U-value of a solid wall battened out with insulation

PIR between 50mm battens on a 9-inch wall, with and without a layer over them.

Battening out a 215mm solid brick wall with 50mm PIR between the battens and plasterboard over them gives about 0.48 W/m²K: the battens bridge 11.8% of the layer (BR 443). Add a continuous 30mm PIR layer across the battens and it reaches about 0.28, inside the 0.30 Approved Document L asks of an internally insulated wall; 25mm over the battens meets 0.30 with nothing to spare (0.2999). Mineral wool between the battens instead of PIR gives about 0.63.

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The build-up (outside → inside)

BR 443 §4.6.2: 47mm battens at 600mm centres with top and bottom rails, 11.8% timber. The pre-filled link opens as a retained solid wall being upgraded, against Approved Document L’s 0.30 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
50mm mineral wool between battens + plasterboard≈ 0.63Needs a vapour control layer
50mm PIR between battens + plasterboard≈ 0.48The battens bridge it
75mm PIR between 75mm battens + plasterboard≈ 0.35
50mm PIR between + 25mm PIR over≈ 0.30Meets 0.30 with nothing to spare (0.2999)
50mm PIR between + 30mm PIR over≈ 0.28As pre-filled
50mm PIR between + 37.5mm PIR over≈ 0.26

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 continuous layer over the battens is what makes this work: it breaks the timber bridge and, as a second layer of insulation, takes BR 443’s air-gap correction to level 0. Thicker battens with more between them do less for the same depth. Use a foil-faced board or an insulated plasterboard over the battens with taped joints, or a vapour control layer: with mineral wool between the battens and nothing on the warm side, U-Monkey’s condensation check finds moisture building up year on year on the old plaster behind it. As pre-filled, with PIR, its ISO 13788 screening finds no condensation, and 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. The battens do leave a services zone, which insulated plasterboard on dabs does not.

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 solid wall battened out with insulation?

Battening out a 215mm solid brick wall with 50mm PIR between the battens and plasterboard over them gives about 0.48 W/m²K: the battens bridge 11.8% of the layer (BR 443). Add a continuous 30mm PIR layer across the battens and it reaches about 0.28, inside the 0.30 Approved Document L asks of an internally insulated wall; 25mm over the battens meets 0.30 with nothing to spare (0.2999). Mineral wool between the battens instead of PIR gives about 0.63.

Battens or insulated plasterboard on a solid wall?

Thermally they end up close: 50mm PIR between battens plus 30mm over comes to about 0.28 W/m²K, the same as a 77.5mm insulated plasterboard on dabs, and both are about 90–95mm deep. Battens suit an uneven or damp-affected wall that needs a fixing background and a services void; dabs are quicker on a sound, flat plastered wall. Either way the joints and edges need sealing, and the moisture behaviour of the solid wall is the real design question.

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