Passive Solar Design FAQ’s

Q: What is passive solar design?

A: Passive solar design uses solar energy (sunlight) to heat the home and provide interior light without relying on electrical or mechanical devices. Using specific materials (like those that can absorb heat, for instance) and specific design techniques (like increased southern windows), passive solar design allows homes to make optimal use of the sun’s energy without having to use a fossil fuel or other energy source to do so.

Successfully designed passive solar buildings are typically quiet and comfortable because their design keeps them free of drafts and cold spots. Instead, passive design techniques make use of convective air currents to provide gentle, natural indoor temperatures.

For instance, because a lot of a structure’s heat is often lost through its windows, most windows in a passive solar building are built in the south wall, along with an overhanging roof or awning.

This allows the structure to capture the most sunlight when it needs it (like during the winter, when the sun is low in the sky and will penetrate the windows) and to block out sunlight when it is best (like during the summer, when the sun is high and will be blocked by the overhang). These passive design choices minimize abrupt temperature variations and the need to mechanically correct them.

Q: What are the parts of a passive solar home?

A: A home that uses passive solar home design will often have each of the following elements:

Aperture (Collector)

The aperture collects the sunlight. Typically it is a large, south-facing window. Since exterior landscaping can affect how efficient the sunlight collector is, there should not be any trees or other structures obstructing the aperture between 9 a.m. and 3 p.m. during the heating season.

Absorber

The absorber is a heat sink, which means that it absorbs heat. Sometimes used interchangeably with the term “thermal mass,” the absorber is typically a hard, dark surface (like a concrete wall or dark floor) that sits directly in the path of sunlight that enters through the aperture. It may also be a water container, since water has excellent heat-absorbing qualities. Sunlight hits the surface of one or more heat sinks and is then absorbed as heat.

Thermal Mass

Similar to the absorber, a thermal mass absorbs heat. How it differs is that a thermal mass is technically not directly exposed to sunlight from the aperture. Instead, a thermal mass sits below or behind an absorber. For instance, the top of a sun-soaked floor would be the absorber and the heat-sinking material below would be the thermal mass.

Distribution Mechanisms

Distribution is the method by which the solar heat that is collected and absorbed in a structure is then circulated throughout that structure. There are three types of transfer modes. Conduction is the direct transfer of heat through two objects that touch, like a warm floor transferring heat to your feet. Radiation is the heat transfer through electromagnetic waves, like sunlight through a window or heat from a fire. Convection is the transfer of heat through the use of air or a fluid, like water. A passive solar home often uses all three natural heat transfer modes.

Control Strategies

Control strategies help control heat buildup by providing shade and preventing overheating. These can include roof overhangs, shutters, blinds, differential thermostats, automatic sensors, and more.

Together these elements collect, contain, control and distribute solar energy throughout a structure.

Q: What is direct gain design?

A: Direct gain design is the least complex passive solar home design technique. As described above, in a direct solar gain system, solar energy enters the living space of the structure directly through a window. It then strikes an absorber with a thermal mass that then stores the heat.

The surfaces of the absorber and/or thermal masses are usually dark. This is because dark colors typically absorb more heat than light colors. Some passive solar home builders may use water-filled containers as thermal masses, since water stores twice as much heat (per cubic foot of volume) than most masonry materials. If you choose to use a water thermal storage design, however, it is important to remember that the water tanks will require some periodic maintenance and treatment (to prevent microbial growth).How much heat a passive solar home can collect depends on the glazing area, and how much heat can be stored depends on the size of the thermal sink. Unfortunately, if you don’t have enough thermal mass, it’s harder to prevent overheating, which reduces the energy efficiency of the structure as a whole.

When designing a home with passive solar design techniques, it is important to remember that the thermal mass must be insulated. If it is not insulated from the outside temperature, the collected solar heat can quickly drain away to the outside, instead of radiating inward to the living spaces. Solar homes can be particularly prone to loss of heat like this if the thermal mass is connected to the ground or in contact with outside air that is cooler than the desired temperature of the thermal mass.

The use of insulated, triple pane, HSHG (high solar heat gain) windows is recommended to maximize the heat-collecting potential of a direct gain design. It is important to keep all of your windows clean, and install window treatments that reduce nighttime heat loss and summer overheating.

