U-Values Explained: What Is a Good U-Value for Windows?

A U-value measures how much heat escapes through a window — the whole thing, frame and glass together — in watts per square metre per degree (W/m²K). Lower is better. It’s the single most useful number when comparing windows for warmth.

What counts as a good U-value?

• Old single glazing: around 5.0 W/m²K — very poor.
• Standard modern double glazing: roughly 1.2–1.6 W/m²K.
• Building Regs minimum for a replacement window: 1.4 W/m²K (WER Band B).
• High-performance / triple glazing: around 0.8–1.0 W/m²K.

So a “good” replacement window comfortably beats 1.4; an excellent one approaches 1.0 or lower.

U-value vs Window Energy Rating

You’ll also see a Window Energy Rating (A++ down to E). That rainbow label is more holistic than U-value alone because it also accounts for useful solar heat gain and air leakage — a window can have a middling U-value but still rate well if it captures free warmth from the sun.

Is a lower U-value always worth paying for?

Up to a point. Going from single glazing or old units to modern A-rated double glazing makes a big, noticeable difference to comfort and bills. The jump to triple glazing brings smaller gains for most UK homes — worth it for very exposed properties, noise, or new builds, less essential elsewhere. The Energy Saving Trust has useful background on savings.

Frequently asked questions

What’s the lowest U-value I can get?

Triple-glazed, high-spec windows can reach around 0.8 W/m²K or lower, but most homes see the best value in good double glazing near 1.2.

Does the frame affect the U-value?

Yes — the U-value is for the whole window, so the frame material and design matter as much as the glass.

Where do I find a window’s U-value?

Ask the installer to state it in writing on the quote, or look for the BFRC energy label.