In most of North America, sunlight can help heat your home on winter days, if you let it in and hold on to its heat. This simple, direct use of the sun’s heat is called passive-solar heating. A passive-solar heating system requires two things: south-facing windows to admit sunlight and interior thermal mass materials that can store that solar warmth. Insulating shades on your windows will help keep heat indoors at night, and shading those windows in summer will prevent overheating.
The winter sun rises in the southeast and arcs low through the southern sky to set in the southwest. So the best place for windows to let in solar heat is the south side of the house, where winter sun can reach deep into the living space. Even if your home wasn’t designed to take advantage of the sun’s warmth, if the midday winter sun shines anywhere on your house’s south walls, you’ve got a chance of improving your passive-solar heating.
Next, consider thermal mass. Have you ever leaned against a brick wall on a cool evening, after the sun has warmed it all afternoon? Then you’ve experienced the power of thermal mass. Brick, stone, tile, and concrete can store the sun’s heat and release it in the evening.
South-facing glass and thermal mass work together for your winter comfort. The more glass you have, the more surface area of thermal mass you need to soak up the solar heat. Skimping on thermal mass will give you overheated afternoons on sunny winter days, and cold nights. Even with a good balance of south glass and thermal mass, a long run of sunny winter days could overheat your south-facing rooms. You can spread the warmth to north-facing rooms by opening an interior door or window, or by installing a quiet fan.
Maintain Solar Access
Do hills, trees, or buildings keep that midday sun from falling on your south walls? Any obstructions in your home’s solar access zone—from about 30° east to 30° west of due south—can block the winter sun when it is strongest, between 10:00 am and 2:00 pm.
How densely built is your neighborhood? If it’s mostly single-story homes on medium-to-large lots, then your neighbor’s home probably doesn’t block your winter sun. But the higher and closer together the buildings are, the harder it may be to get adequate winter sun.
If your home has good solar access, modifying your landscaping might help you get more winter sun. Keep trees out of the solar access zone; even the bare branches of deciduous trees can block some sun, and trees that hold on to their leaves through January are really a problem. When selecting new plants, be sure to take their mature dimensions into consideration—and don’t plant evergreen trees on the south side of your house!
If your south-facing glass area is less than 8% of the room’s floor area, don’t expect to get much heat from the sun unless you add windows. If it’s more than 24%, the room might overheat on a sunny winter day. Thermal mass will soak up some of that excess heat, but you may find yourself taking off clothes or opening windows near sunset.
Choose south-facing windows with a SHGC of 70% or more to admit solar heat, and a U-factor of .33 or less, so you’ll lose less of that heat (Chapter 7). Many codes now require a low SHGC for all windows. That works well for west, east, and north windows, but it will work against your passive-solar heating scheme. You may need to make the case for high-SHGC windows with your building department.
Incorporating Thermal Mass
Thermal mass—the storage part of your passive-solar heating system—refers to materials that have a high capacity for storing heat, like masonry or stone. The darker their surface color, the more heat they absorb. If you don’t have thermal mass to soak up the day’s solar warmth, your house will get too hot during the day and too cold at night. Thermal mass helps even out those temperature swings.
Designers of passive-solar homes typically specify 6 times as much thermal mass area as south glass area, with thermal mass about 4 inches thick. Concrete slabs and brick walls can meet these specifications, but what about readily available thinner mass—such as mortar-set tile floors, granite countertops, or thickly plastered walls? Those will work too, but you’ll need more area. If most of your interior mass is significantly less than 4 inches thick, aim for 9 times the area of the south glass.
Remodeling projects give you a chance to expose any masonry or stone already built into your house to direct sunlight, and to add new thermal mass. If your house has an insulated slab foundation, exposing the slab floor near south windows by removing carpets or wood flooring may provide all the mass you’ll need. (If you have an uninsulated slab, you may not want to remove that insulating carpet.) Or maybe you have a brick fireplace, and can add a south window to let the sun shine onto it.
If your house sits over a crawl space or a basement, you can still add thermal mass. Concrete floors can be attractive, durable, and easy to build. Some house structures can’t take the added weight, so consult a structural engineer.
Don’t cover thermal-mass floors with carpet! Leave them as bare as possible to soak up and release heat. For aesthetics, you can polish or otherwise finish the concrete surface, or cover it with paving bricks, slate, quarry tiles, or dark ceramic tiles.
Another strategy is to replace a wood-framed interior wall that receives direct sunlight with a thermal-mass wall. Dark surfaces are best, as light colors reflect much of the sunlight the wall could otherwise absorb. Such mass walls are particularly effective when they run perpendicular to the south wall; the east side gets morning sun, and the west side gets afternoon sun.
Unlike passive-solar heating, which has few moving parts, active-solar heating—also known as solar thermal—requires a large solar collector, a pump, a large storage tank, and a way of distributing the heat. The solar collector heats a liquid, which can provide both space heating and domestic water heating, saving you the cost of a separate water heater and boiler. This approach is best suited to a cold, sunny climate and a well-insulated home.
It’s also possible to have solar forced-air heating. You can use a fan to blow air through coils that are heated by water from a solar storage tank. Or you can use a collector that heats air, which is then blown into your heating-duct system.
Active-solar systems are usually expensive to maintain, so seek simple solutions that have been used successfully in your area for a long time. Consult local solar water-heating contractors and your utility company for a locally appropriate design.
Solar thermal probably won’t meet all your heating needs year-round, so you’ll want to couple your solar-heating system with a backup mechanical heating system.