U-value of aerogel internal wall insulation

A solid 9-inch brick wall (215mm, plastered) has a U-value of about 2.1 W/m²K; 20mm of aerogel bonded to 12.5mm foil-backed plasterboard, fixed on 25mm battens, takes it to about 0.58, and 10mm to about 0.82 — the figures Proctor’s own tables give for its Spacetherm Wallboard. That is the point of aerogel: a big cut in heat loss for little lost floor space. It is not magic per millimetre, though: at λ 0.0195 it is only about 11% better than PIR (λ 0.022), and on battens a PIR board of the same overall depth does as well; reaching the 0.30 Approved Document L sets for an upgraded wall takes about 55mm on battens. Its strengths are thinness where there is no room for battens or dabs, and a blanket that is itself non-combustible (Euroclass A1) — though Proctor notes that its laminated boards, the Wallboard included, no longer carry that rating.

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The build-up (outside → inside)

215mm solid brick
13mm plaster
25mm battens (11.8% timber, BR 443 §4.6.2)
20mm aerogel blanket (Spacetherm A1, λ 0.0195)
12.5mm foil-backed plasterboard (vapour control: 78.5 MN·s/g)

Aerogel as Proctor’s Spacetherm A1 (λ 0.0195 at 23 °C), the blanket in its Spacetherm Wallboard, which is supplied on 12.5mm foil-faced plasterboard with a vapour control rating of 78.5 MN·s/g (Spacetherm brochure, August 2025); that foil is entered on the board here (sd 15.7m), because U-Monkey’s generic foil-backed board carries no vapour resistance of its own. Proctor’s tables, for 220mm of brick with 13mm plaster (2.11), are in the notes column. 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, 13mm plaster	≈ 2.10	Proctor’s base wall: 2.11
10mm aerogel + 12.5mm plasterboard on 25mm battens	≈ 0.82	Proctor: 0.82
15mm aerogel + plasterboard on battens	≈ 0.68	Proctor: 0.68; inside the 0.7 backstop
20mm aerogel + plasterboard on battens	≈ 0.58	As pre-filled; Proctor: 0.58
30mm aerogel + plasterboard on battens	≈ 0.45	Proctor: 0.45
55mm aerogel + plasterboard on battens	≈ 0.29	Meets the 0.30 for an upgraded wall; Proctor: 0.29
10mm aerogel + plasterboard fixed straight to the wall (generic)	≈ 0.96	22.5mm deep in all
20mm, fixed straight to the wall	≈ 0.65	32.5mm deep
40mm, fixed straight to the wall	≈ 0.39
For comparison: 37.5mm PIR insulated plasterboard (25mm PIR) on dabs	≈ 0.56	52.5mm deep, against 57.5mm for 20mm of aerogel on battens
For comparison: 20mm PIR + foil-backed plasterboard on the same 25mm battens	≈ 0.55	The gap beside the foil counts as low-e

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

On battens the board gains a sheltered air gap, worth about another 3–4mm of aerogel even with the battens bridging it, so a battened board does better than the same board fixed straight to the wall (0.58 against 0.65 for 20mm) — Proctor fixes its Wallboard to timber straps. Depth for depth, though, the battens take away aerogel’s advantage: a 37.5mm PIR insulated plasterboard on dabs, 52.5mm deep in all, gives 0.56 against 0.58 for 20mm of aerogel on 25mm battens (57.5mm), and 20mm of PIR on the same battens gives 0.55, because the gap beside a foil-faced board counts as low-emissivity. Where aerogel earns its price is where there is no room for battens or dabs — window reveals, narrow rooms and listed interiors where the joinery limits how far a wall can come forward — and, as a bare blanket, in its make-up: Spacetherm A1 is Euroclass A1, non-combustible, though Proctor notes that its laminated boards, the Wallboard included, no longer carry that rating (its plasterboard is A2-s1,d0). In the government’s Thin Internal Wall Insulation trial (BEIS, 2021), a 14mm aerogel board took measured solid brick walls from 2.11 to 0.76, with no notable performance gap, and the time its joist ends spent at mould or rot risk in the modelling stayed far below that of 70mm of conventional internal insulation (2.8% of the time, against 11.3%, and 0.2% for the bare wall). Approved Document L asks an upgraded solid wall for 0.30 (Table 4.3), allows a lesser standard where meeting it would cut the room’s floor area by more than 5% (note 6), and adds that a thermal element once upgraded should generally be no worse than 0.7 (paragraph 4.13) — which 10mm on battens (0.82) misses and 15mm (0.68) meets.

Condensation risk with aerogel internal insulation

Any internal insulation leaves the brick colder and slower to dry, and BS 5250 and current guidance treat a monthly condensation calculation as a screening for this, not a verdict: a solid wall is wetted by driving rain and moves water as liquid, which the calculation can’t see. U-Monkey’s free BS EN ISO 13788 check says so. For every aerogel row on this page it finds no interstitial condensation in any month, with the board’s foil facing as the vapour control layer, and rates the wall “screening only”, with a BS EN 15026 hygrothermal assessment by a specialist as the next step.

The check’s surface-mould test is where thin boards shine: the bare wall fails it (fRsi 0.58, below the 0.75 limit for dwellings), and even 10mm of aerogel fixed straight to the wall passes (0.79). Before insulating, cure any damp and repoint or repair the outside so rain stays out — the BEIS trial found wetter bricks had higher U-values — and seal the board’s joints and edges, because that foil only works as a vapour control layer if it is continuous.

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 aerogel internal wall insulation?

Is 10mm of aerogel worth it on a solid wall?

It roughly halves the heat loss through the wall: 10mm of aerogel on a 12.5mm board, fixed on battens, takes a 9-inch solid brick wall from about 2.1 to 0.82 W/m²K, and the 14mm aerogel board in the government’s thin internal insulation trial measured 0.76. That is short of the 0.30 Approved Document L sets for an upgraded wall in England, and of its general 0.7 backstop for an upgraded element (paragraph 4.13), which takes 15mm on battens (0.68); the guidance does allow a lesser standard than 0.30 where meeting it would reduce the room’s floor area by more than 5%. Even the thinnest boards leave the wall’s inside face warmer and less prone to mould: U-Monkey’s check puts the bare wall below the surface-mould limit (fRsi 0.58, against 0.75) and the 10mm board on battens comfortably above it.

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.