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Low-E glass explained

Low-E glass — short for low emissivity — is ordinary glass carrying a microscopically thin, transparent coating that reflects heat. It is the single most important factor in a modern sealed unit's thermal rating, and it is the reason today's double glazing outperforms the units of twenty years ago even at the same cavity width. The coating is almost invisible in daily use, yet it changes how the glass handles radiant heat completely.

What emissivity actually means

Every surface radiates heat, and emissivity is a measure of how readily it does so. Plain glass has high emissivity: it happily radiates warmth from your heated rooms straight out through the window. A Low-E coating is a layer of metal oxide, only a few atoms thick, that has very low emissivity. It lets visible daylight pass but reflects long-wave radiant heat back into the room. In effect, the warmth your radiators produce is bounced back indoors instead of escaping through the glass.

Soft coat and hard coat

There are two families of Low-E coating. Hard-coat (pyrolytic) glass has the coating baked into the surface as it is made; it is durable and can be used as a single pane. Soft-coat (sputtered) glass has a more advanced multi-layer coating applied afterwards; it performs better thermally but must sit inside a sealed unit to protect it. Most modern high-performance units use a soft-coat Low-E on the inner face, where it is shielded by the cavity.

Working with argon and spacers

Low-E is powerful, but it is designed to work as part of a set. The coating tackles radiant heat loss; an argon gas fill tackles convective heat loss across the cavity; and a warm-edge spacer reduces loss at the edge. Specify all three and the unit's rating improves substantially without any change to how the window looks. This is why the coating alone is never the whole story on a quote.

Solar gain and orientation

Low-E coatings also influence how much free solar heat comes through the glass, described by the G-value. Some coatings are tuned to keep more solar gain (helpful on north-facing rooms that need the warmth), others to reject it (helpful on large south-facing glazing prone to overheating). The balance between keeping heat in and letting solar heat through is exactly what U-value and G-value describe. If you want to understand how these specs affect energy savings across a whole house, it is worth reading alongside this page. According to the Energy Saving Trust, replacing older glazing with modern energy-efficient units typically reduces heat lost through windows, though the exact figure depends on your home.

What to check on a quote

Ask whether the Low-E is soft coat or hard coat, and which face it sits on. A good installer states the coating clearly and confirms it on a home survey. Combined with the gas fill and spacer, it lets you compare two quotes on the physics rather than the sales pitch.