Low-E Glass (Low-Emissivity Window)

A layer of metal or metal oxide so thin it is invisible to the eye is all that separates Low-E glass from ordinary window glass. "Low-E" stands for low emissivity, meaning the surface gives off very little radiant heat. The coating reflects heat back toward its source while still letting visible light pass, so the window stays clear but moves much less heat. The U.S. Department of Energy reports low-E coatings can reduce energy loss by 30% to 50% compared with plain glass and typically add only about 10% to 15% to the window cost.

The coating works by reflecting infrared energy. In winter it reflects indoor heat back inside; in summer it reflects the sun's heat back outside. A spectrally selective low-E coating goes further, filtering out 40% to 70% of the heat normally transmitted through the glass while still allowing the full amount of daylight. That is the version that matters in Arizona, because it lowers both the window's U-factor and its SHGC without making the home dark.

For example, two new homes in Apache Junction with the same west-facing windows will feel different at 4 p.m. in July. Plain double-pane glass lets the low sun pour heat into the room. Spectrally selective low-E glass reflects most of that heat back outside, so the room stays cooler and the air conditioner cycles less, while the view and daylight look the same.

Low-E glass is the coating, not the whole window. Its real performance shows up in the NFRC-certified U-factor and SHGC numbers on the label, which is what a builder should actually compare. Not all low-E is tuned the same way: a coating optimized for a cold climate captures winter sun, while a desert home wants the spectrally selective type that rejects it. Matching the coating to Arizona's cooling-dominated climate is the whole point.