Passive Solar Home Design
Passive solar design can make a home more comfortable in every season. The winter sun can warm a home´s interior, while simple shading and thermal mass strategies can prevent summer overheating. Consider these best design bets.
1) Site Right. To maximize winter sun and summer shade, orient the home´s south face to within 10 degrees of true south. Even though orienting the house 30 degrees from true south reduces winter solar gain by only 13 percent, the cooling penalty can be greater. Homes facing from 30 to 45 degrees east or west of south may need longer roof overhangs. This is especially true if the home´s orientation favors the west, because overhangs quickly become much less effective as the hot western sun, low in the sky, strikes the house. In most locations, a slight orientation to the east is desirable to increase winter morning sun and decrease summer afternoon sun.
2) Proper Window Selection & Placement. Heat from the sun entering south-facing windows and doors with glass can provide between 20% and 80% of the heat required to keep a house warm in winter.
South-facing glass should equal at least 5% and usually no greater than 12% of the conditioned square footage of the house. Place just enough windows on the north, east, and west walls to balance interior light levels, capture any views, create an attractive house, and allow for natural cooling. Limit the use of skylights, substituting sun tubes (also known as tubular skylights) in interior rooms without windows, which let in some light, but less heat.
For passive solar space heating, south-facing windows should have a high SHGC (at least 0.52) to maximize the amount of the sun´s heat that passes through the glass. A window with a SHGC of 0.33 lets in only 33% of the sun´s heat energy. Alternatively, install clear (uncoated) double-paned glass and use insulated blinds at night to minimize heat loss. Alternatively, triple-paned clear glass will let in a large amount of sun while limiting heat loss.
3) Seek Summertime Shade/Passive Cooling. Overhangs, awnings, and porches can shade windows in certain seasons and prevent the home from overheating. For cold climates, design overhangs for a long season of full sun striking the south glass. Overhangs and shading devices should shade south-facing windows during the summer months, and allow full sun on windows during the wintertime. For hot climates, design overhangs for a long season of full shade on the south glass.
South window overhangs should be sized for the height of the windows, wall height, and the construction detail of where the roof meets the wall. West and east windows require much longer overhangs, and these windows are best shaded by other methods, such as porches or trees. Computer simulation software, such as the shareware at Sustainable by Design, and especially the online Overhang Design Tool, can make sizing overhangs a snap if you, your builder, or design professional understand roof/wall construction details.
4) Make It Massive. Materials with high thermal mass, such as brick, stone, ceramic tile, and concrete, absorb direct solar gain in the winter and indirect heat during the summer. Although it is best to locate thermal mass in the path of direct sunlight, other mass in contact with the material that receives direct solar gain can serve the same function. Including thermal mass is especially important for homes with glass above 7% of the home´s square footage. Locate the mass as close to south-facing windows as possible. Strive for a minimum of 2 inches (and a maximum of 4 inches) of thermal mass. Less than 2 inches does not store sufficient heat and more than 4 inches can absorb so much heat that it will be too slowly released. The maximum amount of floor mass area that should be used is 1.5 times the south-facing window area, since the sun cannot hit large areas all at once.
For cost effectiveness, use concrete, concrete masonry, and earthen plasters as thermal mass. Slab-on-grade construction, where a concrete floor is poured over insulation, can economically combine the foundation with a heat-absorbing floor. ICF (insulating concrete form) foundations, which sandwich concrete between expanded polystyrene foam panels, are very compatible with cold-climate slabs, even when the upper part of the house is framed with studs.
5) Take Advantage of Natural Light. Daylighting in homes is typically accomplished using windows, translucent doors, skylights, light pipes (tubular skylights), and clerestories. Using sunlight in your home can decrease heating and cooling loads through passive solar design techniques, as well as eliminate most lighting needs during the day.
Designing a daylit home can be very simple. In most regions, provide a long south wall of windows and locate the main living spaces along the south side to take advantage of direct solar gain in the wintertime. In the summer, adequately deep, fixed overhangs will block heat gain but still allow indirect light to enter the windows.