Windows are a major source of heat loss or gain. This means that during the warm times such as summer, your home will tend to gain heat through your windows, which is usually undesirable, as it is not only a discomfort, but it will cost you more to keep your home cool. During cold times, like winter, your home will lose heat making it colder inside, which of course you want to avoid, as this will cost you more to keep your home warm.

It is not possible to totally prevent heat loss or gain, but it is possible to significantly reduce it by having well insulated windows. For this reason, double pane and triple pane windows are much preferred over single pane windows. In this article, we will look at how to compare the energy efficiency of windows.

First let us understand the five main factors that determine energy efficiency of windows.

1. Number of layers of glass. The more layers the window has, the greater the insulation. So one would naturally expect that double panes would perform better than single panes, and triple better than double.

2. How much air space is allowed between the glass panes. The width of the air space matters because to get the best efficiency, they should fall between specific sizes. Going smaller or bigger will reduce the energy efficiency. What is used to fill the “air” space will also affect the conductivity of the glass. Filling with an inert gas like argon or potassium will give better resistance to heat transfer, than just filling it with air.

3. The type of glazing. There are different types of treatments that can be applied to glass which would impact on the energy efficiency of the window. For eg. Low emissivity (low-e) glass allows the light to pass through fully, but it reflects 40% to 70% of the heat. Triple panes allow an extra pane to be treated with a low-e coating, thereby giving it even greater potential for higher energy efficiency. There is also heat-absorbing glass, which can absorb up to 45% of the solar energy.

4. The type of frame and spacer materials used. Some frames conduct heat better than others. For eg. Aluminum frames are not as good insulators as wooden and vinyl window frames.

5. The soundness of the installation. A poorly installed window could defeat all your efforts to enhance energy efficiency. For if the window and frame are not tightly fitted to the space where they should go, then air and heat will be transmitted freely through the gaps.

Windows are given ratings that indicate how well they resist heat conduction, or how well they conduct heat. The R value of a window measures how well it resists heat. Therefore the higher the R value, the better its energy efficiency will be. The U value of a window is actually the inverse of the R value [ie. 1/(R value)]. It measures conductivity. So the higher the U value, then the more heat transfer is allowed. This means the lower the U value, the better it will be for energy efficiency.

R values for windows generally range from around 0.88 to 3.0. However triple pane windows can go beyond an r value of 5.0. When you are getting windows, you should check to see what the R values are, so you can compare which ones are more energy efficient. You should also make sure you are comparing like with like. This means you should take into consideration the frames for each comparison. Having triple pane windows using an aluminum frame might not be better than having a dual pane with a fiberglass frame. So you should try to get an idea of the R value when all is factored in and make sure that you’re not only being told the R value of the center of the window.

To ensure that your comparisons are valid, you should also ensure that the R values of the different windows are set using the same standards.

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