Window Energy Ratings Explained

What the WER rating tells you

The Window Energy Rating, or WER, is a scale similar to the familiar energy labels on appliances, running from A++ at the top down through A, B, C and beyond to G. It gives a single, simplified score for a whole window based on how much heat it lets in from the sun, how much heat it loses, and how much air leaks through it, making it a useful shorthand for comparing windows without needing to understand every technical detail behind it.

A higher-rated window will generally keep more heat inside your home during colder months, which can help with comfort and heating demand, though the WER label is a guide rather than a guarantee of performance in any specific property, since real-world results depend on installation quality and the rest of the building too.

U-values in plain terms

The U-value measures how much heat passes through a window, expressed in watts per square metre per degree Kelvin (W/m²K). Unlike the WER letter rating, a lower U-value is better, because it means less heat is escaping through the window. Whole-window U-values include the frame as well as the glass, since heat can escape through both.

When comparing quotes, ask for the whole-window U-value rather than just a glass-only figure, as the frame's performance can meaningfully affect the overall result, particularly with older or less efficient profiles.

G-values and solar gain

The g-value measures how much of the sun's heat energy passes through the glass into the room, expressed as a figure between 0 and 1. A higher g-value lets in more free solar heat, which can be useful in a north-facing room, while a lower g-value reduces overheating risk in a south-facing room or conservatory that already gets a lot of direct sun. Balancing U-value and g-value appropriately for each elevation of a property is something a good installer will consider rather than applying one specification everywhere.

Gas fills and warm-edge spacers

Modern double and triple glazed units are usually filled with an inert gas, most commonly argon, between the panes rather than ordinary air. Argon is denser than air and conducts heat less readily, which reduces the U-value of the sealed unit and improves thermal performance.

The spacer bar around the edge of the sealed unit, which separates the panes and holds the gas fill in, also matters. Older aluminium spacer bars conduct heat readily and create a cold edge around the glass where condensation is more likely to form. Warm-edge spacers, typically made from composite or foam-based materials, reduce this cold bridging and improve both thermal performance and edge condensation resistance.

Low-E coatings

Low emissivity, or low-E, glass has a microscopically thin coating applied to one surface of the glass, invisible to the eye, which reflects heat back into the room while still letting light through. This is one of the most significant single improvements in modern glazing, and it is standard on virtually all new double glazing installed to current regulations, though the exact type and position of the coating can still affect performance.

Current building regulations in England

Building regulations in England set minimum energy performance standards for replacement windows and doors, and these have tightened in recent years. Installers registered with a competent person scheme, such as CERTASS, can self-certify that replacement windows meet the required standard without a separate council building control application, which is one of the practical benefits of using an accredited installer.

Specific numerical thresholds in the regulations are updated periodically, so rather than quoting a figure that may go out of date, the key point for homeowners is to check that your installer is working to current regulations and can confirm the specific U-value being supplied meets or exceeds what is currently required.

Reading the energy spec on a quote

A properly detailed quote should state the WER rating, the whole-window U-value, the type of gas fill, whether low-E glass and warm-edge spacers are included, and confirmation that the specification meets current building regulations. If a quote is silent on these details, it is reasonable to ask for them in writing before comparing prices with another company.

• WER rating (A++ to G)
• Whole-window U-value in W/m²K
• Gas fill type (commonly argon)
• Spacer bar type (warm-edge preferred)
• Confirmation of current building regulations compliance

Realistic expectations on savings

Upgrading from old, poorly performing windows to modern, correctly specified double glazing can meaningfully improve comfort, reduce draughts and cut down on condensation, and is likely to reduce heat loss through the windows themselves. We would rather not put a specific pound figure on annual savings, since this depends heavily on your existing windows, heating system, insulation levels elsewhere in the property, and how the home is used, and any figure not based on your actual property would simply be a guess.

Common questions

What is the best WER rating I can get?

A++ is currently the top of the scale, though A rated windows are also a strong, widely available performance level and often represent good value against the very top tier.

Is triple glazing always better than double glazing?

Triple glazing typically has a lower U-value than double glazing, but the practical benefit depends on your specific property, budget and priorities, and double glazing remains a strong performer when properly specified.

Do I need to check building regulations myself?

No, a CERTASS registered installer can self-certify compliance on your behalf, though it is reasonable to ask them to confirm this in writing as part of your quote.

Why does my old double glazing mist up between the panes?

This usually indicates the seal on the sealed unit has failed, allowing moisture in and the gas fill to escape, which reduces both clarity and thermal performance and generally means the unit needs replacing.

Does the frame material affect the energy rating?

Yes, the whole-window U-value includes the frame, so a poorly insulating frame can pull down the overall rating even if the glass itself is high specification.