Low-E Glass Explained: What the Coating Actually Does
Low emissivity glass carries a metallic-oxide layer a few atoms thick. It is transparent to visible light but reflective to long-wave infrared — the heat your radiators, your body and your room surfaces radiate. That heat gets bounced back inside instead of being absorbed by the glass and lost.
Soft coat versus hard coat
Hard coat (pyrolytic) is applied while the glass is still hot in manufacture and is durable enough to face a cavity or even the weather. Soft coat (sputtered) performs significantly better — lower emissivity, better U-value — but is more delicate and must face into a protected cavity. In a secondary glazing unit the cavity is on the room side of the original, which is a protected position, so soft coat is what we specify.
- Soft coat: better performance, must face the cavity
- Hard coat: tougher, lower performance
- Coated face orientation matters — get it wrong and you lose most of the benefit
What it does to the numbers
Adding soft-coat Low-E to a sealed secondary unit takes achievable U-values down to around 1.868 W/m²K, against roughly 5.4 W/m²K for a bare single-glazed sash. More usefully for how the room feels, the room-side surface runs several degrees warmer, which is what removes the downdraught.
When we advise against it
Low-E adds cost per square metre. On a landing window, a hallway, a WC or a room you barely occupy in winter, we will normally suggest clear float and putting the money into the rooms you actually sit in. We would rather specify honestly than uplift every opening.
Frequently asked questions
There is a very slight neutral cast visible at certain angles. In practice nobody notices it once the unit is in and the room is furnished.
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