By Katherine Salant
When selecting an energy efficient window, dual glazing, a low e coating on one of the glass surfaces, and argon gas between the panes will always serve you well. But these energy saving features do add to the cost. By going one step further and tailoring the energy efficiency of your windows to your specific climatic circumstances, you can reap the energy and comfort benefits without over spending to get them.
For example, a window that works in Vermont with its frigid winters and mild summers is overkill in a mid-Atlantic state like Virginia. And in regions like Florida where cooling is the main concern, a window that works for Vermont would be inappropriate.
With a climate as severe as Vermont's, you can even tailor the glass to the orientation of the window opening, said Alex Wilson, editor of the Environmental Building News and himself a Vermont resident.
For the north, east and west facing windows that will get little or no winter sun, he would get windows with the lowest U factor you can afford (the U-factor is a measure of the rate at which heat passes through a window). By adding a third pane of glass or a storm window combination, for example, you can bring the U factor down to as low as .21 to .25.
For the south facing windows, which will get plenty of direct sun, Wilson would specify a hard coating low e glass with a high solar heat gain coefficient (SHGC) of .55 to .60 (the solar heat gain coefficient is a measure of how much solar heat passes through a window).
A high SHGC increases the amount of solar heat than can pass through the glass and heat up the space. At the same time, because of its other properties, the low e glass will keep this solar heat inside as well as that generated by your furnace. Since the sun's heat can be substantial (and it's free), you may be able to install a smaller, less expensive furnace than would otherwise be required.
In the summer when the sun is high in the sky, the unwanted solar heat can be kept out of the house with judicious placement of overhangs or awnings, Wilson added.
In the central midwestern region with its long, cold winters and brief, hot summers a strategy that is weighted towards the cold season is appropriate. The U-factor of the windows should be .40 or less but, as with Vermont, the lower the U-factor the better.
Also like Vermont, a hard coat low e on the south facing windows (SHGC .55 or higher) will allow you to take advantage of the free solar heat coming through the windows and lower your heating bills. But the free solar heat will also pour in during the summer and raise your cooling bills. If your summers veer towards the torrid, a soft coat low e with a lower SHGC (around .40 or less) may be a more sensible strategy. This will reduce the summer heat inside but eliminate the potential benefits of that free solar heat in winter. Since cooling costs are generally about three times as much as heating costs, this could be a sensible strategy, Wilson said.
The mid-Atlantic region, including the Washington, DC area, has a cold winter, but nothing like a midwestern one, and a long, stinko hot and humid summer. To get through both seasons comfortably, a window with a low U factor (.40 or less) and a low e with a low SHGC (.40 or less) is recommended.
In Florida, where the weather is hot and hotter, the comfort goal is to keep heat out of the house all year long. Since as much as fifty per cent of the heat load can be traced to solar heat pouring in through the windows, the most effective cooling strategy is reducing the amount of sunlight coming in.
For Florida, the lower the solar heat gain coefficient (SHGC), the better--.40 or below is preferable. The U factor is not so important because the problem is not heat loss (heating going out of the house) but heat gain. A window with a U factor as high as .75 is can still get the Energy Star rating, but Wilson recommended a lower U factor of .50.
For the inland desert areas of Southern California, a similar strategy to Florida's would be appropriate. A pilot program in the Simi Valley sponsored by Southern California Edison found that adding low e with a SHGC of .39 to a clear glass window which already had two layers of glass reduced the solar heat gain and cooling costs significantly. The U-factor of the modified window was .54. Based on preliminary data, these more energy efficient windows, which added about $800 to the cost of the 3,000 square foot house, would reduce it's annual cooling costs by 12.4 per cent.
Air infiltration can also be a troublesome issue with windows, though much of this is attributable to the installation of the window, and not the window itself. When a window is installed properly, and it carries the Hallmark WDMA or the AAMA gold certification sticker, it has an air infiltration rate of .3 or less. This is a tight fit and would not be noticeable, Carl Wagus, technical director for the American Architectural Manufacturers Association said.
But in cold climates, such as Vermont's and Chicago's where winter gales are significant, a lower air infiltration rate of .10 is preferable, Wilson said.
For information on energy-saving window strategies for 48 US and 4 Canadian cities, check "window selection" at www.efficientwindows.org.
To cross reference window type with framing materials, climatic region and manufacturers that participate in the Energy Star Windows Program, check "windows" at www.energystar.org. Note that "double hung," the most commonly used window in residential construction and what you are likely to have on your new house, is listed on this site as "vertical slider."
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