Window Energy Ratings Explained

What Is a Window Energy Rating?

A Window Energy Rating, commonly abbreviated to WER, gives an overall indication of the energy performance of a window.

The BFRC scheme rates windows and doors using a scale running from A++ down to E, with A++ representing the highest rating.

The coloured rating label makes energy performance easier to understand than looking at several individual technical figures.

However, there is one important point:

A Window Energy Rating is not the same as a U-value.

The Quick Answer

A U-value mainly tells you how readily heat passes through a window.

A Window Energy Rating considers a broader combination of factors.

This means you should not assume:

“An A-rated window has a specific U-value.”

or:

“The window with the lowest U-value must have the highest Window Energy Rating.”

They measure different aspects of performance.

What Does a Window Energy Rating Consider?

A Window Energy Rating takes account of factors including:

• Thermal transmittance
• Solar heat gain
• Air leakage

The result is converted into an overall energy-performance rating.

Solar gain is included because useful heat from sunlight entering the building can reduce heating demand during colder periods.

This is one reason WER and U-value results do not always follow the same pattern.

Understanding the Rating Scale

Window Energy Ratings typically run through:

• A++
• A+
• A
• B
• C
• D
• E

Manufacturers often promote A-rated or better products.

However, builders should always check the actual project requirements rather than selecting windows solely by their energy label.

What Does the Rating Label Tell You?

A recognised Window Energy Rating label provides an indication of the energy performance of a particular product configuration.

The rating may depend on:

• Frame system
• Glass specification
• Spacer bar
• Reinforcement
• Seals
• Product configuration

Do not assume every window made from the same profile automatically achieves the same rating.

Window Energy Rating vs U-Value

These two measurements answer different questions.

U-Value

Measures the rate of heat transfer through the window.

A lower U-value generally means better insulation.

Window Energy Rating

Provides an overall energy-performance rating that also considers solar gain and air leakage.

Therefore:

A low U-value is good thermal performance, but it is not the same thing as a high Window Energy Rating.

Example

Imagine two windows.

Window A

• Very low U-value
• Lower solar gain

Window B

• Slightly higher U-value
• Higher useful solar gain
• Very low air leakage

Depending on the rating calculation, Window B may achieve a similar or even better Window Energy Rating despite having the slightly higher U-value.

This is why you should compare products using the performance figure actually required by the project.

Window Energy Ratings and Part L

Part L covers the conservation of fuel and power.

For replacement windows, Window Energy Ratings have historically provided one recognised method of demonstrating suitable energy performance.

However, Building Regulations change over time.

Before ordering, establish:

• Which regulatory standard applies
• Whether the project requires a U-value
• Whether an energy-rating route is acceptable
• Whether a project-specific energy calculation overrides generic requirements

Do A-Rated Windows Automatically Comply With New-Build Requirements?

No.

New-build compliance considers the overall performance of the dwelling.

The project may specify:

• Maximum whole-window Uw
• Specific g-values
• Air-permeability requirements
• Particular glazing
• Energy-model assumptions

Therefore:

Do not substitute an A-rated window simply because the project specifies a particular U-value.

Follow the actual specification.

Aluminium Window Energy Ratings

Modern aluminium windows can achieve strong energy performance when appropriately designed.

Performance can depend on:

• Thermal breaks
• Frame geometry
• Insulation
• Glass specification
• Warm-edge spacers
• Airtight seals

Do not assume aluminium is automatically thermally inferior simply because aluminium itself is conductive.

Modern thermally broken systems perform very differently from older aluminium windows.

uPVC Window Energy Ratings

uPVC frames generally provide good thermal insulation because the material itself has relatively low thermal conductivity.

Multi-chamber frame designs can further improve performance.

Again, compare the actual product data rather than making decisions purely from frame material.

Does Triple Glazing Give a Better Window Energy Rating?

Not automatically.

Triple glazing can improve thermal insulation, but Window Energy Ratings also take account of other factors.

Triple glazing may also alter:

• Solar gain
• Glass weight
• Light transmission
• Frame requirements

Therefore specify the required performance rather than simply asking for triple glazing.

Warm-Edge Spacer Bars

The spacer around the edge of an insulated glass unit affects heat loss at the perimeter of the glazing.

Warm-edge spacers reduce this thermal bridge and can contribute to improved:

• U-values
• Window Energy Ratings
• Internal edge temperatures
• Condensation resistance

Low-E Glass

Low-emissivity coatings reduce heat transfer through glazing.

They are a major component of modern energy-efficient sealed units.

Typical high-performance glazing combines:

• Low-E coating
• Gas-filled cavity
• Warm-edge spacer
• Suitable frame system

Air Leakage

Window Energy Ratings also consider air leakage.

A thermally efficient window that leaks air around the opening sash will not perform as effectively as a well-sealed product.

This is why:

• Hardware adjustment
• Gaskets
• Manufacturing quality
• Installation

all matter.

Common Window Energy Rating Mistakes

Mistake 1 – Assuming A-rated means a specific U-value

It doesn’t.

Mistake 2 – Assuming A++ is required everywhere

Check the applicable regulation and project specification.

Mistake 3 – Assuming all windows using the same profile have the same rating

Glass, reinforcement and configuration can affect performance.

Mistake 4 – Comparing WER and U-value as if they are identical

They are different measurements.

Mistake 5 – Assuming triple glazing automatically gives the best rating

Not necessarily.

Real-World Example

A builder receives two quotations.

Supplier A offers:

A-rated windows

Supplier B offers:

Uw 1.2 W/m²K windows

The builder asks:

“Which is better?”

There is not enough information.

The correct question is:

“What performance does my project actually require?”

If the architect specifies Uw 1.2 or better, ask for the whole-window Uw.

If the replacement-window compliance route allows a particular energy rating, check the relevant rating.

Compare like with like.

Frequently Asked Questions

What is the highest Window Energy Rating?

A++ is currently the highest commonly used BFRC rating.

Is WER the same as U-value?

No.

Is an A-rated window energy efficient?

Yes, but suitability still depends on the project requirements.

Can aluminium windows achieve high Window Energy Ratings?

Yes.

Does triple glazing guarantee an A++ rating?

No.

Should I ask my supplier for WER or U-value?

Ask for whichever performance measure your project requires.

Related Guides

• Window U-Values Explained
• Part L Explained
• Warm Edge Spacer Bars
• G-Values Explained
• Solar-Control Glass
• New-Build Window Regulations

Trade Tech Technical Support

When requesting a quotation, provide the actual required performance:

• Uw
• WER
• g-value
• Acoustic requirement
• Security requirement

Rather than simply asking for:

“The most energy-efficient window.”

That allows the correct product to be selected without unnecessary overspecification.

Important: Window Energy Ratings are product-performance indicators. They do not replace project-specific energy calculations or Building Regulations requirements.