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Solar Design Basics
1. Choose a site that receives south sun during winter. Obstructions to the south of the site, such as tall evergreen trees, buildings or hillsides, need to be kept at least 1.7 times their height away from the home. When in doubt, visit the site around December 21, when the sun is the lowest in the sky. The site should receive full sun from 9 a.m. to 3 p.m.
If you're choosing to build on a small lot, select one that is deep from north to south, to ensure good solar access. Locating the house's septic drainage field within the solar access zone is another strategy for maintaining good solar access, since that area will need to be kept clear of trees and shrubs, which would otherwise block the southern sun.
2. Choose a home design with few projections below the roofline and no porches on the south. Projections shade adjacent windows; porches on the south prevent the sun from entering. Porches on the east and west can be beneficial by shading windows from the hot summer sun.
3. Orient the longest wall of the house so it faces true south. Rectangular floor plans minimize the exposure of east and west walls to summer sun, which is especially helpful in hot climates. The front, back, or side of the house can be the south wall. (True south is not the same as the magnetic south shown by compasses. Check with a local surveyor's office to find out how many degrees to adjust from magnetic south.)
Can you deviate from a due south orientation? Sure, but you'll pay a price in dollars and thermal comfort. Straying from a solar-south design reduces wintertime heat gain and may increase summertime solar gain, leading to overheating.
The more rooms that have some south windows, the better. This helps eliminate the need for fans or ducts to move warm air from one area to another. Place rooms that require less heat, such as workshops, bedrooms and kitchens, on the north side of the house.
4. Maximize windows on the south side. South glass should be a minimum of 7 percent of the house's square footage for a sun-tempered home and a maximum of 12 percent for fully passive-solar designs. Don't go wild on windows. North, east and west windows should be no more than 4 percent of the house's square footage. Exceeding the 12 percent guideline for south windows may cause the home to overheat in summer, and may allow excess heat loss from the windows during the night and during long, cold, cloudy periods.
Coleman recommends choosing south glass carefully for your climate. Many of the new low-emissivity (low-e) coatings reduce heat loss and gain, but for south windows you do not want glass that keeps out the solar heat. She recommends south glass that has a Solar Heat Gain Coefficient (SHGC) of at least 0.5. In warmer climates with properly designed south overhangs, uncoated double-pane glass is preferred.
5. Design the roof overhangs to shade windows properly from the high summer sun. Overhangs are key to successful solar homes. Judkoff says the exact geometry of overhangs is critical for balancing the need to admit maximum sunlight in winter and minimize solar heat gain in summer. Generally, the warmer and sunnier the climate, the deeper the overhang should be. A 2-foot overhang nicely shades an 8- to 9-foot wall in most locations. Coleman recommends the Web site www.susdesign.com/overhang/index.html to help you design overhangs properly. If overhangs aren't possible, use insulated shutters to keep out the summer sun.
6. Provide thermal mass (tile floors and brick or masonry walls) in the south side of the house. Mass absorbs and stores heat when sunlight strikes it or when its temperature is lower than the air temperature. As the room's air temperature drops below the mass' surface temperature, heat is released and the air is warmed. Temperatures indoors remain relatively stable and comfortable, despite dramatic oscillations in outdoor temperatures.
The mass in floors, framing, wallboard and furniture is usually sufficient to accommodate the solar heat in sun-tempered homes. But when you build a full passive-solar design with up to 12 percent south glass, you need to add extra thermal mass in the form of tile, concrete floors, or masonry walls or planters. For optimal results, some mass should be in direct contact with the incoming sunlight throughout the day. The mass also should be distributed throughout the house. Add about 7 square feet of 4-inch-thick mass for every 1 square foot of south glass above the 7 percent minimum. One easy, inexpensive way to add this mass is to choose a concrete slab-on-grade foundation.
In addition to these solar design factors, follow these two principles that apply to all energy-efficient homes:
7. Insulate and seal the structure well. Careful attention to detail is essential. Insulation should not be compressed and air should not leak in. If you can't get as much south glass as you would like, adding extra insulation can result in the same overall lower-energy consumption. Judkoff recommends insulating at least to the level prescribed by the International Energy Conservation Code or ASHRAE 90.2, which are region-specific recommendations for the building-envelope elements and mechanical systems. Choose energy-efficient windows and consider using insulated shades to keep heat from escaping at night (and to keep the heat out in the summer), especially in cold climates. Entryways separated from the main living space by an inner door are especially helpful in preventing cold air from rushing in whenever the outside door is opened.
8. Correctly size the heating and cooling system. Many solar homes require almost no additional heat, so you will need a much smaller, less expensive heating system. A woodstove or natural gas wall heater may be sufficient, or you can connect baseboard radiators to your hot water heater. The insulation and thermal-mass features of a solar home make it more resistant to summer overheating, and in some climates you may get by with using ceiling fans instead of air-conditioning.
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