat can be stored. It is possible to undersize the thermal mass, which results in the house overheating. There is a diminishing return on oversizing thermal mass, but excess mass will not hurt the performance. The ideal ratio of thermal mass to glazing varies by climate.

Another important thing to remember is that the thermal mass must be insulated from the outside temperature. If the thermal mass is not insulated, the collected solar heat can drain away rapidly. Loss of heat is especially likely when the thermal mass is directly connected to the ground or is in contact with outside air at a lower temperature than the desired temperature of the mass.

Even if you simply have a conventional home with south-facing windows without thermal mass, you probably still have some passive solar heating potential (this is often called solar-tempering). To use it to your best advantage, keep windows clean and install window treatments that enhance passive solar heating, reduce nighttime heat loss, and prevent summer overheating.

Indirect Gain (Trombe Walls)

An indirect-gain passive solar home has its thermal storage between the south-facing windows and the living spaces.

Using a Trombe wall is the most common indirect-gain approach. The wall consists of an 8–16 inch-thick masonry wall on the south side of a house. A single or double layer of glass is mounted about 1 inch or less in front of the wall's surface. Solar heat is absorbed by the wall's dark-colored outside surface and stored in the wall's mass, where it radiates into the living space.

The Trombe wall distributes or releases heat into the home over a period of several hours. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall's surface, heat begins to radiate and transfer into the room. For example, heat travels through a masonry wall at an average rate of 1 hour per inch. Therefore, the heat absorbed on the outside of an 8-inch-thick concrete wall at noon will enter the interior living space around 8 p.m.

Isolated Gain (Sunspaces)

The most common isolated-gain passive solar home design is a sunspace. A sunspace—also known as a solar room or solarium—can be built as part of a new home or as an addition to an existing one.

The simplest and most reliable sunspace design is to install vertical windows with no overhead glazing. Sunspaces may experience high heat gain and high heat loss through their abundance of glazing. The temperature variations caused by the heat losses and gains can be moderated by thermal mass and low-emissivity windows. For more information, see sunspace orientation and glazing angles.

The thermal masses that can be used include a masonry floor, a masonry wall bordering the house, or water containers. The distribution of heat to the house can be accomplished through ceiling and floor level vents, windows, doors, or fans. Most homeowners and builders also separate the sunspace from the home with doors and/or windows so that home comfort isn't overly affected by the sunspace's temperature variations. For more information, see sunspace heat distribution and control.

Sunspaces may often be called and look a lot like "greenhouses." However, a greenhouse is designed to grow plants while a sunspace is designed to provide heat and aesthetics to a home. Many elements of a greenhouse design that are optimized for growing plants, such as overhead and sloped glazing, are counterproductive to an efficient sunspace. Moisture-related mold and mildew, insects, and dust inherent to gardening in a greenhouse are not especially compatible with a comfortable and healthy living space. Also, it is difficult to shade sloped glass to avoid overheating, while vertical glass can be shaded by a properly sized overhang.

Passive Solar Home Design for Summer Comfort

It makes little sense to save money on winter heating just to spend it on summer cooling. So in most climates, a passive solar home design must provide summer comfort as well. The solar heat in the summer must be blocked by an overhang or other devices, such as awnings, shutters, and trellises.

Overhangs

The physical dimensions of an overhang are an important element because overheating will occur unless the overhang provides enough shade. Many variables—including latitude, climate, solar radiation transmittance, illuminance levels, and window size and type—need to be considered for properly sizing an overhang in a specific locale. Therefore, it's best to have an experienced solar designer or builder calculate the proper overhang dimensions. For more information, see roof overhangs for shading building elements. 

Sunspace Heat Distribution and Control

When designing a sunspace, two important passive solar design considerations are how heat will be distributed and controlled.

Heat Distribution

Warm air can be blown through ductwork from the sunspace to other living areas. It can also move passively from the sunspace into the house through doors, vents, or open windows between the sunspace and the interior living space.

Strategically placed openings in the common wall can distribute the warmed air from the sunspace to the house by the "thermosiphoning" circulation of the air. In a thermosiphon, warm air rises in the sunspace and passes into the adjoining space through an opening. Cool air from the adjoining space is drawn into the sunspace to be heated.

The minimum opening should be about 8 square feet (0.7 square meters) per 100 square feet (9.3 square meters) of glazing area. If the design calls for two openings—one high in the sunspace and one low—the minimum area for each opening is approximately 2.5 square feet (0.2 square meters) per 100 square feet (9.3 square meters) of glazing, with 8 vertical feet (2.4 meters) of separation. Again, these are rules-of-thumb that should be refined through computer modeling or confirmed with local experts. An uninsulated masonry wall between the house and the sunspace will also transfer some heat into the living space by conduction.

Climate Controls

Overheating can kill plants and make the sunspace unlivable. To control overheating, some designers place operable vents at the top of the sunspace where temperatures are highest and at the bottom where temperatures are lowest. For times when you are not home to open vents manually, thermostatically controlled motors can be installed to automatically open them.

If passive (i.e., nonmechanical) circulation is not possible or practical, fans with thermostatic controls can be used to circulate air to the rest of the house. Other types of climate controls include window coverings that can be operated with electric timers or sensors.