Using the Sun: Using Windows Wisely
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The points that follow assume that you live in a climate cold enough in winter that you want to capture heat from the sun, and warm enough in summer that you want shade. In some areas, one of these concerns will be much greater than the other. Remember, too, that few houses are aligned directly north, south, east, and west;. so you may need to extrapolate from these suggestions to fit your own situation.
The north side is where multiple panes and airtight windows are most crucial in winter. The north side gets little direct sunshine, and the north wind is often the coldest. When the only windows available were drafty, single-pane units, it was best to keep north-facing glass to a minimum. However, low-e windows with good insulating values can face north without losing much heat. More importantly, north glass provides consistent color and intensity of daylight, without annoying glare. Low-glare daylighting can reduce energy consumption in a home office or studio by eliminating the need for electric lights during the day. Contrary to the design philosophy of the '70s, solar-conscious design should include some carefully selected north glass.
East and West Glass
Windows on the east and west receive light and heat, but they're hard to shade from the summer sun. East windows are usually welcome even in summer; they let in morning sunshine and chase off the nighttime chill. However, west windows are almost never energy winners—they overload the house with heat on summer afternoons.
East-facing and west-facing glass work best if you choose glass with the right characteristics. In any climate, multiple panes and gas fills make the window insulate better. Glass with a low solar heat gain coefficient (SHGC) has built-in shading, meaning that it reduces the amount of solar energy that gets in. Check the visible transmittance as well, so you don't block solar heat at the expense of the light and view you want from the window.
Window films stick onto clear glass to reduce the amount of heat the window lets through. The quality of these films has improved greatly over the past few years. They don't block as much daylight as they used to, and some even incorporate a low-e coating. The films are inexpensive, but they should be professionally installed to prevent unsightly bubbles and cracks.
You can use more east- and west-facing glass if you shade the windows. Outdoor vegetation works well to shade the low-angle sun. Tall shrubs, hedges, and arbors, as well as shade trees, all do a nice job.
For east, west, or south windows, consider exterior roll blinds in rooms that are unoccupied during the day. These blinds roll down over the outside of the window, preventing any radiant energy from entering. They have drawbacks: they block the light, views, and ventilation. Also you have to adjust them manually. If you're home a lot during the day, choose exterior shades that are suspended away from the windows, to allow ventilation and let in some light.
Shade screens are exterior shades that install like insect screens on the outside of the window sash. They shade the window but let in air, some light, and the view. Shade screens come with different SHGCs, so you can choose how much heat you want to block. The ones that block the most heat also block the most light.
Interior window shades are less effective than exterior ones because they stop sunlight after it has entered the home. All the same, new blinds or draperies can help reduce summer heat gain and winter heat loss. For better winter comfort, heavy insulating drapes are helpful for single-pane windows if new efficient windows are out of the budget. In summer, light-colored shades can reduce a window's SHGC by as much as 43%. Dark blinds will soak up more sun, releasing the energy as heat inside the house.
Awnings can reduce heat gain through an east or west window by 77%. However, to be this effective, the awning has to slope down over the top two thirds to three quarters of the window, depending on latitude. Slatted awnings let you see outside through the covered area, but they provide less shade. Retract or remove awnings in the winter to let in more sun and to keep snow from damaging the awnings. Awnings should have a small vent space where they meet the house to prevent heat from building up against the window. Be sure that awnings don't obstruct emergency access or egress. Also, most building codes require awnings to end at least 6 feet 8 inches above the ground if they extend over a walkway. And if you must penetrate the wall to install the awnings, be sure to water seal and air seal the holes.
In the Northern Hemisphere, the winter sun rises in the southeast and passes low through the southern sky to set in the southwest. Thus the best place for windows is the south side of the house, so the winter sun can reach deep into the living space, warming the house when you need heat most. South windows are easy to shade from the summer sun. In the summer, the sun rises in the northeast, passes almost overhead at midday, and sets in the northwest.
At one time, it was conventional wisdom to use deciduous trees to shade south windows. However, according to researchers at the National Renewable Energy Laboratory, the trunk and branches of a leafless tree can still block 40% of winter sunshine. They now recommend removing vegetation from the south side of the house and using overhangs to manage summer sun.
Awnings or long roof overhangs are good for shading south windows. It's generally easier and cheaper to install awnings than to build an overhang on an existing house. On a south-facing window, awnings will block as much as 65% of the summer sun's heat. Solar-conscious roof overhangs extend the eaves farther than normal. The drawback of overhangs is that they are difficult to add unless you are going to be working on the roof anyway. But they are a permanent improvement, so the house will continue to benefit from them long after awnings would have broken down.
How big should your awnings or overhangs be? That depends on your home's location and on the climate. The Passive Solar Industries Council publishes this information in locally customized builder guides for hundreds of cities and towns across the United States.
Exterior roll blinds or shade screens can be used on south windows as well. But remember to remove the shade screen for the winter to get the benefit from solar gain.
Excerpted with permission from No-Regrets Remodeling by Home Energy (1997)