U-value of a cavity wall with PIR partial-fill insulation

A partial-fill cavity wall with 100mm PIR and a 100mm aircrete inner leaf 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. 90mm shows as 0.18 but misses it once the mortar joints in the blockwork and the plaster dabs are counted, at 0.185, and the older 60mm-PIR spec manages about 0.24. Open it pre-filled below and adjust the board and blocks to your spec.

Open this build-up in U-Monkey — pre-filled, free

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The build-up (outside → inside)

102.5mm brick outer leaf
~40mm residual (clear) cavity
60–100mm PIR partial-fill board
100mm aircrete block inner leaf (mortar joints counted)
Plasterboard on dabs

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
60mm PIR + 40mm clear cavity	≈ 0.24	Common 2013-era spec
90mm PIR + 40mm clear cavity	≈ 0.18	Just misses the 0.18 notional once the mortar joints and dabs are counted (0.185)
100mm PIR + 40mm clear cavity	≈ 0.17	Current new-build notional spec

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

Board thickness and lambda dominate; keeping the residual cavity clean matters for both thermal performance and moisture. Swapping generic PIR (0.022) for phenolic (0.020) buys a little more, and the inner-leaf block grade still matters — as do its mortar joints, which U-Monkey counts for you (BR 443).

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 cavity wall with PIR partial-fill insulation?

Partial-fill PIR or full-fill mineral wool — which is better?

PIR gets more performance per millimetre (λ≈0.022 vs 0.038), so partial-fill hits 0.18 in a slimmer wall. Full-fill mineral wool is cheaper, more forgiving to install and better acoustically. Both are mainstream — model each in U-Monkey and compare like-for-like.

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.