U-values explained: what the glazing numbers actually mean for a period home
20 July 2026
Window specifications love a number, and the U-value is the one worth understanding — it's the heat-loss score, and it's the main reason new windows change how a room feels.
What it measures. A U-value measures how fast warmth escapes through a square metre of glazing — formally, watts lost per square metre for each degree of difference between inside and out. You don't need the units; you need the direction: lower is better. A low U-value means the warmth your radiators make stays in the room.
The scale, with landmarks. The single glazing in most period windows scores around 5 — it's a radiator running in reverse. Early double glazing from the 80s and 90s typically manages high-2s. Current Building Regulations require 1.4 or better for replacement windows [VERIFY threshold at publication]. Our standard double-glazed units are rated 1.1 — comfortably beyond the regulations, and roughly four times better at holding heat than the single glazing they usually replace.
What's actually doing the work. Counter-intuitively, not the glass. Our units are "4/16/4": two 4mm panes either side of a 16mm sealed cavity, and it's the cavity that does most of the insulating — trapped, sealed, still. That's why a slim, elegant timber window can outperform a much chunkier plastic one: performance lives in the sealed unit, not the bulk.
What you'll notice at 1.1. Three things, in order: the cold-radiating zone next to the window disappears (you can put the sofa back); condensation on the glass largely stops, because the inner pane stays close to room temperature; and heating bills ease, because you've closed the building's leakiest thermal hole.
One honest footnote. Quoted U-values usually describe the glazing unit itself; the whole window — glass plus frame — is its own calculation, and timber is a naturally insulating frame material, which helps. If you're comparing quotes, just check both numbers are describing the same thing: unit-to-unit, or window-to-window. Ask any supplier who won't say which they're quoting.