Best energy-rated windows

Windows are one of the biggest routes for heat to escape a home, so getting the glazing right pays off in comfort as well as running costs. According to the Energy Saving Trust, replacing single glazing with energy-efficient glazing can noticeably cut the heat lost through windows. The trick is knowing which figure to trust.

Window Energy Ratings (WER)

The familiar A-to-G “rainbow” label rates a whole window on a balance of heat lost and useful solar heat gained. Modern replacement windows are commonly A or A+ rated, with A++ at the top of the scale. It's a genuinely useful shorthand — but because it blends several factors, two A-rated windows can still perform differently in practice.

U-values — the number that matters most

The U-value measures how much heat passes through the window: the lower the figure, the better the insulation. Crucially, ask for the whole-window U-value, not a glass-only figure that flatters the product. Current Building Regulations require replacement windows in England to achieve a whole-window U-value of 1.4 W/m²K or better (or a WER of band B or above), so treat that as your floor, not your target.

What makes a window energy-efficient

• Low-emissivity (low-E) glass. A microscopically thin coating reflects heat back into the room.
• Gas fill. Argon between the panes insulates better than plain air.
• Warm-edge spacer bar. A non-metal spacer around the unit cuts cold-bridging at the edges.
• A well-insulated frame. uPVC's chambers and a thermally broken aluminium frame both help; see best upvc windows for how to specify the frame.
• Triple glazing, where it's justified. Excellent for very cold or exposed sites, though a good A-rated double unit suffices for most homes.

Reading past the badges

A shiny “A++” sticker means little if the frame is poorly reinforced or the unit is fitted badly, so the installer matters as much as the product — our best window materials hub covers how each frame performs. Energy efficiency also isn't the only reason to upgrade; if noise is your issue, our sister guide on acoustic glazing is the better read. And if your frames are sound but draughty, it's worth checking repair or replace first? before a full replacement.

Funding and contribution options may be available, subject to eligibility and a home survey, and it's worth exploring funded glazing options alongside your quotes when planning an efficiency upgrade.