U-value of an SFS infill wall with rainscreen cladding

Steel-frame infill with mineral wool between the studs and a rainscreen slab outside: where the cladding brackets decide the answer.

A steel frame system (SFS) infill wall — 100mm steel studs at 600mm centres with 100mm of ROCKWOOL NyRock Frame Slab 032 between them, 12.5mm Siniat Weather Defence sheathing, and 150mm of NyRock Rainscreen 032 outside behind ventilated cladding — has a U-value of about 0.14 W/m²K before the cladding brackets are counted, and about 0.16 with the χ values certified for one low-conductivity bracket system. BR 443 says brackets that pass through the insulation must be counted: from the bracket system’s own point thermal transmittances (χ) where they exist, or by adding a conservative 0.30 W/m²K where they don’t — which takes the same wall to 0.44 and fails the 0.26 limiting value at any thickness. So the U-value of a rainscreen wall is as much a question of which brackets, and whose numbers, as of insulation.

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The build-up (outside → inside)

The brackets are entered as the Passive House Institute component certificate for the BILDA Rainscreen System gives them for heavy (stone) cladding: supporting brackets at 0.12 and 0.20 per m² (χ 0.0353 W/K) and restraining brackets at 0.49 per m² (χ 0.0053 W/K), Σχ·n 0.0139 W/m²K, entered as their mean χ (0.01715 W/K) over 0.81 brackets per m² — use your own system’s figures. The certificate’s χ values were calculated for its reference façade, with 240mm of insulation at λ 0.035, and it was valid to the end of 2025; a bracket’s χ changes with the insulation depth and the wall behind it, so the figures here are an example, not a property of this wall. The steel frame is BRE Digest 465’s hybrid case in U-Monkey’s light steel frame panel (50mm flanges, 600mm centres). The cladding and its ventilated cavity are disregarded and the outside surface resistance becomes 0.13 (BS EN ISO 6946 §6.9.4; BR 443 §4.8.5).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
No brackets counted (U0): 150mm rainscreen slab≈ 0.14Where BR 443 does not let you stop: brackets through the insulation must be counted
Example certified brackets (Σχ·n 0.014): 100mm rainscreen slab≈ 0.20Meets the 0.26 limit; misses the 0.18 notional
Example certified brackets: 150mm rainscreen slab≈ 0.16As pre-filled (0.156)
Example certified brackets: 200mm rainscreen slab≈ 0.13
Brackets with no calculated χ: BR 443’s +0.30 default, 150mm≈ 0.44Fails 0.26 — and no thickness of slab can pass with 0.30 added
Brackets with no calculated χ: BR 443’s +0.30 default, 200mm≈ 0.42
150mm studs + 150mm frame slab, 150mm rainscreen, example certified brackets≈ 0.14
Warm frame: nothing between the studs, 150mm rainscreen, example certified brackets≈ 0.21All the insulation outside the steel: plain ISO 6946, no Digest 465 weighting (BR 443 §4.10)
RainScreen Duo (λ 0.035) 150mm instead, example certified brackets≈ 0.160.164
Studs at 400mm centres (0.3% steel), 150mm, example certified brackets≈ 0.160.163

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 brackets come first. BR 443 (2019) §4.8.5 says the brackets or rails that fix rainscreen cladding “must be taken into account” where they penetrate the insulation, in one of four ways: a detailed calculation of the whole wall; a linear thermal transmittance for a rail; a point thermal transmittance (χ, W/K) for each bracket, added as U = U0 + Σχ·n with n brackets per m²; or, where no calculated result exists, U0 plus a conservative 0.30 W/m²K. The Annex F fixings correction of BS EN ISO 6946, used for wall ties and screws, is not the method for them. A χ comes from a three-dimensional calculation (BS EN ISO 10211) by the bracket or subframe maker, and how much it adds varies far more than the insulation does: the Passive House Institute’s certificate for the BILDA rainscreen system, for heavy stone cladding, gives Σχ·n 0.0139 W/m²K (the pre-filled figure); ROCKWOOL’s own NyRock Rainscreen 032 tables, modelled in 3-D with FastFrame brackets and 8mm cladding, read 0.18 at 145mm and 0.17 at 160mm, about 0.035 above U-Monkey’s U0; and Hilti’s rainscreen brochure takes one wall from 0.161 without brackets to anywhere between 0.165 and 0.357 with them, depending on the bracket system. The steel studs are BRE Digest 465’s hybrid frame — insulation between and outside the studs — which U-Monkey’s light steel frame panel applies, weighting the upper and lower limits by p; keeping most of the insulation outside the steel keeps the studs warm (“the majority of the insulation should be external”, SCI P283), and the warm frame row, with nothing between the studs, shows the other extreme. Results below about 0.20 sit outside the range of walls Digest 465 was derived from, and the calculator says so. Slab edges, head tracks and window reveals are linear thermal bridges, not part of the U-value — model them in ΨMonkey. And in England, on a relevant building — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes — regulation 7(2) of the Building Regulations limits the materials in an external wall to A2-s1,d0 or A1: the mineral wool and the Weather Defence board here qualify; PIR and phenolic rainscreen boards (Kooltherm K15 is C-s2,d0) do not.

Condensation risk in an SFS rainscreen wall

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 every assumed vapour value tested across its plausible range and the vapour control layer at a third of its rated resistance, one at a time and all together: the polythene layer inside, the vapour-open sheathing (μ 10) and membrane outside, and two-thirds of the insulation outside the steel.

The check is one-dimensional, through the insulation between the studs; the studs themselves and the brackets are two- and three-dimensional effects it cannot see. Keep the vapour control layer continuous and sealed at the head and base tracks and around openings, and keep the bulk of the insulation outside the frame — a cold frame, with wool only between the studs, is where condensation on the steel becomes a risk.

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 an SFS infill wall with rainscreen cladding?

A steel frame system (SFS) infill wall — 100mm steel studs at 600mm centres with 100mm of ROCKWOOL NyRock Frame Slab 032 between them, 12.5mm Siniat Weather Defence sheathing, and 150mm of NyRock Rainscreen 032 outside behind ventilated cladding — has a U-value of about 0.14 W/m²K before the cladding brackets are counted, and about 0.16 with the χ values certified for one low-conductivity bracket system. BR 443 says brackets that pass through the insulation must be counted: from the bracket system’s own point thermal transmittances (χ) where they exist, or by adding a conservative 0.30 W/m²K where they don’t — which takes the same wall to 0.44 and fails the 0.26 limiting value at any thickness. So the U-value of a rainscreen wall is as much a question of which brackets, and whose numbers, as of insulation.

What χ value should I use for rainscreen brackets?

The one your bracket or subframe supplier calculates for your brackets, cladding weight and spacing, to BS EN ISO 10211 — normally one χ per bracket type (fixed and sliding points, or supporting and restraining brackets) with the number of each per m², summed as Σχ·n and added to the wall’s U-value (BR 443 §4.8.5(c)). As a sense of scale, the Passive House Institute–certified BILDA system lists χ 0.0353 W/K for its supporting brackets and 0.0053 for its restraining brackets — at 0.32 and 0.49 per m² for stone cladding, 0.0139 W/m²K in all. With no calculated χ, BR 443 §4.8.5(d) adds 0.30 W/m²K, enough on its own to fail Approved Document L’s 0.26 limit. In U-Monkey, enter brackets in the rainscreen layer’s fixings sheet: “Certified χ per fixing or bracket”, or “Rainscreen brackets, no χ” for the default.

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

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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.

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