U-value of a light steel frame wall (SFS)

A brick-clad light steel frame wall — 100mm steel C-studs at 600mm centres with mineral wool between them, 80mm of PIR outside the sheathing board and plasterboard inside — has a U-value of about 0.17 W/m²K by the method BR 443 sets for steel, BRE Digest 465. 60mm of PIR gives about 0.20 and 100mm about 0.15, as SCI’s design guide P402 tabulates them; P402 gives 0.18 for the 80mm wall. The steel is thin, but it conducts nearly 400 times better than timber, so BS EN ISO 6946’s plain average is replaced by a weighting, p, that Digest 465 works out from the frame: 0.65 for this wall.

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The build-up (outside → inside)

102.5mm brick outer leaf
50mm cavity
Breather membrane
80mm PIR board, continuous outside the steel
12mm cement particle sheathing board
100mm steel C-studs at 600mm centres (1.2mm web: 0.2% steel) with 100mm mineral wool slab between
Polythene VCL
15mm plasterboard

Studs entered as BR 443 asks, as a bridged layer: a 1.2mm web at 600mm centres is 0.2% steel (λ 50). U-Monkey’s steel-frame panel supplies the flange width (50mm), the stud centres (600mm) and the stud depth (the bridged layer, 100mm) for Digest 465’s p. Digest 465 was derived from walls of about 0.20–0.45 W/m²K (Passivhaus Trust, 2022), so the app flags results below that range as less certain.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
60mm PIR outside the studs	≈ 0.20	SCI P402 Table 2.8: 0.20
80mm PIR outside the studs	≈ 0.17	As pre-filled (p 0.65); P402: 0.18
100mm PIR outside the studs	≈ 0.15	P402: 0.15
80mm PIR, studs at 400mm centres	≈ 0.18	More steel, lower p (0.53): 0.1752
80mm PIR, plain ISO 6946 average instead of Digest 465	≈ 0.17	0.1747 — not BR 443’s method for steel
80mm PIR, nothing between the studs (warm frame)	≈ 0.23	Normal ISO 6946 (p 0.5)
Mineral wool between the studs only (cold frame)	≈ 0.48	p 0.41: the steel bypasses the wool

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

Most of the work is done by the board outside the steel, which no stud crosses; the wool between the studs is heavily short-circuited, which is why a cold frame — wool between the studs and nothing outside — is so poor. BR 443 (2019) §4.10 sorts steel walls by where the insulation is: outside the studs only is a warm frame and takes the normal ISO 6946 calculation; between them, or between them and outside (a hybrid, like this one), takes BRE Digest 465. Its p depends on how far apart the two ISO 6946 limits are and on the stud depth, centres and flange width, which U-Monkey asks for in the steel-frame panel that appears when a layer carries a steel bridge (here 50mm flanges, 600mm centres, 100mm deep): closer studs mean more steel and a lower p. Note the names: SCI calls this build-up a “warm frame”, where BR 443 calls it a hybrid. Rails, noggings and the brackets that carry cladding add bridging that a single figure cannot show, which is why published figures for similar walls vary.

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 light steel frame wall (SFS)?

Why does a steel frame wall need a different calculation?

Because steel conducts so well that the two limits BS EN ISO 6946 averages end up far apart, and the plain average is then no longer reliable. BR 443 (2019) §4.10 therefore uses BRE Digest 465 for cold and hybrid frames: the same two limits, weighted by a factor p instead of half and half. Here that gives a slightly lower U-value than the plain average (0.166 against 0.175); with the wool between the studs only, Digest 465 is the harsher of the two. Only a warm frame, with all the insulation outside the steel, keeps the normal method — including a correction for the fixings that hold that insulation on.

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.