If designed properly, a home can be heated with minimal additional cost through the use of passive-solar design. By simply siting the home and allocating glazing properly, a home can take advantage of our free, readily available heating source: the sun. Here are six steps to get you started. (See "Fact Sheet: Air Sealing" and "Fact Sheet: Insulation.")

1. Build an energy-efficient building envelope. The first component of any passive-solar home is to make sure the building envelope is as energy efficient as possible. Make sure the home is well sealed and insulated. By reducing these energy losses, you can more easily meet the heating and cooling needs of the home.

2. Orient the home 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 in the summer. 

3. Size glazing and thermal mass properly. Passive-solar homes are typically either suntempered or direct-gain systems. Suntempered homes do not have thermal mass, a material that stores heat. These designs should have no more than a 7 percent ratio of glazing to floor area on the south side of the home. Direct gain system should have 7 to 12 percent glazing to floor area of south-facing glass. For each square foot of glass above 7 percent, it should be accompanied by 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, minimize the amount of east and west facing walls and glass to reduce overheating in the summer. East and west glazing should be less than 5 percent of the floor area to prevent overheating. 

4. Choose windows wisely. 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.)

5. Size overhangs properly. South-facing windows should be accompanied by properly sized overhangs to prevent overheating in the summer. “As a rule of thumb in North Carolina to prevent summer gains, the angle ‘α’ between a line ‘S’ from the edge of the overhang to the bottom of the window and a vertical line ‘V’ should be approximately equal to the latitude minus 18.5 degrees. To prevent wintershading, the angle ‘β’ between a line ‘W’ from the edge of the overhang to the top of the window and a vertical line should be approximately equal to the latitude plus 18.5 degrees. For more detailed calculations, use computer simulation software or procure the services of a professional solar designer.” (NCSC) Mature deciduous trees are also a great option—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. (See the diagram at the N.C. Solar Center Web site listed below.)

6. 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 fact sheet include NC Solar Center: Passive Solar Home Design Checklist, http://www.ncsc.ncsu.edu/information_resources/factsheets/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.