How Low-E Coatings Keep Heat In Without Blocking Daylight

Low-emissivity glass is the single biggest reason a modern sealed unit outperforms one from twenty years ago. The coating is thinner than a soap bubble, invisible in normal light, and does one job brilliantly.

The physics in one paragraph

Warm objects in a room — radiators, people, furniture — radiate heat as long-wave infrared. Ordinary glass absorbs and re-emits much of that energy outwards. A low-E coating is a metallic layer engineered to reflect long-wave infrared back into the room while letting visible light and the sun’s short-wave energy straight through. Heat stays in; daylight is barely touched.

Hard coat vs soft coat

• Hard coat is fired into the glass surface during float production. Durable, slightly higher emissivity, faint haze in some lights.
• Soft coat is sputtered on afterwards in vacuum chambers. Lower emissivity — better insulation — but must face into the sealed cavity where it is protected.

Nearly all high-performance units use soft coat on face 3 (the cavity side of the inner pane) in a DGU.

What it does to the numbers

An air-filled unit with plain glass sits around 2.7 W/m²K. Add a soft-coat low-E and argon and the centre-pane value drops close to 1.1 W/m²K — calculated to EN 673. Try build-up combinations yourself in the U-value calculator.

Pairing it well

Low-E works hardest alongside argon fill and a warm edge spacer. If you are replacing misted units anyway, specifying all three costs little more and transforms winter comfort near the glass.