Low-E Glass Explained: What the Coating Does and Which Kind You Need

Low-E glass is window glass with a microscopically thin, nearly invisible coating of metal or metal-oxide layers ³ that limits heat moving into or out of a building while letting daylight through ². "Low-E" stands for low emissivity: the coating reflects long-wave infrared energy, or heat, instead of emitting it ². In winter that keeps room heat inside; in summer, depending on the coating, it keeps much of the sun's heat out.

The coating is the main reason a modern double-pane window performs so differently from an older clear one. It lowers the U-factor (heat loss) and, depending on the type, the solar heat gain coefficient, or SHGC (solar heat let in) ⁴. Which type you want depends on your climate: a home in Phoenix and a home in Minneapolis want different coatings.

How the coating works

Emissivity is a number from 0 to 1 that describes how readily a surface gives off radiant heat ¹. Vitro, a glass maker, puts ordinary clear glass at about 0.84 and says some high-performance low-E glass reaches about 0.02 ¹. The coating reflects long-wave infrared energy, the heat given off by people, furniture and heaters, while visible light passes through ¹².

Inside a sealed insulating unit, the coating sits on one of the inner glass surfaces. Glass makers number those surfaces from the outside in, and Cardinal, another glass maker, places its standard low-E coatings on the inner surfaces of double- and triple-pane units ². You do not need to know the surface number to buy a window, but it explains why two windows with "low-E" on the quote can perform differently: the coating type and where it sits both change the result ¹.

DOE's consumer guide also notes that low-E glass reflects ultraviolet light, which can reduce fading of furnishings by as much as 75% ⁶.

Hard coat vs soft coat

There are two ways to put the coating on glass ¹:

• Hard coat (pyrolytic). Applied while the glass is being made, so the coating bonds to the surface and is very durable. Its emissivity is somewhat higher ¹. Cardinal lists different glass surfaces for its hard coats than for its standard coatings, including the outermost faces of the unit ².
• Soft coat (sputtered). Applied later in a vacuum chamber, it reaches very low emissivity and high solar control performance ¹. Soft coats are typically sealed inside the insulating unit, between the panes ¹.

The practical question for you is not which process was used but what the coating does to the window's NFRC numbers.

Why the right low-E depends on your climate

Low-E coatings are designed either to let in more of the sun's heat or to limit it, depending on climate ¹. That shows up in the SHGC on the label, which runs from 0 to 1: low numbers resist solar heat, high numbers allow it ⁴.

ENERGY STAR's criteria make the climate split concrete ⁵:

In the two southern zones, a low SHGC is required, so you want a solar-control coating that blocks much of the sun's heat ⁵. In the Northern zone, ENERGY STAR sets a minimum SHGC instead and even accepts a slightly higher U-factor when SHGC is 0.35 or higher ⁵. That favors a coating that keeps heat in without blocking too much sun.

DOE adds that orientation matters: in a cold climate, a south-facing window benefits from the highest SHGC you can get, while east and west windows usually do better with a low SHGC or shading ⁶. Some installers can supply different glass for different sides of the house; it is worth asking.

Gas fills and low-E work together

A low-E coating handles radiant heat. Gas fills handle conducted heat. Argon or krypton is sealed between the panes in place of air; NFRC describes argon as an inert, nontoxic gas used to reduce heat transfer ³. Most efficient windows combine the two, which is why quotes often list them together as a "glass package." If a quote lists low-E but not the gas, ask.

Is low-E glass worth it?

DOE lists low-E coatings among the features of ENERGY STAR windows, alongside multiple panes, gas fills and better spacers and frames ⁶. For a home replacing clear glass, the coating is usually part of the glass package being quoted rather than an exotic extra. On price, Fixr lists $300 to $1,100 per window for a window with low-E glass and $450 to $2,000 for double-pane windows in general; the ranges overlap because size, frame and brand move the price as much as the coating ⁷.

Two side effects are worth knowing:

• Outside condensation. Because low-E glass keeps heat in, the outer pane stays cooler. Andersen notes that this can cause dew on the outside of the glass on cool, humid mornings, which usually evaporates during the day and does not affect the inside of the home ⁸. More in window condensation.
• Tint and light. Some solar-control coatings lower visible transmittance (VT), the share of daylight that comes through ⁴. Compare VT on the label if you want bright rooms.

Low-E vs tempered glass

They are different things. Low-E describes a coating that controls heat. Tempered describes glass that has been heat-treated for strength and safety. A pane can be both, and a quote may list both.

How to tell if glass is low-E

The surest way is the paperwork: the NFRC label on a new window and the quote or order should name the glass package and its numbers ⁴. On older windows, the manufacturer may be able to identify the glass from the window's model and series.

For the full label and every rating on it, see energy-efficient windows explained and window U-factor explained. When you are ready, you can compare quotes from window installers and ask each one which coating they are quoting.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Vitro description of how low-E coatings work, the 0 to 1 emissivity scale, clear glass at about 0.84 and high-performance low-E near 0.02, and hard-coat vs soft-coat processes, updated August 20, 2026, checked October 22, 2026.
• ² Reported: Cardinal glossary definition of low-E glass and emissivity, and the glass surfaces used for its standard coatings and its hard coats, updated July 24, 2026, checked October 22, 2026.
• ³ Official record: NFRC glossary definitions of low-E coatings, gas fills, argon and krypton, checked October 22, 2026.
• ⁴ Official record: NFRC label descriptions of U-factor, SHGC (0 to 1) and visible transmittance, checked October 22, 2026.
• ⁵ Official record: ENERGY STAR U-factor and SHGC limits by climate zone, including the Northern SHGC floor and the 0.35 equivalent-performance row, read in the official criteria PDF, checked October 22, 2026.
• ⁶ Official record: DOE consumer guide (2021 PDF) listing low-E coatings among ENERGY STAR window features, low-E glass reflecting UV light and reducing fading by as much as 75%, and glass choice by orientation, checked October 22, 2026.
• ⁷ Reported: Fixr per-window ranges for low-E ($300 to $1,100) and double-pane ($450 to $2,000) windows, updated January 21, 2026, checked October 22, 2026.
• ⁸ Reported: Andersen help article on exterior condensation on low-E insulating glass, checked October 22, 2026.