U-value of built-up (twin-skin) metal wall cladding

Site-assembled metal cladding — liner, rails and brackets, quilt, outer sheet — for sheds, factories and warehouses, with the spacer system counted.

Built-up (twin-skin) metal wall cladding — a 0.4mm steel liner sealed at its laps, rails and brackets at SCI P312’s default spacing (rails every 1.2m, 0.8 brackets per m²), 180mm of glass mineral wool cladding roll (λ 0.040) and a 0.7mm profiled steel outer sheet — has a U-value of about 0.25 W/m²K, inside the 0.26 Approved Document L Volume 2 sets for a wall. 160mm reads 0.28 and fails. The rails and brackets add about 0.02: a layer-by-layer sum gives 0.22 at 180mm, but BR 443 says metal cladding “needs to take account of … any metallic components within the insulation”, and for rails and brackets it gives SCI P312 as the alternative to a numerical calculation.

Open this build-up in U-Monkey — pre-filled, free

Every layer editable · live U-value · Part L Volume 2 targets (buildings other than dwellings) · free PDF report

The build-up (outside → inside)

The pre-filled link opens in the England · non-domestic regime with Metal cladding set to Built-up (SCI P312) at the method’s Table 2 defaults — rails 40 × 40mm at 1.2m centres, 0.8 brackets per m² — with its worked example’s 1.2mm rail steel (one thickness crossing the quilt) and 60mm² bracket (50 × 1.2mm), steel at λ 60 as P312 takes it; no liner ribs pressing the quilt. The air round the rails takes BS EN ISO 6946’s 0.18 for horizontal heat flow; the quilt BR 443’s air-gap level 1.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
140mm, SCI P312 defaults≈ 0.31Fails 0.26
160mm≈ 0.28Fails 0.26 (0.276)
180mm≈ 0.25As pre-filled: meets 0.26 (0.247)
200mm≈ 0.22
220mm≈ 0.21Still short of the 0.18 the notional building takes for a side-lit building (0.205)
160mm, rails every 1.8m and 0.46 brackets per m²≈ 0.26Still fails (0.264); Tata Steel’s Trisobuild wall table gives 0.26 at 160mm at this spacing
180mm, rails every 1.8m and 0.46 brackets per m²≈ 0.240.237; Tata Steel gives 0.23
180mm FactoryClad Roll 35 (λ 0.035)≈ 0.220.221
180mm, rails and brackets not counted≈ 0.22Not allowed: BR 443 §4.9 (0.223)

Figures computed by SCI P312 for the rails and brackets (BR 443 §4.9.1) — the quilt’s rail zone bridged by the rail steel and the air round the rails, the limits weighted by P312’s p, the bracket, air-gap and any liner-rib corrections added — exactly as the pre-filled calculator works them out with Metal cladding set to Built-up, with any pass or fail judged on the unrounded U-value. Indicative for P312’s default rails and brackets: your system’s spacing, bracket size and quilt will move the number, and the maker’s own 3-D figure is the one to use where it has one.

