Replacement Window GuideLow-emissivity, or low-e, glass carries a coating too thin to see that changes how a window handles heat and light. The Department of Energy describes a low-e storm coating as a thin layer that reflects radiant heat, and ENERGY STAR reports that the coatings on certified windows reduce fading by up to 75 percent. This guide explains what the coating does and how it works with the rest of the glass package, with every figure tied to its source.
What low-e glass is
Low-e glass is standard window glass with a microscopically thin coating that controls radiant heat. The NFRC consumer guide on glazing describes low-emissivity coatings and gas-filled spaces between panes as advancements that improve a window's ability to control heat transfer.
The coating does not block your view. It targets the invisible infrared energy that carries heat, letting a window keep more warmth inside in winter and reject more solar heat in summer without going dark.
How the coating reflects heat by season
The Department of Energy, describing low-e storm windows, explains that a coating with low emissivity reflects heat back inside the house during the winter and reflects it outside during the summer, keeping the home more comfortable.
The same source describes a low-e coating as a thin layer deposited directly on the surface of one or more panes of glass, where the coating increases the window's energy efficiency by reflecting radiant heat. The same physics applies whether the coating sits on a storm window or on a modern insulated glass unit.
How much heat a glass package controls
Coatings work alongside panes and gas. The NFRC glazing guide states that proper glazing can reduce heat transfer by up to 50 percent, and that homes can lose up to 30 percent of heating and cooling energy through inefficient windows.
The same guide describes double-glazed units as two layers of glass separated by a layer of inert gas that significantly reduces heat transfer. A low-e coating is what pushes a sealed, gas-filled unit toward the top of that range.
Low-e and fading protection
Low-e coatings also protect interiors from sun damage. ENERGY STAR reports that the coatings on certified windows reduce fading by up to 75 percent from ultraviolet light exposure.
ENERGY STAR also notes that a standard double-pane window allows approximately 75 percent of the sun's heat into your home, while certified options substantially reduce that heat gain. The coating is the reason a low-e window can hold light while cutting both fading and unwanted solar heat.
Matching low-e to your climate
Not every low-e coating suits every climate. ENERGY STAR requires that windows meet U-Factor and, where applicable, Solar Heat Gain Coefficient requirements based on climate zone, so the right coating in a cold zone differs from the right one in a hot zone.
A cold-climate coating favors a low U-factor to keep heat in, while a hot-climate coating favors a low SHGC to keep solar heat out. Reading the NFRC label, not the marketing name, tells you which balance a given low-e product strikes.