Decoding Your New Windows

New windows can make a huge impact on your home. Not only can installing new windows make your place look better, but those windows can also make it a lot easier to heat and cool as well. Sure, replacing old windows can eliminate drafts, but that’s only a small part of how installing new windows can increase your home’s energy efficiency.

Information about the efficiency of new windows is printed on a sticker that’s attached right to the glass. Unfortunately, if you don’t know what you’re looking at then these stickers may raise more questions than anything. If you need a little help understanding exactly what you’re looking at on your window sticker, here’s a rundown of everything you need to know.

What’s the U-Factor?

When looking at window stickers, two values are listed as “Energy Performance Ratings.” The first of these is the U-Factor, which provides information about the insulating ability of the window. This is similar to the R-Value that you find on insulation, and the U-Factor value will usually be somewhere between 0.20 and 1.20 on new windows. The lower this value is, the better the window is at insulating your home and preventing heat transfer between the inside and outside. If you want to think about this in terms of R-Value instead, simply divide 1 by your U-Factor value and you’ll end up with the corresponding R-Value (so a U-Factor of 0.20 would correspond to an R-Value of 1 ÷ 0.20 = 5, while a U-Factor of 1.20 would correspond to an R-Value of 1 ÷ 1.20 = 0.83.)

What About Solar Heat Gain Coefficient?

The other value listed under “Energy Performance Ratings” is the Solar Heat Gain Coefficient (SHGC). This measures how much heat is transferred through the window from sunlight (as opposed to the air heat transfer that is indicated by U-Factor.) The SHGC scales between 0 and 1, with lower values indicating a greater ability to block heat transfer from sunlight.

What Is the Visible Transmittance?

Beyond the “Energy Performance Ratings” entries on a window sticker, three other values are also provided to help you choose the window that best fits your needs. One important listing among these additional performance ratings is Visible Transmittance (VT). As with SHGC, the VT of a window scales between 0 and 1. In most cases you will want a high VT, however, as it indicates how much light passes through the window glass to provide daylight for your home.

What’s the Condensation Resistance Rating?

As the name suggests, the Condensation Resistance (CR) rating of a window indicates how well it can resist the formation of condensation on its surface. This not only indicates how likely you are to experience “fogging” and liquid condensation but can also indicate the likelihood of frost formation in the winter as well. This rating ranges from 1 to 100, with higher CR numbers indicating a greater resistance to condensation.

What Does Air Leakage Mean?

Another important performance rating is Air Leakage (AL). As the name implies, this measures how much air can leak through the window and affect the internal climate of your house. These values typically scale between 0.1 and 0.3, with lower values indicating a smaller amount of air leakage. One thing to keep in mind is that this is considered an optional rating, meaning that not all manufacturers will provide AL data; as a result, some window stickers might only have ratings for the other four values.