A window's U-value measures how easily heat passes through it, and lower is better. A Window Energy Rating, from A++ to E, combines heat loss, solar gain and air leakage into one grade. In England, replacement windows should achieve a U-value of 1.4 W/m²K or a Window Energy Rating of band B. Wales, Scotland and Northern Ireland set their own standards.
What is a U-value?
A U-value is the rate at which heat passes through a building element, measured in watts per square metre for each degree of temperature difference between inside and outside, written W/m²K. The Glass and Glazing Federation describes it as how easily heat can pass through the unit. The lower the U-value, the less heat escapes and the better the window insulates.
Check which U-value you are being quoted. A centre-pane figure describes only the middle of the glass, where performance is highest. A whole-window U-value includes the frame and the edge of the sealed unit, which lose more heat, so it is always higher than the centre-pane figure. Building regulations for replacement windows use the whole-window value, so ask for that when comparing quotes.
What is a Window Energy Rating?
A Window Energy Rating, or WER, grades the energy performance of a whole window on a scale from A++ to E, with A++ the most efficient. The British Fenestration Rating Council, which runs the scheme, says the calculation takes three factors into account: heat loss, solar gain and air leakage. A window with a good U-value can still score lower if it lets in little solar heat or leaks air.
Doors have an equivalent Doorset Energy Rating. Ratings are usually given for a standard window size, so the figure on a product label may not match the exact size you are buying. In England, the method for calculating ratings is set out in the Glass and Glazing Federation's Glazing Manual Data Sheet 2.3, which Approved Document L refers to.
The two measures answer slightly different questions. The U-value tells you how well a window keeps heat in. The energy rating tells you the overall balance of heat lost and heat gained over a year, which is closer to what you notice in a heated home.
What makes a window energy efficient?
Several parts of a window work together, and each is worth checking on a quote. The glass usually makes the biggest difference, but the frame, the spacer around the edge of the sealed unit and the quality of the seals all affect heat loss and air leakage. A well-specified sealed unit in a poorly fitted frame will not perform as rated.
Low-E glass has an invisible coating that lets short-wave heat from the sun in and reflects long-wave heat from the room back inside. The coating normally sits on one of the inner faces of the sealed unit. Hard coats are durable, while soft coats generally perform better but are more delicate and are protected inside a gas-filled unit.
- Low-E coating on one pane to reflect heat back into the room
- Argon gas in the gap, which insulates better than air; krypton is used in some thin or high-performance units
- Warm edge spacer bars with little or no metal, which cut heat loss and condensation at the edges
- Frames with good thermal performance and effective seals
- Careful fitting so the window is well sealed and draught-proof
| Element | Maximum U-value | Or minimum energy rating |
|---|---|---|
| Window | 1.4 W/m²K | Window Energy Rating band B |
| Rooflight | 2.2 W/m²K | Not given |
| Door more than 60 per cent glazed | 1.4 W/m²K | Doorset Energy Rating band C |
| Other doors | 1.4 W/m²K | Doorset Energy Rating band B |
Should I choose double or triple glazing?
Triple glazing adds a third pane and a second gap, which lowers the U-value further, and the Energy Saving Trust notes it can be worth considering in cold or noisy areas. It is heavier and thicker than double glazing, so the frame, hinges and any opening sashes must be designed to carry it. Not every existing frame or window style suits triple glazing.
A good double-glazed unit with low-E glass, argon fill and warm edge spacers can meet the replacement window standards in England. Triple glazing admits a little less light and solar heat, and it is more likely to show condensation on the outside on cold, clear mornings because so little heat reaches the outer pane. That is a sign of good insulation, not a fault.
Where windows cannot be changed, such as in many listed buildings, secondary glazing fitted on the inside is an alternative. It adds a second layer of glass behind the existing window, which cuts draughts and heat loss and adds a barrier against noise, while leaving the original windows in place. Low-E glass in the secondary panes improves heat retention further.
What is a g-value, and why does solar gain matter?
The g-value, also called the solar factor or total solar energy transmittance, is the fraction of the sun's energy that passes through the glass into the room. It ranges from 0 to 1. A higher g-value lets more free heat in, which helps in winter and is part of what a Window Energy Rating rewards.
Solar gain is not always welcome. Large south- or west-facing glazed areas, conservatories, roof lanterns and glass roofs can overheat in summer. The Glass and Glazing Federation notes that low-E glass can increase solar gain and suggests considering low-E combined with solar control glass for large glazed areas. Solar control glass has a lower g-value, so it lets in less heat while keeping the view.
The right balance depends on orientation and use. A north-facing bedroom window benefits from low heat loss and does not need a low g-value. A glass roof over a kitchen may need solar control to stay comfortable. A glazier can explain how each option changes the U-value, g-value and light transmission.
What must replacement windows achieve in England?
