A picture window is mostly glass, and glass lets in far more heat than an insulated wall. A big pane on a west wall takes the full afternoon sun all summer. On a picture window, the glass you pick matters more than anything else.
Our energy-efficient windows page explains how these ratings work together.
- Low-E glass. Low-E is a thin, clear metal coating that reflects the sun’s heat. Coatings differ a lot. Low-E 366, for example, has three layers of coating and still lets about 66% of visible light through. Our glass types guide compares them.
- SHGC. Solar heat gain coefficient is the share of the sun’s heat that gets through the window. Lower is better in Phoenix. SRP’s rebate requires 0.23 or lower.
- U-factor. U-factor measures how easily heat moves through the whole window. Lower is better. It works 24 hours a day, not only when the sun is on the glass.
- Which wall it faces. West-facing glass gets the hottest sun of the day. South-facing glass gets less summer sun when a roof overhang shades it. On a west wall, a patio cover, a tree or a sun screen helps along with good glass.
Our energy-efficient windows page explains how these ratings work together.
