Do Your Windows Pass the “Miles-Per-Gallon” Test?
If Your Windows Are 10 Years Or Older, They’re Probably Sucking More Cash Out Of Your Wallet Than A Gas-Guzzling SUV Would!
Similar to how many miles per gallon a car gets, windows are also measured on how efficient they are.
But while cars are rated on how fuel efficient they are (miles per gallon), windows are rated on energy efficiency in these areas: U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage, and Condensation Resistance.
This sounds highly technical and confusing, but it’s not. In fact, the only rating you really need to concern yourself with is U-Factor. Here’s why.
U-Factor measures the rate of heat loss between 0 and 1. The lower the number, the better the window insulates.
A well-insulated window is going to keep your home a comfortable temperature year round, while cutting your energy costs.
According to Efficient Windows Collaborative, Missouri homeowners want a window with a U-Factor of 0.32 or less.
Consider that windows eight years and older have an average U-Factor of 0.45, and you’ll see just how much more energy efficient new windows are. Since you can save approximately $30 per year in energy costs for every .01 you lower the U-value of your windows, you’d be keeping at least $390 in your bank account every year.
What about the other factors?
Visible Transmittance (VT)
VT rates the amount of light that can enter a window on a scale of 0 to 1. In the case of VT, the higher the rating, the better. Most windows are between 0.3 and 0.7.
Solar Heat Gain Coefficient (SHCG)
SHGC rates how well a window blocks out heat on a scale of 0 to 1. The lower the number, the more heat the window blocks. SHGC is more important in hot climates like Arizona, where the temperatures reach well over 100 degrees. As a Virginian, however, you don’t have to pay as much attention to SHGC as you do U-Factor. Just know a good SHGC is 0.4 or less.
Air Leakage (AL)
AL is the rate at which cubic feet of air passes through square foot of window area, expressed between 0 and 1. The lower the AL, the less air will pass through cracks in the assembly. AL isn’t as important as U-Factor and SHGC and is an optional measurement on the NFRC label. Generally, though, a rating of 0.3 or lower is considered good.
Condensation Resistance (CR)
CR measures how well a window resists condensation on the inside surface. CR is expressed as a number between 1 and 100. The higher the number, the better. Fifty to 60 is considered average, while anything above 60 is considered good. (CR is also an optional rating, and does not appear on some NFRC labels.)
