Insulated plasterboard on a solid wall: what thickness reaches a U-value of 0.30?

Thermal laminate board stuck to a 9-inch wall, and the core thickness that meets 0.30.

On a 215mm solid brick wall, insulated plasterboard fixed on dabs needs about 65mm of PIR core (a 77.5mm board) to meet the 0.30 W/m²K that Approved Document L asks of an internally insulated wall: that gives about 0.28. A 60mm core shows as 0.30 but is 0.302 before rounding, so it misses; 50mm gives about 0.35 and 40mm about 0.41. The plaster dabs behind the board bridge its 15mm air gap, which BR 443 counts and U-Monkey adds for you.

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The build-up (outside → inside)

The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 for internal insulation (2021 edition, Table 4.3). The dab gap is BR 443’s 15mm air space with 20% plaster dabs; an ordinary foil-backed board gets no low-emissivity credit there.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
25mm PIR core (37.5mm board)≈ 0.56
40mm PIR core (52.5mm board)≈ 0.41
50mm PIR core (62.5mm board)≈ 0.35
60mm PIR core (72.5mm board)≈ 0.30Misses: 0.302 before rounding
65mm PIR core (77.5mm board)≈ 0.28As pre-filled: meets 0.30
80mm PIR core (92.5mm board)≈ 0.24

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 core thickness and its λ do the work: a phenolic core (around λ 0.019) gets there thinner, and the products in U-Monkey’s library carry their own λ and, where certified, the emissivity of their facing. Room loss is the trade: about 92mm comes off every lined wall, and Approved Document L allows a lesser standard where meeting 0.30 would cut a room’s floor area by more than 5% (Table 4.3, note 6). The bigger question on a solid wall is moisture. Lining it makes the brick colder and wetter; as pre-filled, U-Monkey’s ISO 13788 screening finds no condensation, but it reports that as a screening result only, because PAS 2035 notes that an ISO 13788 check might not be sufficient for internal insulation on solid masonry; a BS EN 15026 hygrothermal assessment is the fuller test. Seal the board’s joints and edges, and treat window reveals and floor junctions as part of the job.

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

Insulated plasterboard on a solid wall: what thickness reaches a U-value of 0.30??

On a 215mm solid brick wall, insulated plasterboard fixed on dabs needs about 65mm of PIR core (a 77.5mm board) to meet the 0.30 W/m²K that Approved Document L asks of an internally insulated wall: that gives about 0.28. A 60mm core shows as 0.30 but is 0.302 before rounding, so it misses; 50mm gives about 0.35 and 40mm about 0.41. The plaster dabs behind the board bridge its 15mm air gap, which BR 443 counts and U-Monkey adds for you.

How thick is insulated plasterboard for a solid wall?

For 0.30 W/m²K on a 215mm brick wall, a 77.5mm PIR board (65mm core + 12.5mm plasterboard) on dabs, with about 15mm of dab behind it, so roughly 92mm in front of the old plaster. A 62.5mm board gives about 0.35 and a 52.5mm board about 0.41. Phenolic boards reach the same U-value a little thinner. Open the build-up below and use the − / + buttons on a product to step through its real board sizes.

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