Expensive new windows have a limited potential for reducing your energy bill. Even if you replaced single-pane wood windows with the smartest windows tested, you would see only a modest drop in heating and cooling costs. But you may need new windows because you have remodeled or because the old windows are past their prime. Then it makes sense to choose durable windows that keep out wind and water and that offer high thermal performance.
An old window, such as a pane of glass in a wood frame, lets heat escape through the glass itself and through gaps and cracks in the frame. A double-glazed (insulated glass) window with a frame of vinyl or wood clad in aluminum or vinyl is a smart window. The new frame and the extra pane of glass cut heat loss. Smarter still is the window made with special glass panes coated to optimize your home's heat in winter: double glass with inert gas between the panes, or a heat-blocking film sandwiched between the glass. You also can make a dumb window smarter with weather-stripping and caulking, or add a storm window--either a permanent one or a temporary one. But what if you need to replace the window because the frame has deteriorated or because you are remodeling? Here are some tips:
The frame has a significant effect on a window's thermal performance, price, and upkeep. Wood frames, plain or clad in vinyl or aluminum, tend to be more expensive than all vinyl. Plain wood, of course, needs to be painted. Clad wood requires minimal maintenance.
Aluminum is a good heat conductor. Even an aluminum-framed window that is "thermally broken," with insulation between the interior and exterior parts, conducts more heat than does vinyl-framed or wood-framed windows. In cold weather, heat inside the house travels readily through the frame to the outdoors making the indoor side of the window feel cold to the touch. In a temperate climate, an aluminum frame may be a practical choice, but it won't offer the best thermal protection in cold winters.
Better quality vinyl windows have welded corners. Other windows may have corners that are screwed together. These are best avoided since they are less likely to be airtight and watertight, and the corners may start to pull apart after being exposed to heat and cold.
The glass you choose affects a window's price and performance:
In cold climates and hot ones, single-pane windows are best reserved for garages and other spaces that don't require heating or cooling. However, single-panes may be adequate in areas with brief heating and air-conditioning seasons.
Most new homes have this type. It consists of two sealed panes, usually separated by an aluminum spacer that includes a desiccant to keep moisture from condensing between the panes. Once moisture does condense between the panes, the only way to get rid of it is to replace the glass.
In a regular double-glazed window, air fills the gap between the panes. A step up in thermal performance and price are windows filled with an inert gas, usually argon. Argon-filled glazing achieves its optimum performance with a half-inch space between panes. Air-filled windows perform best with a space of one-half to one inch.
Low-e, shorthand for low-emissivity, refers to a coating that alters the way the glass transmits visible and invisible light. Much of the sun's radiant energy passes through ordinary glass and warms the objects in a room. In the winter, a warm room re-radiates energy back through the window to the outdoors as long-wave infrared radiation. Some experts say that such long-wave infrared accounts for as much as 60 percent of the heat lost through a window. A low-e coating helps reduce heat loss in the winter and heat gain in the summer by blocking nearly all the long-wave infrared. Some low-e coatings, designed for hot climates or southern exposures, reduce the buildup of heat from the sun. Other low-e coatings meant for cold climates allow more of the sun's warmth into the house.
Triple-pane windows provide better insulating ability than a plain double-pane, but few manufacturers offer them because they are heavy and costly.
These are used by some window manufacturers and are a variation of the triple-pane. It sandwiches a polyester film between two pieces of glass. The film, which has a low-e coating, provides extra insulation without adding significantly to the double-pane window's weight and thickness.
Most manufacturers use the term "U-value" as a measure of insulating ability. We've used the more familiar R-value (the U-value divided into the number 1). Some manufacturers quote numbers that are supposed to tell you how well a window retains heat. The makers have different ways to test windows or report the results, so the numbers are difficult to compare. Only California now requires windows to be certified according to a standard test method developed by the National Fenestration Rating Council.
Glass makers participating in a program run by the Insulated Glass Certification Council subject insulated glass to tests that expose the glazing and its seals to heat, cold, and water. Glazing that passes three test segments earns a "CBA" mark which is etched on a corner of the glass or stamped on the edge of the spacer. Models with a "CB" have passed two identical tests; and models marked "C" have passed one test.
Literature for some windows lists a shading coefficient--a measure of the window's ability to control the amount of sunlight that passes through. Low numbers provide more shade; higher numbers, less shade. Floridians, whose cooling bills are taxed by the summer sun, should look for a low shading coefficient. Northerners, who want to make the most of the sun's warmth in winter, should look for a high shading coefficient.
Low-e coatings and similar treatments can affect the amount of light entering a room. Some manufacturers provide a "visible light" or "daylight transmission" number to help you compare different types of glazing. Untreated double-pane windows typically admit about 82 percent of visible light; low-e glass emit no more than 79 percent. But numbers like those may be hard to relate to real life. If you think new windows might block too much sunlight, ask the retailer whether you can take a sample of the glass home.
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