What Is a Window Energy Rating?

 

A Window Energy Rating, often shortened to WER, assesses the energy performance of the complete window rather than the glass alone.

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The British Fenestration Rating Council grades certified windows from A++ to E, using the familiar coloured scale seen on many energy-related products. Genuine certified products should carry information identifying the rating, certified company and licence details.

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The calculation considers three main factors:

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• Heat passing through the window

• Useful heat gained from sunlight

• Air leaking through the window and its opening sections

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This makes the WER a broader measure than a U-value, which concentrates on heat transfer.

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Understanding the Rating Scale

 

The BFRC window-rating scale runs from A++ at the higher-performing end to E at the lower-performing end.

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Typical bands include:

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• A++

• A+

• A

• B

• C

• D

• E

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A higher rating generally indicates a stronger overall energy balance. However, homeowners should still check the complete specification because the rating does not describe security, noise reduction, frame appearance or installation quality.

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A highly rated window may not automatically be the best choice where acoustic glazing, heritage detailing or solar control is particularly important.

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What Are A-Rated Windows?

 

A-rated windows have achieved Band A under a recognised Window Energy Rating scheme.

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They are designed to provide a positive overall balance when heat loss, solar gain and air leakage are assessed together. Higher classifications, including A+ and A++, are also available.

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An A rating does not mean that no heat escapes through the window. It also does not guarantee a specific reduction in household energy bills.

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The eventual benefit will depend on:

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• The condition of the windows being replaced

• The number and size of the windows

• The property’s wall and roof insulation

• Heating arrangements

• Window orientation

• Household temperatures

• Installation quality

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Replacing single glazing or old draughty frames is likely to produce a more noticeable improvement than replacing relatively recent double glazing.

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What Is the U-Value for Windows?

 

A U-value for windows measures the rate at which heat passes through the product.

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It is expressed as:

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W/(m²·K)

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This means watts of heat transfer through each square metre for every degree of temperature difference between inside and outside.

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With U-values, lower is better. A window with a U-value of 1.2 allows less heat to pass through it than a comparable window with a value of 1.6.

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It is important to establish exactly which part of the window the quoted figure covers.

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Whole-window U-value

Usually shown as Uw, this includes both the glass and frame. It is the most useful figure when comparing complete window products.

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Glass U-value

Often shown as Ug, this measures the centre-pane performance of the glazing rather than the complete window.

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Frame U-value

Usually shown as Uf, this relates specifically to the frame.

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A low Ug-value can look impressive in a brochure, but the complete window may perform less effectively once the frame, spacer bars and opening sections are included.

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Window Ratings Versus U-Values

 

A Window Energy Rating and U-value measure related but different aspects of performance.

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A U-value considers:

• Heat transfer through the window

• The insulating performance of the frame and glass

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A Window Energy Rating considers:

• Heat transfer

• Solar heat entering through the glass

• Air leakage through the window

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BFRC describes its rating as an assessment of the complete window or door, including thermal transmittance, solar factor and effective air leakage.

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This means two windows with similar U-values can receive different energy ratings if one admits more useful solar energy or controls air leakage more effectively.

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Neither figure should be considered in isolation. The Window Energy Rating provides an accessible overall comparison, while the U-value gives a direct measure of thermal insulation.

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What Is the Best U-Value for Windows?

 

There is no single best U-value for windows suitable for every project.

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Very low U-values can be beneficial, but achieving them may involve:

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• Triple glazing

• Heavier sealed units

• Thicker frames

• Additional low-e coatings

• More expensive gas fillings

• Reduced solar gain

• Increased product cost

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For many existing homes, a well-made double glazed window with a strong whole-window U-value can provide an effective balance between cost, insulation and practicality.

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Triple glazing may be worthwhile for highly insulated homes, exposed locations or projects with demanding energy targets. However, a properly installed double glazed window may outperform a more expensive product that is badly measured, inadequately sealed or incorrectly adjusted.

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Ask the installer to explain the improvement being offered and whether the additional cost is proportionate to the likely benefit.

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Building Regulations for Replacement Windows

 

Replacement windows in England must meet the applicable Building Regulations requirements.

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Current Approved Document L guidance for new and replacement windows in existing dwellings sets a maximum whole-window U-value of 1.4 W/(m²·K) or a minimum Window Energy Rating of Band B. The published 2026 edition, due to take effect from 24 March 2027 for most building work in England, retains the same replacement-window limit.

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These are limiting standards rather than a statement that every compliant window offers identical performance.

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Building Regulations also consider matters such as:

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• Ventilation

• Safety glazing

• Means of escape

• Structural support

• Protection from falling

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A high energy rating does not remove the need to satisfy these other requirements.

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Different regulations and certification systems apply in Wales, Scotland and Northern Ireland.

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Double Glazed Window Ratings

 

The rating belongs to the complete tested window configuration rather than double glazing generally.

