Window U-values and Part L

What a U-value is

The rate at which heat passes through a square metre of the window for each degree of temperature difference, in watts per square metre per kelvin (W/m²K). Lower is better. A single-glazed Victorian sash is around 4.8–5.0. An old non-thermally-broken aluminium window is worse still. A modern double-glazed unit in a thermally broken frame is around 1.2–1.4.

The number that matters is the **whole-window** U-value (Uw) — glass and frame together, weighted by area. A quote that only gives you the centre-pane figure (Ug) is quoting the best number it has, not the number the regulations use. Ask for Uw.

The three levers on the number

• Low-E coatingA microscopically thin metal-oxide layer on the cavity face of the inner pane, reflecting long-wave heat back into the room. Soft coat outperforms hard coat. This is the single biggest contributor.
• Cavity fillArgon instead of air, typically worth around 0.1–0.2 W/m²K. Krypton for narrow cavities. Cheap, and standard on any decent unit.
• Spacer barA warm-edge polymer spacer instead of aluminium. Reduces heat loss at the edge of the glass and — just as usefully — reduces the condensation that forms around the perimeter of the pane on cold mornings.

Reading a quote honestly

Any quote for replacement windows should state the whole-window U-value it will achieve. If it doesn't, ask — and if the answer is vague, that is your answer.

Two quotes at 1.4 and 1.2 W/m²K are not the same product. The 1.2 will normally be a better glass spec, and it will cost more. Whether it is worth it depends on how long you are staying and how cold the room is now.