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U-value and G-value explained

Two numbers describe how a window handles heat: the U-value and the G-value. Between them they tell you how much warmth the window lets escape and how much free solar heat it lets in. Once you can read these figures, you can compare double glazing quotes on measured performance rather than on marketing language, which is the whole point of understanding the spec.

What the U-value measures

The U-value measures how quickly heat passes through the window, in watts per square metre per degree of temperature difference (W/m²K). Lower is better: a lower U-value means less heat escaping. It is the headline energy number on most quotes, and it already reflects the whole build — the panes, the Low-E coating, the argon fill and the warm-edge spacer. When you compare two units, you are largely comparing their U-values.

Glass U-value versus window U-value

There is an important distinction. The centre-pane or glass U-value describes only the sealed unit, while the whole-window U-value includes the frame and the edge effects, and is usually a little higher (worse). Make sure you are comparing like with like: two quotes should both quote the whole-window figure, or both the glass figure — not one of each.

What the G-value measures

The G-value (or solar factor) describes how much of the sun's heat passes through the glass, on a scale from 0 to 1. A higher G-value lets more free solar warmth in, which is welcome on north-facing or shaded rooms. A lower G-value rejects more solar heat, which helps large south- or west-facing windows that would otherwise overheat in summer. The right balance depends on the room's orientation, and the coating choice is where it is tuned.

Window Energy Ratings

You may also see a Window Energy Rating (WER), an A++ to E band that rolls heat loss and solar gain into a single letter, a bit like the label on a fridge. It is a handy shorthand, but the underlying U-value and G-value are the figures that let you compare quotes precisely. According to the Energy Saving Trust, replacing older glazing with modern energy-efficient windows typically cuts the heat lost through them, though the saving for any home depends on its size, heating and existing windows.

Using the figures on a quote

Ask for the whole-window U-value and, if solar gain matters for the room, the G-value too. Together with the unit build, they let you rank two quotes objectively. An installer confirms the final figures for your chosen specification on a home survey, so the numbers you compare are the numbers you get.