U-value of a solid aircrete wall (thin-joint blockwork)

A single leaf of 215mm aircrete on thin joints: what the block gives on its own, and what external insulation adds.

A single leaf of 215mm Solar-grade aircrete (λ 0.11) laid on 2–3mm thin joints and rendered has a U-value of about 0.49 W/m²K — nearly twice the 0.26 limiting value for a new dwelling — so a solid aircrete wall needs insulating: 100mm of grey EPS external wall insulation (λ 0.032) takes it to about 0.20, and 150mm to 0.16. The thin joints matter on the bare wall: laid on conventional 10mm mortar joints, the same wall reads 0.59, because BR 443 counts mortar as a bridge whenever the block is far more insulating than the joints. Behind 100mm of external insulation the joints cost only about 0.01.

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The build-up (outside → inside)

The thin joints are entered as a bridge at BR 443 §4.2’s general fraction for 3mm joints around a 630 × 215mm block face (0.019), with its mortar λ (0.88 behind external insulation, 0.94 on a rendered-only wall), because BR 443 has no thin-joint figure; without it, U-Monkey adds its standard 6.7% for 10mm joints. The block is H+H’s Celcon Plus in Solar grade (λ 0.11, the declared maximum on NBS Source, 460 kg/m³). External insulation fixings as U-Monkey’s “EWI screw fixings” preset (6 per m², λ 17, 19.6mm²).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Rendered only: 215mm Solar block, thin joints≈ 0.49Fails the 0.26 limit for a new dwelling
Rendered only, 10mm mortar joints instead≈ 0.59BR 443’s standard 6.7% of joints
Rendered only: 300mm Mannok Aircrete Super (λ 0.12), thin joints≈ 0.40The widest above-ground aircrete block made as standard
215mm Solar + 60mm grey EPS≈ 0.26Just misses the 0.26 limit (0.261)
215mm Solar + 100mm grey EPS≈ 0.20As pre-filled: meets 0.26, misses the 0.18 notional
215mm Solar + 120mm grey EPS≈ 0.18Just over 0.18 (0.181)
215mm Solar + 150mm grey EPS≈ 0.16Meets the 0.18 notional and the 0.18 for an extension
Standard-grade block (λ 0.15) + 100mm grey EPS≈ 0.22
215mm Solar + 100mm mineral wool EWI (λ 0.036)≈ 0.21Non-combustible insulation
215mm Solar + 100mm grey EPS, 10mm mortar joints≈ 0.21The joints cost about 0.01 behind insulation (0.212)
215mm Solar + 100mm grey EPS, fixings left out≈ 0.20The screws add about 0.006 (0.196 without them)

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

Three things move a solid aircrete wall: the block’s grade, its joints and the insulation added to it. Grades run from about 0.11 (H+H Solar, Thermalite Turbo) through 0.15 (standard) to 0.18 for the high-strength blocks, and most makers stop at 215mm above ground; Mannok makes a 300mm Aircrete Super (λ 0.12) as standard, which still gives only about 0.40 rendered on thin joints (0.48 on 10mm joints) — reaching 0.26 with a block alone would take about 450mm at λ 0.11. The joints are the subtle one. BR 443 §4.2 counts mortar joints as a bridge unless the block and mortar differ by less than 0.1 m²K/W, which aircrete fails by a wide margin, and gives 6.7% of joints for 10mm mortar around a 440 × 215mm block; it says nothing specific about thin-joint work, so the joints here are its general formula at 3mm around H+H’s 630 × 215mm face: 1.9% of the wall, at BR 443’s mortar λ (0.88 behind insulation, 0.94 where the block is only rendered). A thin-joint mortar’s λ is rarely published: a European thin-bed mortar declares 0.76 and Wienerberger’s Porotherm thin-layer mortar (for clay blocks) 1.17, which would move the rendered-only wall by about 0.01 either way and the insulated one by under 0.002. The block λ needs care on a rendered-only wall: BR 443 takes masonry as exposed unless it has external insulation or cladding, and the 0.11 here is the maker’s declared maximum with no stated moisture basis. External insulation is the realistic route — it keeps the block dry and warm, and covers the thin joints, which can’t be raked out to key a render (H+H recommends a stipple bonding coat or a metal lath). Approved Document L sets 0.26 for the walls of a new dwelling (Table 4.1) and 0.18 for a new wall in an existing one (Table 4.2).

Condensation risk in a solid aircrete wall with external insulation

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with the EPS tested across BS 5250’s published range of vapour resistance: the insulation outside keeps the block warm, and the block is vapour-open. The render does the weathering, so the system’s details at openings and at the base track matter more than the calculation does.

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 a solid aircrete wall (thin-joint blockwork)?

A single leaf of 215mm Solar-grade aircrete (λ 0.11) laid on 2–3mm thin joints and rendered has a U-value of about 0.49 W/m²K — nearly twice the 0.26 limiting value for a new dwelling — so a solid aircrete wall needs insulating: 100mm of grey EPS external wall insulation (λ 0.032) takes it to about 0.20, and 150mm to 0.16. The thin joints matter on the bare wall: laid on conventional 10mm mortar joints, the same wall reads 0.59, because BR 443 counts mortar as a bridge whenever the block is far more insulating than the joints. Behind 100mm of external insulation the joints cost only about 0.01.

Can a solid aircrete wall meet Building Regulations without insulation?

Not at any thickness the makers produce. 215mm of Solar-grade aircrete (λ 0.11) on thin joints, rendered and plastered, comes to about 0.49 W/m²K, and Mannok’s 300mm Aircrete Super, also on thin joints, about 0.40, against the 0.26 Approved Document L sets for a new dwelling’s walls; 0.26 would need about 450mm of Solar-grade block (λ 0.11). That is why solid aircrete walls are built with external insulation — 100mm of grey EPS gives about 0.20 — or lined inside (H+H’s thin-joint brochure gives 0.24 for a 200mm solid wall with 62.5mm of phenolic insulated plasterboard).

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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