Low-E coating
A low-E (low-emissivity) coating is a microscopically thin metallic layer on one glass surface that reflects heat back toward its source while staying visually clear. It is how an insulating glass unit cuts heat loss in winter and solar heat gain in summer without tinting the view.
Heat leaves a warm room in three ways: conduction through materials, convection in the air, and radiation, the infrared energy every warm surface gives off. Plain glass does little to stop radiation. A low-E coating does. The layer is only a few atoms to a few hundred atoms thick, made from silver or metal oxides, and it reflects infrared back to where it came from while letting visible light pass.
The "E" stands for emissivity, a measure of how readily a surface radiates heat. Uncoated glass has an emissivity around 0.84. A good low-E coating brings that down to about 0.05-0.15 on the coated surface.
Where the coating sits
In a double-glazed unit the four glass surfaces are numbered from the outside in. The coating goes on one of the inner surfaces, facing the gas space, where it is protected from weather and cleaning.
| Position | Surface | What it does | Typical use |
|---|---|---|---|
| Surface 2 | Inside face of the outer pane | Reflects solar heat before it enters, also reflects room heat back | Hot and mixed climates, low SHGC |
| Surface 3 | Outside face of the inner pane | Lets more solar heat in, still reflects room heat back | Cold climates, sunny south glass |
Triple glazing adds two more coated surfaces, and the glass maker chooses which to coat. The result is a tuning dial: the same frame can be a heat-blocking window or a heat-collecting one, depending only on the coating stack.
What it changes on the spec
Low-E coating moves two numbers on a window schedule. The first is U-factor, because reflecting room heat back lowers the heat loss through the glass. The second is SHGC, because a low-solar-gain coating cuts how much sun gets through. In IECC zones 1-3 the residential code caps SHGC at 0.25, and the glass coating is how a window meets that without a dark tint.
What to ask for
- The SHGC and U-factor of the whole window with that coating, not a glass-only number.
- Which coating type is on which surface, if you care about winter solar gain on a south face.
- Whether the coating is the same on every window in the schedule. Mixing by orientation is common and worth doing on purpose.
The glass options for each system are in the estimator. European double and triple units come with low-E coatings as standard, and gas fill sits alongside them: see argon fill.
Common questions
- Is low-E glass the same in every window?
- No. Low-E is a family of coatings, not one product. A low-solar-gain coating lets in little sun and suits hot climates. A high-solar-gain coating lets in more and suits cold climates with sunny south glass. Both are low-E, and the SHGC on the label tells you which one you have.
- Does low-E glass look tinted or mirrored?
- Modern coatings look clear. Some have a faint cast of green, blue or gray, visible mostly at an angle or in a reflection, but a good coating does not read as dark or mirrored from the street.
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