Checklist: A primer for passive solar
By Maggie Leslie on 03/22/2010
If designed properly, a home can be heated with minimal additional cost by using passive solar. By simply siting the home and allocating glazing properly, a home can take advantage of our free, readily available heating source: the sun.
- The building envelope is energy efficient. A well-sealed and insulated home is the first component of any passive-solar home. By reducing these energy losses, you can more easily meet the heating and cooling needs of the home using passive heating techniques.
- The home is oriented to the south. To maximize the amount of solar gain in the winter, site the home so that the longest wall of the home faces within 15 degrees (plus or minus) of true south. If 15 degrees is too much of a design constraint, 30 degrees off of true south can still provide about 85 to 90 percent of the optimal winter heat gain. Make sure there are no large obstacles such as buildings or trees that will block heat gain in the winter. Deciduous trees are acceptable and actually provide an advantage by providing shade in the summer.
- Glazing and thermal mass are sized properly. Passive-solar homes are typically either sun-tempered or direct-gain systems. Sun-tempered homes do not have thermal mass, a material that stores heat. These designs should have no more than a 7-percent ratio of glazing (glass area) to floor area on the south side of the home. With direct gain, the system should have 7- to 12-percent glazing to floor area of south-facing glass. For each square foot of glass above 7 percent, there should also be 3 to 6 square feet of 4-inch-thick masonry to act as thermal mass. However, surface area or square footage of thermal mass is more important than thickness. The surface absorbs heat during the day and slowly releases heat as the temperature drops. Additionally, comfort is improved if the mass is evenly distributed in the room. For either design, to reduce overheating in the summer, minimize the amount of east- and west-facing walls and glass. To prevent overheating, east and west glazing should be less than 5 percent of the floor area.
- Windows are chosen to maximize heat gain and minimize heat loss. On the south side of the home, choose a window with a high Solar Heat Gain Coefficient (about .55 or higher) and a low U-factor (about .35 or less). This will maximize heat gain, but minimize heat loss. On the east and west, choose a window with a lower SHGC and a similar U-factor. This should be accompanied by a vertical shading element, such as an insulating blind (insulating blinds are also a great option for evening use on south-facing windows).
- Overhangs are sized properly. South-facing windows should be accompanied by properly sized overhangs to prevent overheating in the summer. In general, for our area you need about a 2-foot-wide overhang within two feet from the top of an average size window. For an exact measurement see the sizing rules below. Mature deciduous trees are also a great addition to overhangs — they permit most winter sunlight to pass through (60 percent or greater) but provide nice shade in the summer. Evergreen trees, on the other hand, should be placed on the north and west sides of the home to buffer winds and afternoon sun.
Overhang sizing rules:
- Draw the wall to be shaded to scale.
- Draw the summer sun angle upward from the bottom of the glazing.
- Draw the overhang until it intersects the summer sun angle line.
- Draw the line at the winter sun angle from the bottom edge of the overhang to the wall.
- Use a solid wall above the line where the winter sun hits. The portion of the wall below that line should be glazed.
- Design rooms to match the passive-solar design. Place rooms that have minimal heating and lighting requirements (such as garages and storage rooms) on the north side of the home. The kitchen is also a great choice for a room on the northern side because it produces its own heat. Keep in mind that furniture, rugs and tapestries will affect the thermal mass performance. Daylighting is an added benefit of passive solar design. Generally, a ratio of 5-percent glazing to floor area provides enough light for the room. Skylights admit light — but can offer unwanted heat in the summer. Solar tubes may be a good alternative.
Sources for this checklist include N.C. Solar Center: Passive Solar Home Design Checklist,http://www.ncsc.ncsu.edu/include/_upload/media/pubs/PassiveDesignChecklist.pdf; Southface Energy Institute: Passive Solar Design Technology Fact Sheet,>http://www.southface.org/web/resources&services/publications/technical_bulletins/PSD-Passivesolar%2000-790.pdf; and Re-Arch: The Initiative for Renewable Energy in Architecture Fact Sheet, http://www.rearch.umn.edu/factsheets/PassiveSolarFactSht.pdf.
[Maggie Leslie is program director of the WNC Green Building Council. She can be reached firstname.lastname@example.org or at (828) 254-1995.]