U-values and Part L

A U-value measures how fast heat escapes through a square metre of the assembly. Lower is better. It is quoted in W/m²K.

The number you need

In England, a replacement window in an existing dwelling must achieve 1.4 W/m²K or better for the whole window, or a Window Energy Rating of band B. Replacement doors have their own figure. New build is tighter, and Scotland, Wales and Northern Ireland set their own standards — check locally rather than assuming England's number applies.

Whole window, not the glass

This is where specifications mislead. A sealed unit on its own might be 1.0. The whole window figure includes the frame and the edge of the glass, where a spacer bar conducts heat around the seal. It is always worse than the centre-pane number. Compare like with like, and if a quoted U-value looks suspiciously good, ask which one it is.

What moves the number

• Low-E coating — a microscopically thin metal layer that reflects heat back inside. Standard now.
• Argon fill — denser than air, conducts less. Standard now.
• Warm edge spacer — a non-metal spacer at the glass edge instead of aluminium. Meaningful.
• Thermal break — on aluminium, the polyamide strip that stops the frame conducting straight through. Non-negotiable.
• Triple glazing — takes you to around 0.8, at real cost in weight and money.

Is triple glazing worth it?

In most British retrofits, no. Going from single to double is transformative. Going from good double to triple is a modest gain for a large premium, and the extra weight puts more strain on hinges. It earns its place in a new build to a high standard, on a very exposed elevation, or where you are chasing acoustic performance as much as thermal.