Glass U-Value Calculator — Centre-Pane & Part L

Specify any single, double or triple glazed unit and read its centre-pane U-value with an instant Building Regulations Part L pass or fail. Models Low-E coatings, argon & krypton fills, warm-edge spacers and cavity widths to the BS EN 673 method.

Single · Double · Triple Part L verdict Argon & Krypton Instant result

0.6 W/m²K

Best triple-glazed unit

1.4 W/m²K

Part L replacement limit

5.8 W/m²K

Single pane (no insulation)

−0.3 W/m²K

Argon vs air saving

Indicative whole-unit U-values (Ug) calculated to the simplified BS EN 673 method using typical UK trade glass data. Final tested values depend on the exact coating, gas-fill rate and frame — confirm exact figures with us before any Building Control submission.

The five levers

What Changes a Glazing U-Value?

Five things decide whether a sealed unit clears Building Regulations. Change any of them in the calculator above and watch the U-value move in real time — here is what each one is worth.

Glazing type

The number of panes is the single biggest lever. Single glazing sits at 5.8; a sealed double unit drops to 1.1–2.8; a triple unit reaches 0.6–1.9. Everything else fine-tunes from there.

5.8 → 0.6 W/m²K

Low-E coating

A microscopically thin soft-coat layer reflects radiant heat back into the room. Optitherm out-performs Pilkington K, and coating a second pane buys a further ~0.2 on a triple unit.

−1.0 to −1.8 W/m²K

Gas fill

Argon is denser than air, so it slows convection inside the cavity. Krypton is denser still and shines in slim 6–12mm cavities where argon has no room to work.

Argon −0.2 to −0.3

Cavity width

There is a sweet spot: 14–20mm is optimal for argon. Go narrower and conduction creeps up; go wider and the gas starts to convect and undo the benefit.

±0.3 W/m²K

Spacer bar

The bar around the unit edge is a thermal short-cut. Swapping aluminium for a warm-edge spacer (Swisspacer or TGI) cuts edge losses — often the exact margin that turns a fail into a pass.

Warm edge ≈ −0.1

Popular specs

Three Specs Most Customers Choose

From a budget Part-L pass to a passive-house triple unit, these are the build-ups we manufacture most. Each is in stock-spec glass for fast turnaround.

Standard DGU

1.3W/m²K

4-16-4 · Low-E (K) · argon · warm edge

The cost-effective everyday unit that clears the Part L replacement limit with room to spare.

Passes Part L replacement

High-performance DGU Best value

1.0W/m²K

4-20-4 · Optitherm · argon · warm edge

Beats new-build requirements from a single sealed unit — our most-specified trade upgrade.

Exceeds Part L new-build

Triple unit

0.6W/m²K

4-16-4-16-4 · dual Low-E · argon

Passive-house territory for the warmest glazing money can buy. Heavier — check frame and hinge ratings.

Passive-house level

Specification	Gas	Spacer	U-value (W/m²K)
Single 6mm float	—	—	5.8
DGU 4-20-4 float	Argon	Warm edge	2.5
DGU 4-16-4 Low-E (K)	Argon	Warm edge	1.3
DGU 4-20-4 Optitherm ★	Argon	Warm edge	1.0
DGU 4-20-4 Optitherm	Krypton	Warm edge	0.8
TGU 4-16-4-16-4 dual Low-E	Argon	Warm edge	0.6

Compliance

Building Regulations Part L Limits

The maximum window U-value allowed under current UK Building Regulations, by application. Hit the number on the left and you are compliant.

1.2

New-build windows (England)

Part L 2022

1.4

Replacement windows (England)

Part L 2022

1.6

Replacement windows (Wales)

Part L Wales

0.8

Passive house standard

PHI

Good to know

Frequently Asked Questions

For UK replacement windows the unit (Ug) value should be 1.4 W/m²K or lower to meet Building Regulations Part L. A standard 4-16-4 unit with a Low-E coating and argon fill typically achieves 1.2–1.4. High-performance Optitherm units reach 1.0 W/m²K.

Ug is the centre-of-glass value for the sealed unit alone — what this calculator estimates. Uw is the whole window including the frame and edge effects, which appears on a Window Energy Rating. Frames are usually less insulating than modern glass, so Uw is typically 0.2–0.6 higher than Ug.

Yes. Replacing the air in the cavity with 90% argon reduces the U-value by approximately 0.2–0.3 W/m²K because argon is denser and slows convection. Krypton performs even better in narrow cavities but costs more.

Often, yes. Swapping an aluminium spacer for a warm-edge spacer such as Swisspacer or TGI cuts edge-zone heat loss by roughly 0.1 W/m²K — frequently the difference between a borderline fail and a pass.

Need an Exact U-Value for Building Control?

Clear Sealed Units Ltd manufactures sealed units to your exact thermal spec. Trade pricing, fast turnaround, UK-wide delivery.

Get a Quote Call the Team

How the U-value is calculated

Centre-pane U-value measures thermal transmittance through the glazing itself, expressed in W/m²K. A lower value means better insulation. Building Regulations Part L sets a maximum centre-pane U-value of 1.4 W/m²K for replacement glazing in existing dwellings.

This calculator uses cavity conductance values for air, argon (≈ 15% improvement over air) and krypton (≈ 30% improvement). Applying a Low-E hard or soft coat reduces long-wave radiation transfer and typically improves U-value by 0.2–0.5 W/m²K depending on emissivity.

Worked example

A 4–16–4 argon DGU with Low-E soft coat: typical centre-pane U-value ≈ 1.1 W/m²K — well within Part L limits. The same unit with air fill and no Low-E would typically reach 2.7 W/m²K, failing Part L.

Centre-pane vs whole-window U-value

Note that centre-pane U-value differs from whole-window U-value (Uw), which accounts for the frame and edge zone. BFRC Window Energy Ratings and SAP calculations use Uw. This tool gives centre-pane values only; consult your frame supplier for Uw data.

Also see our glass weight calculator for handling and logistics data. For triple glazed unit specifications, visit our triple glazed units page.