U-value of a timber frame wall with PIR insulation outside the studs

Brick-clad timber frame with a continuous PIR board over the sheathing.

A brick-clad timber frame wall with 140mm of mineral wool between the studs and 60mm of PIR fixed over the sheathing achieves a U-value of about 0.17 W/m²K, inside the 0.18 notional value of Part L 2021 and the Future Homes Standard. 50mm of PIR shows as 0.18 but misses it before rounding, at 0.1802. Without the board the same frame is about 0.30, because around 15% of the wall is timber: a continuous layer outside the studs covers every one of them, which is why it is worth more than its thickness suggests. Open the build-up below pre-filled and tune it to your frame.

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The build-up (outside → inside)

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
140mm wool between studs, no board≈ 0.30The plain frame — see the timber frame wall page
+ 25mm PIR over the sheathing≈ 0.22Passes the 0.26 limiting value
+ 50mm PIR over the sheathing≈ 0.18Just misses the 0.18 notional before rounding (0.1802)
+ 60mm PIR over the sheathing≈ 0.17Inside the 0.18 notional value
+ 80mm PIR over the sheathing≈ 0.15Comfortably inside 0.18

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

The studs set the base figure and the outside layer does the rest: timber (λ 0.13) conducts more than three times as well as the wool between the studs (λ 0.038), and a continuous board covers every stud. Foil-faced PIR is vapour-tight, so a board outside the frame needs to be thick enough to keep the sheathing behind it warm, with a sealed VCL on the warm side; as pre-filled, with 60mm, U-Monkey’s free condensation check finds no condensation in any month. The breather membrane is modelled over the board. Leave the foil face open to a clear cavity instead and U-Monkey treats that cavity as low-emissivity automatically (BR 443), which takes the 50mm version from 0.1802 to 0.172.

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of a timber frame wall with PIR insulation outside the studs?

A brick-clad timber frame wall with 140mm of mineral wool between the studs and 60mm of PIR fixed over the sheathing achieves a U-value of about 0.17 W/m²K, inside the 0.18 notional value of Part L 2021 and the Future Homes Standard. 50mm of PIR shows as 0.18 but misses it before rounding, at 0.1802. Without the board the same frame is about 0.30, because around 15% of the wall is timber: a continuous layer outside the studs covers every one of them, which is why it is worth more than its thickness suggests. Open the build-up below pre-filled and tune it to your frame.

Should the PIR go outside the studs or inside them?

Thermally the two are within about 0.01 W/m²K of each other: 50mm of PIR over the sheathing gives 0.180, and the same board as a lining under the plasterboard 0.173. What matters is that the layer is continuous. Outside, it keeps the studs and sheathing warm and leaves the service zone free; inside, it is the easier option on an existing wall. Wherever a foil-faced board goes, the vapour control on the warm side has to be sealed — model both positions in U-Monkey and run the free condensation check on each.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Walls

Roofs

Floors

Different build-up? See the full wall U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

Open this build-up pre-filled or call 01202 623236