U-value of calcium silicate internal wall insulation

A solid 9-inch brick wall (about 2.1 W/m²K) lined with 50mm of calcium silicate board — redstone Clima Redboard pro, λ 0.059 — over a 20mm lime-cork levelling plaster comes to about 0.54 W/m²K, which is what its UK distributor’s table gives; on the old lime plaster, without the insulating levelling coat, the same 50mm gives 0.75, just short of the 0.7 that Approved Document L asks of an upgraded wall as a minimum, and 60mm (0.67) is needed. Calcium silicate is chosen for moisture, not for its λ: it is strongly capillary-active, alkaline and non-combustible, and at 0.059 it would take about 174mm to reach the 0.30 Table 4.3 sets — beyond the 120mm its approval covers. Multipor mineral board (λ 0.040) reaches 0.30 at about 116mm.

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The build-up (outside → inside)

215mm solid brick (old gypsum plaster and paint removed)
20mm Diathonite Thermactive.037 lime-cork levelling plaster (λ 0.037)
5mm board adhesive (modelled as lime plaster)
50mm redstone Clima Redboard pro calcium silicate board (λ 0.059, μ 3)
5mm lime finish on mesh

Calcium silicate at its declared λD(23,50) 0.059 W/mK (ETA-19/0559, 180–187 kg/m³, μ 3); the distributor’s table is for 220mm brick with 20mm of levelling plaster. U-Monkey’s figures use 215mm brick, Diathonite Thermactive.037 as the levelling coat (λ 0.037, μ 3) and a 5mm adhesive bed and 5mm finish entered as lime plaster (neither publishes a λ). Multipor rows: 5mm light mortar (λ 0.18) under and over the board, on 15mm of lime plaster; the stockist’s table is for 225mm brick (its λ basis isn’t stated). The pre-filled link opens as an upgraded solid wall.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Bare wall: 215mm brick, 15mm lime plaster	≈ 2.12
25mm board, on the old lime plaster	≈ 1.10
50mm board, on the old lime plaster	≈ 0.75	Just misses the 0.7 backstop
60mm board, on the old lime plaster	≈ 0.67	Meets the 0.7 backstop
80mm board, on the old lime plaster	≈ 0.55
100mm board (two layers), on the old lime plaster	≈ 0.46
20mm lime-cork levelling coat + 30mm board	≈ 0.66	Distributor’s table: 0.66
20mm lime-cork levelling coat + 50mm board	≈ 0.54	As pre-filled; distributor: 0.54
20mm lime-cork levelling coat + 80mm board	≈ 0.43	Distributor: 0.42
50mm board at the rated λ 0.062, on the old plaster	≈ 0.78	The German “rated value”, about 5% above the declared one
50mm SkamoWall (λ 0.068), on the old plaster	≈ 0.82
500mm sandstone, 30mm levelling coat + 50mm board	≈ 0.49	Distributor: 0.48 for 500mm of stone
Multipor M4 mineral board (λ 0.040): 60mm	≈ 0.50	UK stockist’s table: 0.51
Multipor M4: 100mm	≈ 0.34	Stockist: 0.34
Multipor M4: 120mm	≈ 0.29	Meets the 0.30 (0.290); stockist: 0.29
Multipor M4: 120mm at the rated λ 0.042	≈ 0.30	Just over 0.30 (0.302)

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 levelling coat is the hidden variable. The distributor’s table (Ecological Building Systems: 30mm → 0.66, 50mm → 0.54, 80mm → 0.42, 100mm → 0.37 on 220mm brick) includes 20mm of lime-cork insulating plaster under the boards, worth R 0.54 m²K/W — about as much as 32mm of the board itself; on an ordinary lime base the same boards give 0.75 at 50mm and need 60mm for the 0.7 backstop. The λ basis is the second: the board’s European approval (ETA-19/0559) declares λD(23,50) 0.059, and the German “rated value” printed on data sheets is about 5% higher (0.062); sellers’ tables use either. SkamoWall, another board stocked in the UK, is 0.068 (its newer variants 0.073), and a seller’s “U 2.64 W/m²K” for a 25mm board is just the board’s own conductance (0.066 ÷ 0.025, at λ 0.066), with no wall and no surface resistances. Multipor is a different, much lighter mineral board (90 kg/m³): better λ (0.040), very vapour-open (μ 2), and far less absorbent — at most 2 kg/m² short-term water uptake in its approval, against 17–28 for the calcium silicate boards — and, like wood fibre, it can reach 0.30, at about 116mm, though that takes some 13cm off the room. Approved Document L asks an upgraded solid wall for 0.30 (Table 4.3), allows less where that would cut the room’s floor area by more than 5% (note 6), and expects an upgraded element to be no worse than 0.7 (paragraph 4.13); on traditional buildings it asks for improvements that won’t harm the fabric (paragraph 0.10), 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. Strip gypsum plaster, paint and salts first, fix damp and rain penetration, and finish with breathable coatings only.

Condensation risk with calcium silicate boards

U-Monkey’s free BS EN ISO 13788 check does not pass these boards. For the pre-filled wall it leaves 1,415 g/m² of condensate at the end of the year (peak 3,440 g/m²), against the brick, and almost every board and Multipor row on this page fails the same way; the three that don’t — 25mm of board, and 50mm of the boards at λ 0.062 and 0.068 on the old plaster — still build up 2.7–3.6 kg/m² over the winter before drying out in summer, which it rates “screening only”. The boards are so vapour-open (μ 2–3) that a vapour-diffusion calculation sends a winter’s worth of moisture to the cold brick behind them. The check also says why that is not the last word: a Glaser calculation “models vapour diffusion only: it does not model the capillary transport that carries moisture through brick or stone”, and calcium silicate is chosen precisely because it moves liquid water by capillary action (water absorption coefficient about 46 kg/m²√h).

So the calculation that matters here is a transient hygrothermal one to BS EN 15026 (WUFI or DELPHIN), which the board’s distributor recommends instead of the Glaser method, and which the STBA recommends for any internal insulation on a solid wall; U-Monkey rates internal insulation on solid masonry no better than “screening only” for the same reason. Its surface-mould test is clear: the bare wall fails it (fRsi 0.58, below 0.75), and the pre-filled lining passes comfortably (0.87).

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 (f_Rsi) 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 calcium silicate internal wall insulation?

Why is the U-value of calcium silicate board so modest?

Because its thermal conductivity is about 0.059 W/mK — some 2.7 times that of PIR — so 50mm on a 9-inch brick wall gives only about 0.75 W/m²K, or 0.54 with the 20mm lime-cork levelling coat its distributor uses under it. It is not chosen as insulation per millimetre: it is a strongly capillary-active, alkaline, non-combustible board, sold to keep the wall’s inner face dry and free of mould. If the U-value matters more, Multipor mineral board (λ 0.040) does about as well as wood fibre: 100mm gives about 0.34, 120mm 0.29 (wood-fibre boards 0.33 and 0.28).

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.