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A manufacturer may produce several windows from the same basic frame system with different ratings according to:

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• Window dimensions

• Fixed and opening sections

• Glass specification

• Frame reinforcement

• Low-e coatings

• Gas filling

• Spacer bars

• Glazing-bar arrangements

• Weather seals

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A large fixed window may perform differently from a smaller window containing several opening casements, even where both use the same glass.

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When reviewing double glazed window ratings, confirm that the certificate or specification applies to the window system and configuration included in your quotation.

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What Affects a Window’s Energy Performance?

 

Several components work together to determine the rating.

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Low-emissivity glass

Low-e glass has a transparent coating that reduces radiant heat loss through the glazing.

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Gas-filled cavities

Argon is widely used within sealed double glazed units because it provides better insulation than ordinary air. Krypton may be used in narrower or higher-performance units.

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Warm-edge spacer bars

Spacer bars separate the panes around the edge of the sealed unit. Warm-edge materials are designed to reduce heat transfer around the glass perimeter.

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Window frames

UPVC, timber, aluminium and composite frames can all provide good performance when correctly designed. Aluminium systems normally require effective thermal breaks to control heat transfer.

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Weather seals

Opening sections need continuous seals and correct adjustment. A high-performance glazing unit cannot compensate for air leaking through a badly aligned casement.

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Installation

Gaps between the frame and wall must be properly insulated and sealed. Decorative trims alone do not provide effective thermal performance.

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Solar Gain and Window Ratings

 

Sunlight passing through a window can provide useful warmth, particularly during colder weather. The Window Energy Rating calculation therefore includes solar gain as well as heat loss.

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However, excessive solar gain can cause overheating in rooms with large south- or west-facing windows.

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A window with a high energy rating may not automatically be ideal for:

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• Large glazed extensions

• Conservatories

• Highly exposed west-facing rooms

• Properties already affected by overheating

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Solar-control glass, shading and ventilation may need to be considered alongside the energy rating.

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The correct balance depends on the orientation, room use and amount of glazing.

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Air Leakage and Draughts

 

Air leakage is an important part of the energy-rating calculation.

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Even where the glass and frame provide strong insulation, draughts can enter through:

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• Worn weather seals

• Poorly adjusted hinges

• Faulty locks

• Dropped opening sections

• Gaps around the frame

• Inadequate perimeter sealing

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New windows should close evenly and firmly against their seals without the handles needing to be forced.

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The installation should also be checked after completion to ensure that every opening section operates correctly.

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Energy Ratings for Timber, UPVC and Aluminium Windows

 

Frame material alone does not determine the rating.

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UPVC windows

UPVC profiles commonly use internal chambers to reduce heat transfer. Performance varies according to frame depth, reinforcement, seals and glazing.

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Timber windows

Timber has useful natural insulating properties and can achieve strong ratings when the frames are properly designed and manufactured.

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Aluminium windows

Modern aluminium frames normally contain thermal breaks separating the internal and external sections of metal.

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Composite windows

Composite windows combine materials, such as timber internally and aluminium externally, to balance insulation, durability and maintenance.

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Compare certified product performance rather than assuming one material is automatically more efficient.

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Ratings for Listed and Traditional Buildings

 

Standard energy targets may require a more flexible approach where they would unacceptably alter the character of a historic building.

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Current Approved Document L guidance allows character-sensitive windows that cannot meet the normal complete-window target to use glass with a centre-pane U-value no higher than 1.2 W/(m²·K), or to retain single glazing supplemented by low-e secondary glazing.

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Listed building consent or planning permission may also be required before historic windows are altered or replaced.

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Repair, draught-proofing and secondary glazing can sometimes improve energy performance without removing original frames.

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How to Compare Window Quotations

 

Ask each installer to provide the same information so that you can make a fair comparison:

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• Window Energy Rating

• Whole-window Uw-value

• Centre-pane Ug-value

• Double or triple glazing

• Low-e glass specification

• Gas filling

• Spacer-bar type

• Frame material and profile

• Ventilation provision

• Installation and perimeter sealing

• Building Regulations certification

• Product and workmanship guarantees

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Be cautious where a quotation mentions only a very low glass U-value but gives no rating or whole-window result.

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The glass may perform well while the complete frame achieves a less impressive figure.

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Is the Highest Rating Always Worth Paying For?

 

A higher rating can improve thermal performance, but the financial return will depend on the property and existing windows.

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Before paying for an upgrade from A to A+ or A++, consider:

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• The additional cost

• The actual change in whole-window U-value

• Whether triple glazing is required

• The effect on frame thickness and weight

• The property’s wider insulation

• Whether noise, security or appearance is a greater priority

• The expected length of ownership

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The most suitable window is often the one that provides a strong overall specification at a sensible cost rather than the highest rating available.

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Find Trusted Energy-Efficient Window Installers

 

Window ratings make it easier to compare products, but performance also depends on appropriate specification and professional installation.

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APN Double Glazing can connect homeowners with experienced local installers for A-rated windows, low-e glass, energy-efficient double glazing and high-performance replacement windows.

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Get in touch for expert advice and a free, no-obligation quote for your new windows.