Summary:

Install double pane energy star windows instead of triple pane windows. The vast majority of energy savings related to windows comes in the form of a well sealed frame to outer wall and tight connections between moving parts of the window. Skip triple pane windows. They will save you less than 5 cents annually in energy costs per square foot of glazing.

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All the Facts:

First, let’s start with what is right about replacing windows.

The main benefit to window replacement comes from an increase in air tightness, not the glass itself. Fair and away that biggest energy waster in homes are the cracks and other openings that allow energy to escape in the form of conditioned air which is then replaced with outdoor air that your furnace or AC has to start fresh with.

The standard unit for measuring air leakage is “cfm50″. According to The Homeowner’s Guide to Renewable Energy, “A really good measurement is around 500 to 1,500 cfm50. The older houses we work on typically fall in the 6,500 to 8,500 cfm50 range.” Green Building Advisor’s Dr. Allison Bailes equates cfm50 to air leakage in a clear and easy to understand manor.  He says, “Each 1000 cfm50 is approximately equal to 1 square foot of hole.”  By combining those pieces of information, we find that the average older home has the equivalent of a 6.5 to 8.5 square foot hole in its wall. That hole leaks air, and thus energy, year round.  It isn’t surprising to learn that a large percentage of heating and cooling costs are due to air leaks alone.

By replacing windows, you can decrease the leakage found between moving and fixed parts. However, the other place you want to devote attention when replacing windows is the perimeter. Silicone sealant should always be used to seal gaps 1/4″ or smaller and expanding foam sealant should be used on gaps bigger than that.

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The windows themselves should exhibit tight joints between all moving parts. The more moving parts, the less tight the windows will be. A double hung window, one where both the upper sash and lower sash can move up and down, usually provides the worst air leakage characteristics. At the other end of the spectrum is a casement window. These windows seal very well since after the window is closed they use a latch that compresses the moving portion of the window to a compressible seal.

You will notice that all of the information in this section relates to air sealing and none of it relates to the glazing or frame material. As long as you use an Energy Star certified window, the differences in glazing and frame material do not matter. That means that you should never even consider triple pane windows. They cost significantly more money, but provide hardly any additional energy savings over double pane windows. In a house I built, I asked the home Energy Star rater to switch the double pane windows for triple pane windows in the computer model for the house just to see what the annual energy cost difference would be. It turned out the savings was only 10 dollars for over 300 square feet of windows, 3.3 cents in savings per square foot.

Whatever additional money you would have spent for triple pane windows, will be more effectively spent on energy efficient appliances, LED lighting, or a roof top solar array.