In England, Approved Document L sets limiting values for new and replacement windows, doors and rooflights in existing homes. A replacement window should achieve a whole-window U-value of 1.4 W/m²K or a Window Energy Rating of band B. Rooflights have a limit of 2.2 W/m²K, and doors have their own limits and Doorset Energy Rating bands. A replacement should also be well fitted and reasonably draught-proof.
Replacement windows are controlled fittings, so the work must be approved by building control or carried out by an installer registered with a competent person scheme, who issues a certificate within eight weeks of completion. Local authority guidance notes the thermal standards do not apply if you replace only the glass in an existing frame.
For listed buildings, buildings in conservation areas and scheduled monuments, the energy requirements do not need to be met in full where that would unacceptably alter the character or appearance, but should be met as far as reasonably practicable. A new edition of Approved Document L comes into force in England on 24 March 2027 for standard building work.
How do Wales, Scotland and Northern Ireland differ?
Each nation sets its own energy standards, so a window that meets the rules in one may be assessed differently in another. In Wales, Approved Document L volume 1 sets a limit for windows in existing dwellings of 1.4 W/m²K or a Window Energy Rating of band B, with similar exemptions for listed buildings and conservation areas. A new Welsh edition takes effect on 4 March 2027.
In Scotland, section 6 of the domestic Technical Handbook sets the U-value that replacement windows, doors and rooflights should achieve, and the handbook was revised in February 2023 and again in April 2024, so check the current edition for the figure that applies. A complete replacement window generally does not need a building warrant in Scotland, but it must still meet the standards.
In Northern Ireland, energy requirements come from the Building Regulations (Northern Ireland) 2012, with guidance in the Department of Finance technical booklets, and building control is run by the district councils. Ask the installer which standard their windows meet and how the work will be certified or approved where you live.
What should a window quote say about energy performance?
A clear quote lets you compare like with like. It should state the whole-window U-value or Window Energy Rating for the windows being offered, not just a centre-pane figure for the glass, and say how the windows meet the building regulations where you live. If the quote only says energy efficient or A rated without detail, ask for the figures.
We can quote for replacement windows and sealed units across the UK. Each job starts with a survey, and the written quote fixes the price for the specification shown, including the glass, gas fill and spacer. Manufacturer guarantees apply to new units and windows, and the terms are confirmed in writing.
- Whole-window U-value or Window Energy Rating
- Glass make-up: panes, low-E coating, gas fill and spacer type
- g-value where solar gain or overheating matters
- Trickle vents or other background ventilation
- Who certifies the work or notifies building control
Frequently asked questions
Is a lower U-value or a higher energy rating more important?
Both describe the same window from different angles. The U-value shows how well the window keeps heat in. The energy rating also counts solar gain and air leakage, so it gives a fuller picture of performance in a heated home. In England, a replacement window can meet the building regulations with either a U-value of 1.4 W/m²K or a Window Energy Rating of band B.
Do I need building regulations approval to replace my windows?
Building regulations apply to replacement windows across the UK, and the detail differs between nations. In England and Wales, the work must be approved by building control or done by an installer registered with a competent person scheme, who gives you a certificate. In Scotland, a complete replacement generally does not need a building warrant but must meet the standards. In Northern Ireland, ask district council building control.
What does A++ mean on a window?
A++ is the top grade on the Window Energy Rating scale, which runs from A++ to E. It means that, for a standard window size, the balance of heat lost through the window, heat gained from the sun and air leaking through gaps is the most favourable on the scale. The rating applies to the whole window, so the frame and fitting matter as well as the glass.
Is argon gas in double glazing worth having?
Argon is an inert gas that conducts heat less well than air, so filling the gap between the panes with it improves the insulation of the sealed unit. It is common in modern double and triple glazing and costs relatively little to include. It is normally combined with a low-E coating and a warm edge spacer bar in a well-specified unit, and the quality of the edge seal matters too.
Can I just replace the glass to improve my windows?
Yes, if the frames are sound. Fitting new sealed units with low-E glass, argon fill and warm edge spacers into existing frames can noticeably reduce heat loss through the glass. The frame and seals still affect overall performance, so badly fitting or rotten frames limit the benefit. Local authority guidance in England notes that the thermal standards for new windows do not apply to glass-only replacement.
Sources
- Approved Document L: conservation of fuel and power (GOV.UK)
- Approved Document L: conservation of fuel and power (GOV.WALES)
- Building standards technical handbook 2022: domestic, 6.2 Building insulation envelope (gov.scot)
- Energy performance ratings (British Fenestration Rating Council)
- Energy efficient windows and doors (Energy Saving Trust)
- Low-E (low emissivity) glass and thermal efficiency (Glass and Glazing Federation)
- Competent person schemes: current schemes (GOV.UK)
- Building regulations approval: use a competent person scheme (GOV.UK)
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