Window U-Values Explained

What Is a U-Value?

A U-value measures the rate at which heat passes through a building element.

For windows and doors:

The lower the U-value, the better the thermal performance.

U-values are expressed in:

W/m²K – watts per square metre kelvin.

For most builders, however, the critical issue isn’t the unit of measurement.

It’s knowing which U-value the supplier is quoting.

The Three Figures You Need to Understand

Ug – Glass U-Value

Ug refers to the thermal performance of the glazing.

This is sometimes called the centre-pane value.

Uf – Frame U-Value

Uf refers to the frame.

Uw – Whole Window U-Value

Uw represents the thermal performance of the complete window.

It takes account of:

Glass
Frame
Edge/spacer effects
Relative areas of frame and glazing

Uw is generally the figure you want when a specification asks for the whole-window U-value.

The Sales Trick to Watch For

You ask:

“What’s the U-value?”

The salesperson says:

“1.0.”

You assume the complete window achieves 1.0 W/m²K.

But they may be quoting the glass.

The complete window might perform differently.

Always ask:

“Is that the calculated whole-window Uw value for this size and configuration?”

Size Matters

U-values aren’t necessarily identical for every window manufactured from the same system.

The proportion of:

Frame
Glass
Mullions
Transoms
Opening sashes

affects the result.

A small heavily framed window may therefore have a different Uw from a large predominantly glazed window using identical glass and profiles.

Aluminium vs uPVC

Modern thermally broken aluminium can achieve strong thermal performance, but aluminium and uPVC frames behave differently.

Never select purely on generic claims such as:

“Aluminium is cold.”

“uPVC always has the best U-value.”

Compare the actual calculated performance of the proposed products.

Double vs Triple Glazing

Triple glazing can substantially improve glass U-values, but the final window performance depends on the complete product.

Triple glazing also introduces:

Additional weight
Different hardware loads
Wider sealed units
Different bead requirements
Cost implications
Potential solar/light-transmission changes

Don’t specify triple glazing merely because “triple must be better.”

Specify the performance you actually need.

Warm Edge Spacers

The spacer around the perimeter of the sealed unit influences edge heat loss.

Warm-edge spacer technology can help improve:

Thermal performance
Window Energy Ratings
Internal edge temperatures
Condensation resistance

We’ll cover this separately in the Warm Edge Spacer Guide.

New-Build Windows

This is an area where outdated websites can cause contractors problems.

England now has 2026 Approved Document L guidance, while earlier editions continue to apply to projects subject to previous regulatory standards.

Therefore, don’t take a U-value figure from an old blog and assume it applies to your project.

Establish:

Which regulatory standard applies to the project.
What the energy/SAP specification requires.
What Uw value the actual proposed windows achieve.

Replacement Windows

Replacement work is also governed by the applicable Approved Document L regime.

Again, establish which regulatory standard applies rather than blindly copying a number from another project.

Doors

Door thermal performance may be expressed as Ud.

As with windows, make sure you’re comparing the complete doorset rather than only an insulated panel or centre-pane figure.

Common U-Value Mistakes

Comparing Ug with Uw

This is probably the most common mistake.

Assuming one system has one U-value

Actual product size and configuration matter.

Specifying triple glazing without checking frame compatibility

Not every configuration is suitable.

Ignoring mullions and transoms

More framing can change thermal performance.

Using old Building Regulation figures

Regulations and project standards change.

Ordering before obtaining SAP requirements

On new builds, this can be expensive.

Real-World Example

An architect specifies:

Uw ≤ 1.1 W/m²K

A supplier offers glass with:

Ug = 1.0 W/m²K

Those statements do not demonstrate that the window complies.

The builder should request the calculated whole-window performance for the proposed system, sizes and configurations.

What To Ask Your Window Supplier

Send:

Window schedule
Sizes
Configurations
Required Uw
Glass specification
Frame system
Any colour/finish requirements that affect system selection

Then ask for evidence of the proposed thermal performance.

Frequently Asked Questions

Is a lower U-value better?

Yes.

Is Ug the same as Uw?

No. Ug relates to glazing; Uw relates to the complete window.

Does 1.0 glass mean a 1.0 window?

No.

Is triple glazing always required for new builds?

No. The required performance depends on the applicable standards and project design.

Can aluminium achieve low U-values?

Yes. Modern thermally broken systems can provide strong thermal performance.

Does window size affect U-value?