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Solar PowerFrequently Asked Questions :: Passive Solar Home Design

Q: What is passive solar design?

Passive solar design uses sunshine to heat and light homes and other buildings without mechanical or electrical devices. It is usually part of the design of the building itself, using certain materials and placement of windows or skylights. A successful passive solar building needs to be very well insulated in order to make best use of the sun's energy. The result is a quiet and comfortable space, free of drafts and cold spots. Passive solar design can also achieve summer cooling and ventilating by making use of convective air currents which are created by the natural tendency of hot air to rise.

In the winter when heating is required, the sun is low in the sky, which allows the heat to penetrate into windows on the south face of a structure. In the summer, south-facing windows can be shaded by an overhanging roof or awning to keep out the high hot summer sun. Because much of a building's heat is lost through its windows, the majority of windows in a passive solar building are located on the south wall.

Depending on the climate and the design, as much as 100 percent of a building's heating needs can be provided by the sun. In a climate such as Albuquerque, meeting 80 percent of a building's heating needs through sunlight is a realistic goal given a balanced design. Even if 50 percent or 30 percent is sun-generated, conventional heating bills are cut by that amount. Albuquerque's cold but sunny climate is a very favorable location for passive solar heating. Additionally, summer comfort can often be achieved without the need for air conditioning by employing shading and natural cooling techniques. (Source: New Mexico Solar Energy Association)

Q: What are the main elements of passive solar home design?

A: 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.

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.

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.

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. (Source: EERE)


Q: What is direct gain design?

A: Direct gain is the simplest passive solar home design technique. Sunlight enters the house through the aperture (collector)-usually south-facing windows with a glazing material 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.

This photo shows a mountain home in Colorado that uses passive solar heating, i.e., direct gain. Photo credit: Dave Parsons

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. (Source: EERE)

Q: What is indirect gain (Trombe wall) design?

A: 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. (Source: EERE)

Q: Does landscaping play a role in passive solar design?

A well-designed landscape not only can add beauty to your home but it also can reduce your heating and cooling costs. On average, landscaping for energy efficiency provides enough energy savings to return an initial investment in less than 8 years.

Learn more about the following elements of an energy-efficient landscape design:

  • Climate
    Use energy-efficient landscaping strategies based on your regional climate.
  • Microclimate
    Consider your home's microclimate along with your regional climate in your landscape design.
  • Shading
    Use trees and other plants to help shade your home if needed to help reduce cooling costs.
  • Windbreaks
    Use windbreaks—tree and/or shrub plantings—around your home if needed to help reduce heating costs.
  • Water conservation
    Conserve water along with energy for a more sustainable landscape.
    (Source: EERE)

Q: What are the benefits of passive solar home design?

  • High energy performance: lower energy bills all year round.
  • Investment: independent from future rises in fuel costs, continues to save money long after initial cost recovery.
  • Value: high owner satisfaction, high resale value.
  • Attractive living environment: large windows and views, sunny interiors, open floor plans.
  • Low Maintenance: durable, reduced operation and repair.
  • Unwavering comfort: quiet (no operating noise), warmer in winter, cooler in summer (even during a power failure).
  • Environmentally friendly: clean, renewable energy doesn't contribute to global warming, acid rain or air pollution. (Source: New Mexico Solar Energy Association)

Q: What are passive solar and solar tempered home designs?

A: Passive solar and solar tempering refer to heating and cooling a house with non-mechanical systems that utilize natural forces such as the sun and wind.

 Passive or “integrated” design utilizes the relationships between building components such as south-facing windows and thermal mass. Windows should be oriented within ten degrees of true south.

• Even small deviations from due south can cause significant increases in unwanted heat gain in the cooling months.

• Southeast orientation is better than a southwest orientation in climates like Montana.

• A rectangular building footprint with the long axis in the east-west orientation is best for passive solar designs. This maximizes southern exposure and minimizes east and west exposures.

In the Montana climate, spring tends to be cool and fall tends to be warm and sunny. Overhangs and other shading devices are often designed as a compromise with intermediate-sized shading devices that permit greater sunlight in the spring and then assume that unwanted fall solar gain will be reduced with window shades or deciduous trees and vines.

Thermal mass , usually a concrete slab floor or masonry walls, can be used to store solar heat for use when the sun does not shine. Thermal mass also minimizes overheating in the space when the sun is shining.

Solar tempered designs are the simplest and least expensive solar design strategies because they do not require thermal mass.

• South glass should not exceed 7 percent of total floor area in a house without added thermal mass.

• This strategy can satisfy up to 20 percent of a home’s space heating energy needs.

Passive solar designs can provide over half the space heating energy but require added thermal mass that can also add significantly to the cost of the house.

• Typical designs include south windows totaling 7 to12 percent of floor area . Thermal mass prevents overheating and stores heat for use when the sun is not shining.

• A common rule-of-thumb is that 5 .5 square feet of uncovered and sunlit floor mass is required for each square foot of south glazing.

• The windows, shading devices, and thermal mass must be carefully designed if a passive solar home is to achieve maximum comfort and energy performance.

• Insulating even very energy-efficient windows can significantly increase the effectiveness of passive solar strategies.

The surface area of thermal mass is more important than its depth. Beyond four inches in depth, the effectiveness of masonry thermal mass decreases significantly.

What are the major design strategies for passive solar design?

A: There are five major elements to passive solar design:

• Aperture (collector): large expanse of glass where sunlight enters
• Absorber: a part of the solar storage element; the absorber is a hard, darkened surface that sits in the direct path of the sun and absorbs heat
• Thermal Mass: materials that retain and store heat from the sun
• Distribution: the method for circulating solar heat throughout the house
• Control: roof overhangs that shade the aperture during the summer months to help prevent overheating

For more information see the National Renewable Energy Laboratory (NREL) publication Passive Solar Design for the Home. (Source:


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