U-value of external wall insulation with mineral wool

Rendered mineral-wool EWI on a solid wall, with its fixings counted, and how it compares with EPS.

Rendered mineral-wool external wall insulation, 120mm thick (λ 0.036), on a 215mm solid brick wall gives a U-value of about 0.27 W/m²K once the steel fixings through it are counted — inside the 0.30 Approved Document L asks of an externally insulated wall. 110mm meets it too, at 0.29, and 100mm, at 0.32, does not. Mineral wool needs about a fifth more thickness than grey EPS for the same result (100mm of EPS at λ 0.031 gives 0.27), but it is non-combustible, which is why it is used where combustible insulation is not allowed.

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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 external insulation (2021 edition, Table 4.3). Mineral wool at λ 0.036 (a dual-density EWI slab in U-Monkey’s library); fixings as the app’s “EWI screw fixings” preset, corrected by BS EN ISO 6946 Annex F.3. The brick behind the insulation is taken as protected, λ 0.56 (BR 443 §7), with its mortar joints counted (§4.2).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm mineral wool≈ 0.32Misses 0.30
110mm mineral wool≈ 0.29Meets: 0.294
120mm mineral wool≈ 0.27As pre-filled: meets 0.30
140mm mineral wool≈ 0.24
150mm mineral wool≈ 0.23
120mm, fixings left out≈ 0.26The fixings add about 0.01
120mm slab at λ 0.039≈ 0.29Meets: 0.291
100mm grey EPS (λ 0.031), bonded, no fixings≈ 0.27For comparison

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

λ and the fixings set it. Mineral-wool EWI slabs run λ 0.034–0.039 against 0.030–0.032 for grey EPS, and mineral wool is mechanically fixed: each steel fixing through the insulation is a small thermal bridge, which BR 443 (§4.8.3) and BS EN ISO 6946 Annex F correct for. Here U-Monkey’s typical EWI screw fixings — 6 per m², 5mm stainless steel, λ 17 — add about 0.01; recessed fixings with insulating caps add less, so enter the system’s own fixing data where it is known. The rest is detailing: the insulation has to return into the window reveals, run down past the floor level at the base and up into the eaves, or those junctions undo a good part of the gain.

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 external wall insulation with mineral wool?

Rendered mineral-wool external wall insulation, 120mm thick (λ 0.036), on a 215mm solid brick wall gives a U-value of about 0.27 W/m²K once the steel fixings through it are counted — inside the 0.30 Approved Document L asks of an externally insulated wall. 110mm meets it too, at 0.29, and 100mm, at 0.32, does not. Mineral wool needs about a fifth more thickness than grey EPS for the same result (100mm of EPS at λ 0.031 gives 0.27), but it is non-combustible, which is why it is used where combustible insulation is not allowed.

Why use mineral wool rather than EPS for external wall insulation?

Fire. Mineral wool is non-combustible and EPS is not. In England, regulation 7(2) of the Building Regulations 2010 requires materials that become part of the external wall of a “relevant building” — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes — to be of European Classification A2-s1, d0 or A1, which rules out EPS there. Mineral wool is also chosen on lower buildings, at the cost of a thicker system for the same U-value.

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.

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