Windows are the eyes of the home -- necessary for beauty and function. Recently, however, there has been increasing concern about window function as an energy winner or loser.
Initial reaction to windows as transparent and thermally ineffective wall areas dictated a trend to fewer windows -- an unfortunate restriction on beauty. Home buyers recently had to suppress their desire for daylight in order to conserve energy and save money. More affluent buyers often said, "Hang the cost; give me a view."
But today's window manufacturers offer a wide variety of units to fit nearly every style of home, according to residential energy expert Arthur W. Johnson of Gaithersburg. With proper selection and placement by the builder and proper use by the home buyer, windows can be far more energy effective than a solid wall. Take note when house hunting and be an informed user.
How well windows transfer or block heat flow determines how they affect energy costs and home comfort. Johnson says heat loss is the result of all window elements: the glass and the sash material (which holds the glass in place) as well as the crack space between the sash and window frame.
Glass conducts heat out of your home. Adding a second pane of glass (double glazing) reduces heat loss by nearly one-half. Triple glazing provides proportionally further reduction.
Sash materials are usually wood or aluminum. Wood is a natural insulator. However, for several years metal window manufacturers have been using what's called a thermal break. This involves putting an effective insulation material in portions of the window so that there is no continuous metal-to-metal contact. This construction significantly reduces heat loss by conduction and makes metal sash thermally comparable to wood.
Air infiltration, Johnson points out, is directly affected by the amount of crack space between sash and frame. Assuming equally well-fitted windows, the effectiveness of any style relates to the ratio of the total crack length to the window area. The higher the ratio, the more infiltration there will be. In order of effectiveness: the fixed or permanently closed window is best, but perhaps not the most practical; next comes the singlehung or sliding window, followed by double-hung, casement and awning or hopper windows. Forget jalousies for effectiveness.
For any style window, sash fit should be as tight as practicable. Check for undamaged weatherstripping and good caulking. Keep in mind that adding a second window frame significantly reduces air infiltration and heat loss. The net result is that a single-glazed window plus a storm window will have less overall heat loss than will double glazing (insulating glass) in a single window.
Because windows are transparent they allow radiant heat from the sun to pass through -- solar energy. Solar heat either confounds interior cooling or tremendously assists interior heating, depending on the season.
Heat loss has no relation to whether or not sunlight is striking the window.
However, Johnson says, radiation is directly related to sunlight and the window's orientation is passed through clear gloss. A second pane has little additional effect.
Therefore, solar heat gain from windows in winter may greatly offset any heat loss. But those same windows used for winter solar gain must be protected from summer sunlight. This is where window orientation and seasonal shading become critical. Johnson tells us what to look for:
Northerly windows are effectively in shade year-round. They are always energy losers. Windows here should be minimum in number and size.
East and west windows are usually energy losers depending on the severity of the winter, and whether or not they are well-shaded in summer. Selective plantings may help provide summer shade. Landscape features may provide an effective wind-break for north, east and west windows to reduce air infiltration and cut energy losses.
As window orientation becomes increasingly southern, the potential to become an energy winner increases. However, here's where summer shading by roof overhang, awnings or large deciduous trees becomes the most critical. Because of the sun's summer angle, about 3 feet of roof overhang on Washington area homes is enough to shade floor-to-ceiling glass. Standard height windows would be shaded by just 2 feet of roof projection.
Two other aspects of windows should be considered. One is condensation -- water forming on windows due to a difference in temperature and humidity from one side to the other. This is a natural phenomenon you cannot prevent, but can control. In winter months home humidity should be high enough for health and comfort, but low enough to prevent window condensation.Usually, 30 percent to 40 percent works fine.
The second aspect is ventilation. Washington area windows can be used for effective cooling in the spring and fall. The most effective ventilation comes from opposite-facing windows placement in line with prevailing winds, northwest to southeast.
Finally, Johnson says, proper use of draperies shades, blinds and shutters is necessary for admitting or blocking solar energy. Light-colored opaque shades can block more than one-half of the sun's radiant heat. Lined draperies may block up to a third; blinds and shutter, about 30 percent.