U-value of cork insulation on a solid wall

Expanded cork board inside or outside a 9-inch brick wall: the U-values, and how cork differs from wood fibre.

Expanded cork board (Amorim ICB, λ 0.040) 60mm thick, bonded inside a solid 9-inch brick wall and finished in lime plaster, gives a U-value of about 0.50 W/m²K, from about 2.1 — about the same as 60mm of wood fibre, and the 0.49 the ASBP’s specification generator gives for 60mm of cork. 40mm (0.67) is the thinnest standard board that meets the 0.7 Approved Document L sets as a minimum for an upgraded wall, and its 0.30 takes about 120mm, inside (0.29) or outside (0.29). Cork earns its place on what differs from wood fibre: it does not take up water by capillary action, and it is more vapour-resistant (μ 20, against 3–5).

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The build-up (outside → inside)

Amorim expanded corkboard (ICB) at λ 0.040 and μ 20 (Amorim technical features; ACERMI 0.040, EU declared 0.039). The adhesive (Secil Isovit, an NHL and cork mix) publishes no λ and is entered as lime plaster, like the base and top coats. External rows: 3mm lime top coat and 10mm base coat outside the cork, 10mm adhesive, 215mm brick (protected by the cork outside it: λ 0.56, BR 443 §7, mortar joints counted, §4.2), 13mm plaster inside, with the fixings left out (Tŷ-Mawr names no fixing type). The pre-filled link opens as an upgraded solid wall.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Internal: 40mm cork≈ 0.67Meets the 0.7 backstop
Internal: 50mm≈ 0.58
Internal: 60mm≈ 0.50As pre-filled; ASBP: 0.49
Internal: 80mm≈ 0.41
Internal: 100mm≈ 0.34
Internal: 120mm≈ 0.29Meets the 0.30 (0.293)
Internal: 60mm at the declared λ 0.039≈ 0.500.495
External: 100mm cork, lime base coat≈ 0.33
External: 120mm≈ 0.29Tŷ-Mawr’s external design for 0.30 or better
External: 140mm≈ 0.25
External: 160mm≈ 0.23
500mm sandstone or granite + 60mm cork inside≈ 0.52Fails the condensation check (about 650 g/m² left at the year end)
500mm limestone + 60mm cork inside≈ 0.49Fails the condensation check (about 400 g/m² left)

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

Thickness does the work, as with any board, and cork’s λ (Amorim declares 0.039 for its EU label and 0.040 under ACERMI; 0.040 is used here) puts it level with wood fibre: 60mm takes a plastered 9-inch brick wall from about 2.1 to 0.50, 120mm to 0.29. What sets cork apart is the moisture behaviour. Amorim’s expanded corkboard “does not absorb water by capillary action” and has a vapour resistance factor of 20 — about 1.2m of equivalent air at 60mm, four to seven times less open than 60mm of wood fibre — so it neither wicks moisture away like a capillary-active board nor lets vapour through as freely. That is part of why it is also used outside: Tŷ-Mawr’s external design puts 120mm of cork on a lime levelling coat, with insulated fixings at about 8 per m² and a meshed lime-cork base coat, for 0.30 or better — U-Monkey gives 0.29 with the fixings left out (add them in the fixings sheet to suit the fixing chosen). The board is Euroclass E (Amorim’s external system is B), so in England it can’t form 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 — inside or out (regulation 7(2)). Approved Document L asks an upgraded solid wall for 0.30 (Table 4.3), allows a lesser standard where internal insulation would take more than 5% of the room’s floor area (note 6), and generally no worse than 0.7 (paragraph 4.13); outside, note 6 doesn’t apply. Inside or out, cure damp and keep driving rain out first — Tŷ-Mawr calls external protection from driving rain essential for internal insulation — and use lime adhesives and vapour-permeable paints.

Condensation risk with cork internal insulation

For the pre-filled wall, U-Monkey’s free BS EN ISO 13788 check finds seasonal condensation at the back of the cork, where it meets the lime adhesive — a peak of 463 g/m² in March — that dries out fully over the summer, and rates the wall “screening only”: internal insulation on solid masonry needs a BS EN 15026 hygrothermal assessment before the result can be relied on, because a monthly calculation can’t see driving rain or liquid moisture moving through the brick. It also flags that the condensation forms against a bio-based insulant, which a prolonged wetting can damage. On 500mm of stone the same 60mm fails instead: condensate builds up year on year (about 650 g/m² left at the end of the year on sandstone or granite, 400 on limestone).

The same cork outside the wall — 120mm on the brick, under a lime render — gets a straight pass: no condensation in any month. That is the general pattern for solid walls, and the reason internal insulation is the one that needs the extra care: keep the rain out, the joints and edges sealed, and the finishes vapour-permeable.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 cork insulation on a solid wall?

Expanded cork board (Amorim ICB, λ 0.040) 60mm thick, bonded inside a solid 9-inch brick wall and finished in lime plaster, gives a U-value of about 0.50 W/m²K, from about 2.1 — about the same as 60mm of wood fibre, and the 0.49 the ASBP’s specification generator gives for 60mm of cork. 40mm (0.67) is the thinnest standard board that meets the 0.7 Approved Document L sets as a minimum for an upgraded wall, and its 0.30 takes about 120mm, inside (0.29) or outside (0.29). Cork earns its place on what differs from wood fibre: it does not take up water by capillary action, and it is more vapour-resistant (μ 20, against 3–5).

Is cork better than wood fibre for insulating a solid wall?

Thermally they are level: λ 0.039–0.040 for cork, 0.038–0.040 for wood-fibre boards, so 60mm of either takes a 9-inch brick wall to about 0.49–0.50 W/m²K. The differences are in moisture and use. Wood fibre is capillary-active and very vapour-open (μ 3–5); cork does not take up water by capillary action and is more vapour-resistant (μ 20). In U-Monkey’s screening check, 60mm of cork inside a 9-inch brick wall leaves seasonal condensation that dries out each summer (on 500mm of stone it builds up instead), and 120mm of cork outside the wall none at all. Both are used outside as well as in, on lime-based systems.

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.

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