Energy efficient windows

Energy efficient windows keep heat in with three things working together: coated double or triple glazing, a gas-filled gap between the panes, and a frame that seals well and doesn’t conduct the cold. Every replacement window in England must now reach at least a Window Energy Rating of band B or a U-value of 1.4 W/m²K, so a new window from a registered installer is a big step up from single glazing or early double glazing.

How energy efficient windows save heat

An old window loses heat in three ways. It conducts heat through the glass and frame, it lets warm air leak out round a worn seal, and it lets cold air in. Modern windows tackle all three:

• Low-emissivity glass reflects heat back into the room.
• Argon gas between the panes slows heat crossing the gap.
• Warm-edge spacers keep the edge of the glass from getting cold.
• Compression seals and multipoint locks pull the sash tight against the frame, so draughts stop.

Window Energy Ratings

The Window Energy Rating, usually shown as a coloured label from the British Fenestration Rating Council, grades the whole window from A++ down. It balances heat lost through the window against free heat gained from the sun and air leaking round the frame. So it’s a fair way to compare two windows in a British climate. Our guide to window U-values explains the other number you’ll see on quotes.

Where energy efficient windows make the most difference

The saving depends on what you’re replacing. Swapping single glazing for modern double glazing makes the largest difference, both to bills and to comfort. Replacing early double glazing that has misted or lost its seals still helps. By contrast, replacing good modern double glazing with something slightly better rarely pays back in energy alone.

Comfort counts too. Efficient windows stop cold downdraughts, reduce condensation on the glass and make the area near the window usable in winter.

Double or triple glazing?

Triple glazing lowers heat loss further and cuts noise, but the step up from a good double glazed window is smaller than the price difference suggests. It makes most sense in cold, exposed or noisy rooms. For most homes, A-rated double glazing in uPVC, aluminium or timber gives nearly all of the benefit.

If you can’t replace the windows

In a listed building, a conservation area or a rented flat, secondary glazing and draught-proofing are the practical alternatives. See listed building windows.

Getting the most from new windows

• Fit them properly: a good window badly sealed into the wall still leaks.
• Keep trickle vents open in bedrooms and kitchens to avoid condensation.
• Ask every installer for the energy rating of the exact window they are quoting, and check it on the BFRC website.

When you’re ready, get quotes and compare the ratings as well as the prices.