Basic Passive Solar Design Techniques

Passive solar homes range from those heated almost entirely by the sun to those with south facing that provide some fraction of the heating load. The difference between a passive solar home and a conventional home is design. And the key is designing a passive solar home to best take advantage of the local climate. Elements of design include window location and glazing type, insulation, air sealing, thermal mass, , and sometimes auxiliary heat.

You can apply passive solar design techniques most easily to new buildings. However, existing buildings can be adapted or “retrofitted” to passively collect and store solar heat. In some ways, every home is a passive solar home because it has windows, but designing a home to work in its climate is the basis for these techniques.

Every passive solar building includes five distinct elements: the aperture (or collector), the absorber, thermal mass, the distribution, and the control. But there are three basic types of passive solar design—direct gain, indirect gain, and isolated gain—that differ in how these five elements are incorporated.

Direct Gain

Direct gain is the simplest passive design technique. Sunlight enters the house through the aperture (collector)—usually south with a glazing made of transparent or translucent glass. The sunlight then strikes masonry floors and/or walls, which absorb and store the solar heat. The surfaces of these masonry floors and walls are typically a dark color because dark colors usually absorb more heat than light colors. At night, as the room cools, the heat stored in the thermal mass convects and radiates into the room.

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 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 it's not, the collected solar heat can drain away rapidly, especially when thermal mass is directly connected to the ground, or in contact with outside air whose temperature is lower 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 Wall


Trombe Wall

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 to 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 – Sunspace

A sunspace—also known as a solar room or solarium—is a versatile approach to passive solar heating. A sunspace 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.

The thermal mass 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.

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, such as overhead and sloped glazing, which are optimized for growing plants, 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, to avoid overheating, it is difficult to shade sloped glass, while vertical glass can be shaded by a properly sized overhang.

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 a roof overhang or other devices, such as awnings, shutters, and trellises.

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. (The Solar Radiation Data Manual has appropriate overhang lengths for many U.S. locations.)