Reduce energy consumption at home by replacing your windows

With many projects intended to reduce energy consumption, the costbenefit ratio is relatively small. Weather stripping and caulk may be inexpensive, but they take lots of time and labor to install. Because the gains are hard to see, the effort is harder to make. Replacing an entire window requires much more work than sealing the edges of an existing one, but the benefits are immediate and obvious.

According to research number from Lawrence Berkeley National Laboratory, if you install energy-efficient windows in San Diego, which has a warm and stable climate, you can save nearly 4 million Btu per year, which translates to $65 in current heating costs. That makes the payback time fairly long - unless energy prices go up.

In Great Falls, Montana, where the temperature and humidity change dramatically over the course of a year, an energy-efficient window can bring savings of roughly 62 million Btu, or $695 per year. If energy prices rise, that number will go higher as well. The payback time would be almost immediate.

Building supply centers, lumberyards, and specialty stores can all stock energy-efficient windows, but comparison shopping can be difficult. Few retailers carry more than one brand, which means you have to visit several places to check one set of windows against another. And while the common features listed above apply to most replacement windows, manufacturers try to set themselves apart by giving similar products different names, and emphasizing the qualities their windows offer while minimizing any weak points.

To see how different windows compare, use the current standard for technical information - the Energy Star rating system from the U.S. Environmental Protection Agency (EPA). It will help you check the measurable differences in performance among all products that qualify for Energy Star labeling. These tests and measurements are set by a nonprofit industry group called the National Fenestration Rating Council (NFRC). Any window that qualifies for the Energy Star designation must include information on the following characteristics:

U-Factor: This tells you the rate of heat transfer, which indicates the level of insulation provided. Unlike R-values, where higher numbers mean better performance, you will want to find windows with the lowest U-Factor ratings.
Solar heat gain coefficient: This is a measure of how much heat from sunlight gets through the window and into your house. Values range between zero and one; again, lower numbers mean better performance.

Those two values determine the Energy Star rating for a window. An NFRC label also provides information on other performance characteristics:

Visible transmittance: Similar to solar heat gain coefficient, this is a measure of the light each window allows to pass through. These values also range between zero and one. The difference is that you generally want less heat gain but more light transmittance.
Condensation resistance: In locations where differences in humidity cause excessive condensation, this rating becomes very important. Ranging from 0 to 100, this number indicates a window’s resistance to condensation; higher ratings mean greater resistance to water buildup.
Air leakage(AL): This number shows how much air gets past the window in a set period: cubic feet of air per square foot of window per minute (cfm/ft2). Lower values mean less air leakage. Most building codes require an AL value of 0.3 cfm/ft2.

You don’t need to become an expert on heat transfer to make an informed buying decision. Just remember that lower numbers are better for U-factor, solar heat gain coefficient, and air leakage; higher numbers are better for visible transmittance and condensation resistance.

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Reduce energy consumption at home by replacing your windows

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