Low-E Glass Explained: How It Works in Aluminum Windows and Doors

Windows are usually the weakest part of a building’s envelope. Ordinary glass lets solar heat in during the day and lets indoor heat escape at night, and the air conditioning ends up paying for both.

Low-emissivity glass — usually shortened to Low-E — is one way of cutting that transfer. A very thin metallic coating is applied to the glass during manufacture. The coating reflects infrared energy while still passing most visible light, so daylight comes through but less radiant heat crosses the pane. Emissivity is simply the rate at which a surface radiates the energy it has absorbed; ordinary glass radiates readily, which is why untreated glazing couples so easily to whatever is on the other side. The coating lowers that rate, so less heat moves across by radiation. To be clear: the coating does not change the view or turn the window into a mirror, though some Low-E products do read slightly reflective depending on the coating and the light.

What it changes in practice

It slows radiant heat passing through the pane, in both directions. It reduces the ultraviolet and infrared light entering the room, which helps slow fading of interior finishes. And it keeps most visible light transmission, so daylight is not traded away for the thermal benefit.

These are the general properties of Low-E coatings, not figures for any one product. How much a given window actually saves depends on the whole build-up — coating type, single or double glazing, spacer and gap, the frame itself, and the local climate — so the numbers only mean anything once the configuration is fixed.

Where it gets used

Low-E glazing is common in commercial offices, hotel lobbies and retail spaces, where large glass areas and high cooling loads come together. In housing it tends to appear where the orientation puts a lot of sun on the glazing, or where the owner wants to slow fading.

Before you specify it

If Low-E is part of your requirement, the glazing build-up has to be settled together with the frame, not separately. The coating choice, the number of panes and the spacer all interact with the thermal break and with overall window performance. Send us the project location, orientation and target performance, and we will confirm which glazing configurations are available for it.