The short answer
Low-emissivity, or Low-E, coatings are thin layers applied to glass surfaces to manage radiant heat transfer while allowing visible light through. The words Low-E alone do not identify a window's performance: coating type and placement, pane configuration, frame, spacer, gas fill when used, and the complete NFRC ratings must be compared together.
What the coating changes
A Low-E surface reflects portions of radiant energy. Manufacturers can use different coating formulations and place the coating on different protected glass surfaces, so two windows described as Low-E can have different U-factor, solar heat gain coefficient, and visible-transmittance ratings.
- Complete NFRC label
- Match the model, size, operation, and glazing configuration to the written quote.
- U-factor
- Compare heat transfer through the complete rated product, not the coating alone.
- Solar heat gain coefficient
- Compare how much incident solar heat the complete product admits.
- Visible transmittance
- Check how the rated configuration balances daylight with other goals.
Common questions
Common questions
Does Low-E glass make every window energy efficient?
No. The coating is one part of a complete window. Frame, sash, panes, spacer, operation, air leakage, installation, and the existing opening also affect the finished result.
Sources
Sources
- Window Types and Technologies — U.S. Department of Energy
- Window Energy Performance Ratings — National Fenestration Rating Council
- What Makes a Product ENERGY STAR — ENERGY STAR