Key takeaways
- U-factor measures how well a window holds heat, and lower is better
- SHGC measures how much sun heat comes in
- The best numbers depend on your climate and the wall
U-factor
U-factor tells you how fast heat moves through the whole window, frame and glass included. A lower number means better insulation. In cold climates it is the number to watch most.
Compare units by the same measure on the same label.
Solar heat gain
Solar heat gain coefficient, or SHGC, is the share of the sun's heat that passes through. In hot climates a low number keeps rooms cooler. In cold climates a higher number on south walls lets in free winter warmth.
That is why the right window is not the same on every wall.
Putting it together
Look at your climate, then at each wall's sun. Our Sun Dial and Glass Lab help you test combinations, and we explain the label in plain words before you order.
Visible transmittance, air leakage and condensation ratings are also on many labels and are worth a look.
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Quick answers
What is a good U-factor?
Lower is better, and what counts as good depends on your climate. We show the numbers for your area.
Can I have different glass on different walls?
Yes, and it is often smart to.
Ready to talk it through? Get a free estimate or call 800-224-9245.