Understanding window energy ratings

Window brochures are full of letters and numbers. Most of them come down to one question: how well will this window keep your home warm? This guide explains the main measures and what the rules require.

Window Energy Ratings (WER)

Window Energy Ratings work like the labels on fridges and washing machines, on a scale from A++ (best) down to E. A window's rating takes three things into account:

• how much heat escapes through it
• how much free warmth it lets in from the sun
• how much air leaks around it

Because it looks at the whole picture, the rating is a useful way to compare windows. It applies to a standard-sized window, so the performance of your actual windows will vary a little with their size and shape.

U-values

A U-value measures how easily heat passes through something. It's given in watts per square metre per degree (W/m²K). Lower is better.

Watch out for two different figures:

• Whole-window U-value includes the frame and the glass. This is the one that matters.
• Centre-pane U-value measures only the middle of the glass. It always looks better, so it can make a window seem more efficient than it is.

If a quote only gives a centre-pane figure, ask for the whole-window value.

g-values and solar gain

The g-value shows how much of the sun's heat comes through the glass, on a scale from 0 to 1. A higher g-value lets in more free warmth, which helps in winter but can make south-facing rooms hot in summer. A balance is usually best, and the WER takes it into account.

What the rules require

In England, replacement windows in an existing home must meet a minimum standard under Building Regulations (Approved Document L). For most replacement windows, that's a whole-window U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B or better. Doors have their own standard.

A replacement also shouldn't perform worse than the window it replaces. Wales has similar but separate rules, and the standards can change, so ask your installer to confirm the product meets the current requirements for your home.

Is a higher rating worth paying for?

Moving from old single glazing to any modern A-rated window makes a big difference. Going from a good rating to the very best usually makes a smaller one. Things that are often worth more attention than the last step up in rating:

• Good fitting. Gaps around the frame can undo the benefit of the best glass.
• Draught-proofing elsewhere. Doors, loft hatches and floors all leak heat.
• Ventilation. A warmer, more airtight home still needs fresh air to avoid condensation and damp.

Questions to ask

1. What is the Window Energy Rating and the whole-window U-value of the windows you're quoting for?
2. Do they meet the current Building Regulations for replacement windows?
3. Is the glass low-E and gas-filled?
4. How will you seal the gap between the frame and the wall?