What Is Low-E Glass?

Low-emissivity glass — universally shortened to low-E glass — is float glass coated with an ultra-thin metallic or metallic oxide layer that significantly reduces the amount of infrared (heat) radiation passing through it. The coating is so thin — often just a few hundred nanometres — that it has no visible effect on the appearance of the glass from inside a building.

The concept of emissivity is key here. All materials emit thermal radiation, and emissivity is a measure of how efficiently they do so on a scale from 0 to 1. Ordinary float glass has an emissivity of around 0.84. A high-quality low-E coating can reduce that to as low as 0.02–0.04, meaning it radiates far less heat and reflects far more back into the room.

How the Coating Works

Low-E coatings work by selectively filtering the electromagnetic spectrum. Visible light (which we see and which provides daylight) passes through largely unimpeded. Long-wave infrared radiation — the heat emitted by warm objects indoors, such as radiators, people, and furniture — is reflected back by the coating rather than escaping through the glass.

Solar infrared (short-wave infrared from sunlight) can be either blocked or allowed through depending on the coating type:

  • Hard-coat (pyrolytic) low-E — applied during glass manufacture; more durable but slightly less effective. Often called "online" low-E. Allows more solar gain, making it better suited for colder climates where you want passive solar heating.
  • Soft-coat (magnetron sputtered) low-E — applied in a vacuum after manufacturing; higher performance but must be protected inside a sealed double or triple glazed unit. Better for warmer climates or south-facing windows where solar gain needs limiting.

Low-E Glass in Double Glazing

Low-E glass is most effective when used in a sealed insulating glazing unit (IGU) — what we commonly call a double or triple glazed window. The coating is applied to one of the internal surfaces (typically surface 2 or 3 in a double-glazed unit, counting from the outside in). The argon or krypton gas fill in the cavity further suppresses heat transfer by conduction and convection.

Together, low-E coating plus inert gas fill can achieve centre-pane U-values below 1.0 W/m²K in triple glazed units — compared to around 5.6 W/m²K for a single-glazed window. That's a dramatic improvement in thermal performance.

Real-World Benefits

Lower Heating and Cooling Bills

By keeping heat in during winter and reducing solar heat gain in summer (depending on specification), low-E windows reduce the workload on heating and air conditioning systems. Exact savings depend on your climate, building, and the specific coating used, but upgrading from single glazing to a low-E double-glazed unit typically yields noticeable reductions in energy consumption.

Improved Comfort

Cold windows create radiant cold spots that make occupants feel uncomfortable even when the air temperature is warm. Low-E glass keeps the inner pane surface warmer, eliminating the uncomfortable cold zone near windows in winter.

Reduced Condensation

Because the inner pane is warmer, condensation is less likely to form on the glass surface — helping to protect window frames from moisture damage and mould growth.

UV Protection

Many low-E coatings also filter a significant portion of ultraviolet radiation, reducing fading of furnishings, artwork, and flooring near windows.

How to Check If Glass Is Low-E

A simple test: hold a lit match or cigarette lighter close to the glass. You'll see multiple reflections — one from each glass surface. In a standard double-glazed unit you see four reflections (two surfaces per pane). A low-E coating will cause one of those reflections to appear a slightly different colour (often a shade of blue, green, or purple) compared to the others. Some manufacturers also mark their low-E units in the edge spacer.

Choosing the Right Low-E Specification

When specifying low-E glass, the key performance metrics to look for are:

  • U-value — measures heat loss; lower is better for cold climates.
  • Solar Heat Gain Coefficient (SHGC) / g-value — measures solar heat admitted; higher is better for passive solar heating, lower is better for solar control in warm climates.
  • Light transmittance (LT) — the percentage of visible light let through; higher means a brighter interior.

A glazing specialist or window manufacturer can advise on the best combination of these values for your specific building orientation and climate.