Q: What is a Trombe wall, or indirect gain design?

A: In an indirect solar gain home design, the absorber/thermal mass is placed between the sun and living space, which typically means that the thermal mass sits between the south-facing windows and the living spaces.

The most common indirect gain design approach is to use a Trombe wall. The Trombe wall is a thick, masonry wall that faces the south and has a single or double layer of glass mounted in front of it. Solar heat penetrates through the glass, is absorbed by the outside surface of the wall (the absorber), and then stored in the wall’s mass (the thermal mass).

Trombe walls gently radiate heat into the home over a period of several hours, with heat typically traveling through the masonry wall at a rate of one hour per inch. Heat radiates inward from the outer wall, so when indoor temperatures fall below that of the wall’s surface, the heat stored in the wall begins to radiate and transfer into the living space. Typically, heat absorbed in an 8-inch concrete wall at noon enters interior living spaces roughly 8 hours later.

Q: What is isolated passive solar design?

In an isolated passive solar design system (like a sunroom) solar collection and storage are isolated from the regular living space of a structure. Unlike in a direct solar concept or indirect solar gain home, where the absorber and thermal mass are typically in the living area, in an isolated solar design, both are physically separated but thermally linked. This means that although they are in a different area than the living space, thermal energy (i.e., heat) can still move freely between the two spaces. Sunrooms are a popular type of isolated solar design that use a combination of direct gain and indirect gain solar system features.

Typically, a sunroom is a south-facing room that extends beyond the normal confines of a structure and has three glass walls. Sunlight enters the sunroom, is retained by a thermal mass, and is transferred to the living areas either through a shared mass or vents.

Q: How does landscaping affect passive solar home design?

A: Landscaping can have a substantial effect on passive solar efficiency. In fact, landscaping for energy efficiency can measurably reduce annual heating and cooling costs. Here are a few ways you can increase the effectiveness of a passive solar home:

  • Position trees to maximize shade during the summer and choose trees that lose their leaves during the winter.
  • Position trees to create wind breaks that deflect winter winds.
  • Position shrubs and trees to create wind tunnels that direct summer breezes toward your home.

When considering how you can use landscaping to increase the energy efficiency of your home, you will want to consider things like the climate and microclimate of your region, as well as ways you can conserve water. (For more information and tips, you can visit theEnergy.gov website.)

Q: What are the benefits of having a passive solar home?

A: The benefits of passive solar home design are many. In fact, passive homes are incredibly energy efficient—whether you want to save the earth or you want to save your wallet, passive solar home design can make a real difference. Plus, homes designed for passive solar energy are low-maintenance, offer exceptionally comfortable living environments, and typically have high resale values (probably due to the many other benefits of owning a naturally energy efficient home).

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

A: Passive solar and solar tempering are two different types of home design strategies that use non-mechanical systems and natural forces to increase home energy efficiency.

Passive (also called integrated) home designs use south-facing windows and thermal mass to collect and store solar energy.

Typically:

  • Windows are oriented within 15 degrees of true south.
  • Southern-facing windows that total roughly 7 to 12 percent of the floor area are used.
  • The structure is typically rectangular to maximize southern exposure while minimizing east and west exposure.
  • 5.5 square feet of usable (in other words, uncovered and sunlit) floor mass for each square foot of south-facing glazing over 7 percent.
  • The depth of the thermal mass is less important than its surface area. The effectiveness of masonry thermal masses drops significantly beyond four inches in depth.
  • Insulating windows can have a huge impact on even already-energy-efficient homes.

A solar tempered design is the simplest and the least expensive type of passive solar design strategy because it does not need a thermal mass. Instead of using a thermal mass to store energy, a solar tempered design just uses the sun’s heat to warm living spaces during the day. Typically:

  • Southern-facing windows that total not more than 7 percent of the floor area are used.
  • This strategy does not collect as much heat, but it can still make up to 20% (or more with optimal conditions) of a home’s heating needs.
  • There are no moving parts or maintenance needs.

For more information, contact PEA Builders, Inc. at 262-506-9616.

 

Copyright P.E.A. Builders