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Do Your Windows Pass The “Miles-Per-Gallon” Test?

(If They’re 10 Years Old Or Older, Probably Not!)

Energy Efficient Windows in Lubbock, TX

If you’ve been shopping for new windows or an energy efficient window near Lubbock, you may have been told U-factor is the most important thing to look for.

While this is true for colder parts of the country, U-factor is not what’s most important in hotter states like Texas.

This is because U-factor measures the rate of heat loss (in BTU’s per hour) through glass or other materials in wintertime. And while it’s true we want to keep our homes warm during the few months when it’s cold outside, the much bigger problem is keeping them cool during the long, sweltering summer months.

Since we live in a warm climate, Solar Heat Gain Coefficient (SHGC) is the number one thing you must look for when shopping for an energy efficient window in Lubbock and surrounding areas.

SHGC is the rate at which radiant heat, the heat that accompanies light from the sun, penetrates through a window. This rate is expressed as a number between 0 and 1. The lower the SHGC, the better the window is at blocking heat.

According to Efficient Windows Collaborative, which is sponsored by the Department of Energy, the ideal window for southern Arizona should have a SHGC of less than or equal to 0.27.

You may be thinking, “But won’t blocking out heat also keep out sunlight?”

The answer to this question used to be Yes. (Shade screens, for example, cut down heat by reducing a corresponding amount of light.) But Low-E technology has changed that.

Low-E consists of one or more microscopic coatings of silver applied to glass (in Lubbock it should be applied to the inside of the outer pane of insulated glass). The great thing about Low E is that it’s “spectrally selective” — it can differentiate between longer wavelengths (the heat portion) and shorter wavelengths (the light portion). This technology blocks the former while letting the latter pass through. This, in turn, gives you the best of both worlds: maximum light with minimal warming effect.

When you want to determine how much light a window will let in, you need to look at its Visible Transmittance (VT) rating. This rating is express as a number between 0 and 1, with most windows falling between 0.3 and 0.7. But unlike SHGC, the higher a window’s VT rating, the better.

Remember: You want a low SHGC and a high VT.

What About The Other Factors?

Though not as important SHGC, the following features should still be considered when you’re shopping for an energy efficient window near Lubbock.


The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly, expressed between 0 and 1. The lower the number, the better. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating properties.

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 near Lubbock 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.)