Part L 2022 Window U-Values: 1.4 for New Builds and 1.6 for Replacements

When the Part L 2022 uplift took effect on 15 June 2022, the target U-value for new-build windows dropped from 1.6 to 1.4 W/m²K — a 12.5% tightening that eliminated the cheapest double-glazed specifications from the compliant market. For replacement windows in existing homes, the target moved from 1.6 to a notional 1.6, but the method of demonstrating compliance changed significantly.

The two targets you need to understand

Part L sets different U-value targets depending on whether the work is a new dwelling or a replacement in an existing dwelling.

New dwellings (Part L Volume 1, 2022 edition)

Element	Target U-value (W/m²K)	Notional dwelling value	Limiting fabric value
Windows	1.4	1.2	1.6
Rooflights	1.4	1.2	1.6
Doors (glazed >50%)	1.4	1.2	1.6
Doors (glazed ≤50%)	1.4	1.0	1.6

The “target” is the value your actual building must achieve on average. The “notional dwelling” value is what you would need to specify if you were building to a notional specification (used in SAP calculations). The “limiting fabric value” is the absolute maximum any individual element can achieve — you cannot install a window with a U-value of 1.7 even if your overall building performs better than the target.

Existing dwellings (Part L Volume 1, replacement windows)

For replacement windows in existing homes, the situation is more nuanced. The regulation states:

• Where the existing window has a U-value worse than 1.6 W/m²K, the replacement must achieve a U-value of 1.6 W/m²K or better
• Where the existing window already meets 1.6 W/m²K or better, the replacement must not worsen the U-value — it must meet or exceed the existing performance

In practice, this means most pre-2002 windows (which are typically 2.8-5.8 W/m²K single or early double glazing) trigger the 1.6 W/m²K requirement. But a 2010-era A-rated window that is being replaced due to damage must be replaced with a window that achieves at least the same U-value.

What U-value means for windows

A U-value measures the rate of heat transfer through a building element. For windows, there are two measurements:

1. U-value of the glass only (Ug): The centre-pane thermal performance, ignoring the frame
2. U-value of the whole window (Uw): The combined performance of glass, frame, and spacer — this is what Part L refers to

The frame matters. A 4-16-4 argon-filled unit with a low-E coating achieves a Ug of approximately 1.0 W/m²K in isolation, but when installed in a basic aluminium frame with an aluminium spacer, the whole-window Uw can be 1.6 W/m²K. The same unit in a thermally broken timber frame with a warm-edge spacer can achieve Uw of 1.2 W/m²K or better.

What specifications achieve 1.4 W/m²K

The new 1.4 target for new builds requires a more deliberate specification than the old 1.6. Here are the glazing configurations that reliably achieve 1.4 W/m²K on a whole-window basis:

uPVC frames

Configuration	Ug	Uw (typical)
4-16-4 argon, low-E, warm-edge	1.0	1.2-1.4
4-20-4 argon, low-E, warm-edge	0.9	1.1-1.3
4-16-4 argon, low-E, aluminium spacer	1.0	1.4-1.6 (borderline)
4-16-4 krypton, low-E, any spacer	0.8	1.1-1.3

The critical variable is the spacer. An aluminium spacer adds 0.2-0.4 W/m²K to the whole-window U-value compared to a warm-edge spacer. With the 1.4 target, an aluminium spacer is still possible but leaves no margin — if the frame is slightly wider or the glass is a non-standard size, you may miss compliance.

Aluminium frames

Configuration	Ug	Uw (typical)
4-16-4 argon, low-E, warm-edge, thermal break	1.0	1.3-1.5
4-20-4 argon, low-E, warm-edge, thermal break	0.9	1.2-1.4
4-18-4 argon, low-E, warm-edge, deep thermal break	1.0	1.2-1.3

Aluminium frames need a wider thermal break to achieve 1.4. Entry-level aluminium systems with a 24mm polyamide break may not meet 1.4 on their own; premium systems (Smart Systems Alitherm 600, Reynaers CF 77) with 32-34mm breaks will.

