U-value of rainscreen cladding on a concrete or blockwork wall

Mineral wool and ventilated cladding on a concrete frame or block wall: the brackets decide more than the insulation does.

A concrete wall with rainscreen cladding — 200mm reinforced concrete lined inside with plasterboard on dabs, 200mm of ROCKWOOL RainScreen Duo Slab (λ 0.035) outside it and fibre-cement panels on a ventilated subframe — has a U-value of about 0.17 W/m²K for the insulation alone, and about 0.23 once the aluminium cladding brackets are counted at one per m² with the point thermal transmittance Hilti publishes for its standard bracket into concrete (χ 0.064 W/K). Stainless steel brackets (χ 0.011) give 0.18; with no calculated χ, BR 443 adds 0.30 and the wall reads 0.47, failing Approved Document L Volume 2’s 0.26 at any thickness. On a rainscreen the bracket choice moves the U-value more than the insulation does.

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The build-up (outside → inside)

Brackets as Hilti’s Technical Manual for ventilated façades (Section 6, U-value, 2019) gives them for its MFT-MFI medium aluminium bracket with a polypropylene isolator into 180mm concrete with λ 0.035 insulation: χ 0.0638 W/K at 100mm, 0.0642 at 160mm and 0.0634 at 200mm (interpolated in the source), entered as 0.064 at one bracket per m² — a round figure to show the scale; enter your own system’s χ and count. Stainless rows use its VTR stainless bracket (0.017 at 100mm, 0.0106 at 200mm), the thermally separated row its polyamide-separated brackets (about 0.001–0.0035). The cladding and its ventilated cavity are disregarded and Rse becomes 0.13 (BS EN ISO 6946 §6.9.4; BR 443 §4.8.5); the plaster dabs are bridged as BR 443 §4.6.1 directs. Opens in the England · non-domestic regime.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Insulation alone, no brackets counted (U0)≈ 0.17Where BR 443 does not let you stop: brackets through insulation must be counted
150mm, aluminium brackets, 1 per m²≈ 0.28Fails 0.26 (0.280)
200mm, aluminium, 1 per m²≈ 0.23As pre-filled (0.233)
250mm, aluminium, 1 per m²≈ 0.200.203
200mm, aluminium, 1.5 per m²≈ 0.26Fails 0.26 (0.265)
200mm, aluminium, 2 per m²≈ 0.30
200mm, Hilti’s large aluminium bracket (χ 0.108), 1 per m²≈ 0.28Heavier cladding, bigger brackets
200mm, stainless steel brackets (χ 0.011), 1 per m²≈ 0.180.179
200mm, stainless, 2 per m²≈ 0.190.190; ROCKWOOL’s own 3-D figure for 210mm on 200mm concrete with stainless brackets is 0.18
200mm, thermally separated brackets (χ 0.0035), 1 per m²≈ 0.170.172
200mm, brackets with no calculated χ: BR 443’s +0.30≈ 0.47Fails 0.26 — no thickness passes with 0.30 added
200mm NyRock Rainscreen 032 instead, aluminium, 1 per m²≈ 0.220.220

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

BR 443 (2019) §4.8.5 treats a rainscreen in two parts. The cladding and its ventilated cavity drop out, and the outside surface resistance becomes 0.13; but “the effect of brackets or rails fixing the cladding to the wall behind must be taken into account” wherever they cross insulation — by a calculation of the whole wall, a linear thermal transmittance for rails, a point thermal transmittance χ for each bracket (U = U0 + Σχ·n), or, with no calculated result, “a conservative default value of 0.30 W/m²K”. Few bracket makers publish χ. Hilti’s technical manual for ventilated façades (2019, calculated to BS EN ISO 10211) is the fullest public set, and shows the range: about 0.064 W/K for its medium aluminium bracket with a polypropylene isolator into 180mm of concrete, 0.108 for the large one, 0.010–0.032 for stainless steel brackets and 0.001–0.0035 for brackets with a polyamide thermal separation; into 300mm of brick the same aluminium brackets lose roughly half as much. The count per m² comes from the subframe layout — rail spacing times bracket spacing, set by the cladding’s weight and the wind load — so heavy cladding costs twice. Insulation makers’ tables come from 3-D calculations with one named bracket system: ROCKWOOL gives 0.18 for 210mm of RainScreen Duo Slab on 200mm of concrete with stainless steel brackets, and 0.18 for 200mm of NyRock Rainscreen 032 on 150mm of concrete with Ash & Lacy’s AxiAL aluminium brackets, which come with isolation pads — use such figures for the system modelled, not yours. The concrete itself adds little resistance, but what the brackets fix into changes their χ, as the brick figures above show, so use the χ for your backing wall; slab edges, window reveals and parapets are junction Ψ-values on top. Approved Document L Volume 2 sets 0.26 for a wall (2021 and 2026), and the NCM notional building 0.18 for a side-lit building. Where the fire rules for a building restrict combustible materials in its external walls, check the insulation’s reaction-to-fire class against them.

Condensation risk with rainscreen on concrete

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall: the concrete is on the warm side and the mineral wool outside it is vapour-open to the ventilated cavity, so any vapour that reaches it passes out.

The check is one-dimensional, through the insulation; the brackets are three-dimensional thermal bridges it cannot see. Keep the insulation tight round them, and treat the slab edges and openings as junctions.

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 building'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 rainscreen cladding on a concrete or blockwork wall?

A concrete wall with rainscreen cladding — 200mm reinforced concrete lined inside with plasterboard on dabs, 200mm of ROCKWOOL RainScreen Duo Slab (λ 0.035) outside it and fibre-cement panels on a ventilated subframe — has a U-value of about 0.17 W/m²K for the insulation alone, and about 0.23 once the aluminium cladding brackets are counted at one per m² with the point thermal transmittance Hilti publishes for its standard bracket into concrete (χ 0.064 W/K). Stainless steel brackets (χ 0.011) give 0.18; with no calculated χ, BR 443 adds 0.30 and the wall reads 0.47, failing Approved Document L Volume 2’s 0.26 at any thickness. On a rainscreen the bracket choice moves the U-value more than the insulation does.

Aluminium or stainless steel rainscreen brackets?

For the U-value, stainless or thermally separated. On this wall one aluminium bracket per m² with an isolator (χ 0.064 W/K) adds 0.064 W/m²K, taking 200mm of insulation from 0.17 to 0.23, while a stainless bracket (χ 0.011) adds 0.011, giving 0.18; Hilti’s polyamide-separated brackets add about 0.0035. The χ is the bracket maker’s, for your backing wall and insulation depth. In U-Monkey, enter it with your number of brackets per m² in the insulation layer’s fixings sheet (“Certified χ per fixing or bracket”); with no χ, pick “Rainscreen brackets, no χ” for BR 443’s +0.30.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (Approved Document L Volume 2 for buildings other than dwellings, and the Welsh and Scottish equivalents), 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? For a building other than a dwelling, U-values feed the SBEM calculation that checks the building’s emission and primary energy rates against their targets (BER and BPER against TER and TPER) — and the non-domestic U-value page has the Volume 2 limits. 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.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

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