U-value of insulating (thermal) plaster on a solid wall

Lime-cork and lime insulating plasters on a 9-inch brick wall: what 30–80mm really does, product by product.

Insulating plaster laid straight onto a solid 9-inch brick wall (about 2.1–2.2 W/m²K) gives about 0.64 W/m²K at 50mm of Diathonite Evolution (λ 0.045), applied in two coats — inside the 0.7 Approved Document L sets as a minimum for an upgraded wall, which takes about 44mm of it. How far a thermal plaster goes depends almost entirely on its λ, and products sold under the same name range from 0.037 to 0.12: 50mm of Diathonite Thermactive.037 gives 0.55, close to the 0.54 reported for a mews-house retrofit with it, while 50mm of an NHL insulating render at λ 0.12 only roughly halves the heat loss (1.15). None reaches the 0.30 of Table 4.3 at a practical thickness: it would take about 107mm of the best and well over 300mm of the weakest.

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The build-up (outside → inside)

Plasters at their makers’ declared λ: Diathonite Evolution 0.045 (UNI EN 1745, T1; μ 4) and Thermactive.037 0.037 (μ 3) from Diasen’s data sheets; Baumit NHL Thermo ≤ 0.075 (rated 0.08); Secil ecoCork Lime 0.09 (EN 12667, technical data sheet April 2024); Womersley’s NHL2 0.12 (the figure Carrington Lime lists for the same Cornerstone render). Each on 215mm brick with a 5mm lime finish. The mews-house figure is the ASBP’s account of a Prewett Bizley retrofit. The pre-filled link opens as an upgraded solid wall.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Bare 215mm brick, no plaster≈ 2.23
15mm ordinary lime plaster≈ 2.09
Diathonite Evolution (λ 0.045): 30mm≈ 0.89
Diathonite Evolution: 40mm (one coat)≈ 0.74Just misses the 0.7 backstop; 40mm is the most one coat takes
Diathonite Evolution: 50mm (two coats)≈ 0.64As pre-filled: meets the 0.7 backstop
Diathonite Evolution: 60mm≈ 0.56Mesh needed above 60mm
Diathonite Thermactive.037 (λ 0.037): 30mm≈ 0.79
Thermactive.037: 50mm≈ 0.55A mews-house retrofit reported 2.1 → 0.54
Baumit NHL Thermo (λ 0.075): 40mm≈ 1.01
Baumit NHL Thermo: 80mm (its maximum)≈ 0.66Meets the 0.7 backstop only at its maximum build
Secil ecoCork Lime (λ 0.09): 40mm≈ 1.11
Womersley’s NHL2 insulating render (λ 0.12): 50mm≈ 1.15About 49% less heat loss than the bare wall, against a claimed “up to 60%”

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 product is the whole story. Thermal plasters sold for solid walls run from λ 0.037 (Thermactive.037, natural hydraulic lime with recycled cork, expanded amorphous silica, perlite and pumice) and 0.045 (Diathonite Evolution, cork, clay and lime) through 0.075 (Baumit NHL Thermo) and 0.09 (Secil ecoCork Lime) to 0.12 for NHL renders with a lightweight mineral aggregate — a threefold spread that matters more than any thickness choice: 50mm takes a 9-inch brick wall to 0.55 with the best and 1.15 with the weakest. Only Secil’s is a measurement to EN 12667: Diasen cite EN 1745, whose values can be tabulated rather than tested, Baumit no method at all, and EN 998-1’s T1 class only caps a plaster’s λ at 0.10 — so treat the figures as approximate. Claims are worth checking against the physics: Diasen’s 0.222 m²K/W per centimetre is right; a 50mm NHL2 render taking “up to 60%” off a solid wall’s heat loss needs a λ nearer 0.07 than the 0.12 Carrington Lime lists for the same render (U-Monkey: 49% on bare brick); and a lime plaster sold as cutting heat loss by “more than 75%” at 50mm manages it only if its claimed λ of 0.04 holds, and then only on a poorer wall (U-Monkey: 74% on 9-inch brick, 76% on 500mm of sandstone). The other limits are per coat: Diathonite takes 15–40mm a coat with mesh above 60mm in total, Baumit’s NHL Thermo at least 30mm a coat and 80mm in all, Secil’s ecoCork 20mm a coat. Where thermal plasters earn their place is uneven stone and brick, reveals and junctions — they go on in full contact with the wall, leaving no gap for air to move behind them, and Historic England lists insulating plasters (“typically lime based”, with cork, hemp or aerated lime) among its internal insulation options. Approved Document L’s 0.30 for an upgraded wall (Table 4.3) is out of reach; the 0.7 backstop (paragraph 4.13) and the 5% floor-area allowance (note 6) are the realistic tests, and Historic England reports the DEEP study’s finding that targeting around 0.8 rather than 0.30 cut the moisture risk while keeping about two-thirds of the energy saving.

Condensation risk with insulating plaster

U-Monkey’s free BS EN ISO 13788 check fails the pre-filled wall: 1,215 g/m² of condensate is still against the brick at the end of the year (peak 3,850 g/m²). How much the plaster insulates and how vapour-open it is both count: 50mm of Thermactive.037 (R 1.35, μ 3) leaves 2,681 g/m², while 40mm of Baumit’s NHL Thermo (R 0.53, and more vapour-resistant) gets “screening only”, seasonal condensation that dries out. The Glaser method moves moisture by vapour diffusion alone, so the more a plaster insulates, the colder the brick behind it, and the more vapour-open it is, the more of a winter’s vapour reaches that cold brick.

The check says itself that this is not the whole picture: it “does not model the capillary transport that carries moisture through brick or stone”, and lime plasters in full contact with the wall move liquid water both ways. A BS EN 15026 hygrothermal assessment is the right tool, as for any internal insulation on solid masonry, and the usual precautions apply — fix damp and rain penetration first and keep every finish vapour-permeable. The surface-mould test, which is what most people buy a thermal plaster for, is clear: the bare wall fails it (fRsi 0.56), 30mm of Diathonite Evolution passes it (0.80), and 50mm of the NHL2 render only just misses it (0.748, against 0.75).

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 insulating (thermal) plaster on a solid wall?

Insulating plaster laid straight onto a solid 9-inch brick wall (about 2.1–2.2 W/m²K) gives about 0.64 W/m²K at 50mm of Diathonite Evolution (λ 0.045), applied in two coats — inside the 0.7 Approved Document L sets as a minimum for an upgraded wall, which takes about 44mm of it. How far a thermal plaster goes depends almost entirely on its λ, and products sold under the same name range from 0.037 to 0.12: 50mm of Diathonite Thermactive.037 gives 0.55, close to the 0.54 reported for a mews-house retrofit with it, while 50mm of an NHL insulating render at λ 0.12 only roughly halves the heat loss (1.15). None reaches the 0.30 of Table 4.3 at a practical thickness: it would take about 107mm of the best and well over 300mm of the weakest.

Does insulating plaster actually work?

Yes, within limits set by its λ. On a 9-inch brick wall of about 2.2 W/m²K, 50mm of a good lime-cork plaster (λ 0.037–0.045) brings it to about 0.55–0.64 — a 70–75% cut, inside the 0.7 Approved Document L expects of an upgraded wall — and even 30mm lifts the inside surface clear of the mould-risk threshold (fRsi 0.80, against 0.75). 50mm of a lime render with a lightweight mineral aggregate (λ about 0.12) roughly halves the loss (1.15). No thermal plaster reaches the 0.30 the regulations ask of an upgraded wall at a practical thickness: about 107mm of the best, over 300mm of the weakest.

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.

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