Timber frames

Configuration	Ug	Uw (typical)
4-16-4 argon, low-E, warm-edge	1.0	1.0-1.3
4-20-4 argon, low-E, warm-edge	0.9	0.9-1.2

Timber is the best-performing frame material. Even a basic 4-16-4 specification in a timber frame with a warm-edge spacer will comfortably beat 1.4. This is why heritage and conservation-area replacements using slim-profile double glazing in existing timber frames can sometimes still achieve Part L compliance.

What specifications achieve 1.6 W/m²K (replacement standard)

The 1.6 target for replacements is easier to achieve. Almost any double-glazed unit manufactured after 2002 with argon fill and a low-E coating will meet 1.6 in a uPVC or timber frame. Even aluminium frames with basic thermal breaks achieve 1.6 with a 4-16-4 unit and an aluminium spacer.

The only windows that will not meet 1.6 are:

• Single glazing (5.8 W/m²K)
• Early double glazing with no low-E coating and no gas fill (2.8-3.2 W/m²K)
• Double glazing with failed seals (condensation between panes indicates the gas fill is lost)

The WER alternative

Part L allows you to demonstrate compliance through Window Energy Rating (WER) instead of U-value. The WER system rates windows from A++ to E. For Part L 2022:

• New builds: Minimum WER B (or U-value 1.4)
• Replacements: Minimum WER B (or U-value 1.6)

The WER route can be easier because it takes into account solar gain (the heat energy that comes through the glass from sunlight). A window with a U-value of 1.5 but high solar gain could achieve a WER of B, satisfying Part L even though the U-value alone would not meet 1.4. See our argon vs krypton gas fill guide for how gas fills affect both U-value and WER.

Triple glazing and the path to 0.8 W/m²K

Part L does not require triple glazing. The 1.4 target is achievable with double glazing in all frame materials. However, if you are building to Passivhaus or aiming for an EPC A rating, you will need:

Configuration	Whole-window Uw
4-16-4-16-4 triple, argon, low-E, warm-edge, timber frame	0.7-0.9
4-16-4-16-4 triple, argon, low-E, warm-edge, uPVC frame	0.8-1.0
4-16-4-16-4 triple, krypton, low-E, warm-edge, any frame	0.6-0.8

Triple glazing adds 20-40% to the window unit cost. For a standard 1200×1000mm casement, that means an additional £150-£300 per unit over a compliant double-glazed specification. See our double vs triple glazing guide for the comfort and acoustic trade-offs.

Air permeability and the whole-house test

Part L 2022 introduced a tighter air permeability target for new builds: 5 m³/(h·m²) at 50 Pa, down from the previous default of 10. This matters for windows because the installation quality directly affects the air test result. A PAS 24 door with poorly sealed frame fixings will leak air and can cause the whole house to fail the air test — even if the windows themselves meet the U-value target.

The practical implication: specify that your installer uses expanding foam tape (not just silicone) around all window and door frames, and insist on a pre-test check if you are within 10% of the air permeability limit.

The Future Homes Standard (2025)

The government has confirmed that the Future Homes Standard, expected to come into force in 2025, will require a further U-value reduction. The proposed target for windows is 0.8 W/m²K for new builds — effectively mandating triple glazing as standard. The replacement window standard is expected to remain at 1.6 W/m²K.

If you are specifying windows for a new build that will be completed after 2025, consider triple glazing now to avoid a costly retrofit.

Summary

• New-build windows: 1.4 W/m²K (Part L 2022) — achievable with double glazing and warm-edge spacers
• Replacement windows: 1.6 W/m²K — almost any modern double glazing meets this
• The spacer bar matters more than most homeowners realise — aluminium spacers add 0.2-0.4 W/m²K
• WER B is an alternative route to demonstrating compliance
• Triple glazing is not required by Part L but will be needed for the Future Homes Standard (0.8 W/m²K)
• Installation quality affects air permeability — a poorly sealed frame can fail the whole-house air test