What changes the number

Steel that crosses the insulation is what a plain BS EN ISO 6946 sum cannot handle, and built-up cladding has it at every rail and bracket. BR 443 (2019) §4.9: “The U-value of metal-clad walls and roofs needs to take account of joints between panels and any metallic components within the insulation, including through fixings”. It refers to MCRMA Technical Paper 18 for the calculation, its wall table asks for numerical analysis, and §4.9.1 adds that “the method given in Steel Construction Institute (SCI) Technical Note P312 … can be used as an alternative for rail-and-bracket systems”. SCI P312 (2002) is what U-Monkey uses when Metal cladding is set to Built-up: the quilt is split into the rail zone, bridged by the rail steel and by the air the rail squeezes out of the quilt (“twice the width of the rail”), and the rest, crossed only by the brackets, each costing ΔU = 1.6·λ·A·n·(R_i/R_ub)²/d_i on its cross-section A and number per m² n; the two resistance limits are weighted by P312’s p (its equation 3, based on the one for light steel framing) rather than averaged, and the brackets must sit on thermal break pads. U-Monkey reproduces P312’s worked example (a 170mm wall, U 0.255). The method is for rail-and-bracket spacers: Z-spacers and standing-seam clips need the system maker’s figures. What moves the number: the depth and λ of the quilt first — a 0.035 roll saves about 0.03 at 180mm — then the brackets and rails, which the steelwork spacing and the wind load set (P312’s method covers rails at least 1m apart and up to 1.3 brackets per m²). Makers’ own 3-D figures sit close: Tata Steel’s Trisobuild wall, at 1.8m steelwork and 1.2m bracket spacing, gives 0.26 at 160mm and 0.23 at 180mm, against U-Monkey’s 0.26 and 0.24 at the same spacing. Fill the void without over-compressing the quilt; where shallow liner ribs press it (no deeper than 25mm, at least 250mm apart) BR 443 §4.9.2 adds a correction, and otherwise says to “measure the insulation thickness at its thinnest point”. Approved Document L Volume 2 sets 0.26 for a wall in both its 2021 and 2026 editions; for a new building the real test is the whole-building calculation — the BER and BPER against the TER and TPER — where the NCM notional building has 0.18 walls if it is side-lit and 0.26 if top-lit, the rooflit shed.

Condensation risk in built-up wall cladding

U-Monkey’s free BS EN ISO 13788 check gives the pre-filled wall a conditional pass: no interstitial condensation in any month while the steel liner works as a sealed air and vapour control layer, but condensate builds up year on year if its laps, fixings and penetrations are left unsealed — the check tests the liner unsealed as well as sealed. With steel on both faces nothing can dry out through the outer sheet, so the liner is what keeps the quilt dry.

A perforated (acoustic) liner lets vapour through. With no vapour control layer the same wall collects kilograms of water per m² over a winter in the check, and Knauf’s guidance is plain: “Where a perforated liner sheet is used then a separate vapour control layer should always be installed.” Which one depends on the building’s humidity class and the liner, so take the system maker’s advice. U-Monkey’s check takes the outer sheet as vapour-tight, so it asks a lot of that layer: a polythene VCL still leaves condensate building up year on year, a foil VCL (sd in the thousands of metres) does not.

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 built-up (twin-skin) metal wall cladding?

Built-up (twin-skin) metal wall cladding — a 0.4mm steel liner sealed at its laps, rails and brackets at SCI P312’s default spacing (rails every 1.2m, 0.8 brackets per m²), 180mm of glass mineral wool cladding roll (λ 0.040) and a 0.7mm profiled steel outer sheet — has a U-value of about 0.25 W/m²K, inside the 0.26 Approved Document L Volume 2 sets for a wall. 160mm reads 0.28 and fails. The rails and brackets add about 0.02: a layer-by-layer sum gives 0.22 at 180mm, but BR 443 says metal cladding “needs to take account of … any metallic components within the insulation”, and for rails and brackets it gives SCI P312 as the alternative to a numerical calculation.

Can I work out a twin-skin wall’s U-value layer by layer?

Not for compliance. BR 443 (2019) §4.9 says metal-clad walls need the metal within the insulation taking into account, and its wall table asks for numerical analysis, with SCI P312 as the alternative for rails and brackets. Layer by layer, this wall reads 0.22 at 180mm; by P312, 0.25. In U-Monkey, enter the outer sheet, the quilt at its full depth (as one layer or as the rolls that make it up) and the liner, set Metal cladding to Built-up, and type in your system’s rail spacing and brackets per m² — or use the system maker’s own calculated value.

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, and for built-up metal cladding SCI P312, which BR 443 §4.9.1 gives as an alternative for rail-and-bracket systems; BS EN ISO 13788 for the condensation 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.

Open this build-up pre-filled or call 01202 623236