U-value of a shipping container wall

Spray foam inside a corrugated steel container: the U-value is easy, the condensation is not.

A shipping container’s side wall lined with 100mm of closed-cell spray foam (BASF Walltite, λ 0.025), taken at its thinnest point, then a battened service void and plasterboard, has a U-value of about 0.23 W/m²K — inside the 0.26 limiting value for a new dwelling; 125mm gives 0.19 and 50mm only 0.42. The steel adds almost nothing to the U-value: its part in the calculation is to be vapour-tight on the cold side, and that is why U-Monkey’s condensation check fails every version on this page insulated on the inside — 271 g/m² of condensate is still in the wall at the end of the year with the foam alone, about 30 g/m² with a polythene vapour control layer added — and passes the same container insulated on the outside, or inside behind a sealed foil vapour control layer about as tight as the steel.

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The build-up (outside → inside)

Closed-cell foam at BASF Walltite’s declared λ 0.025, measured at its thinnest point over the corrugations. The side panel is Corten (weathering) steel, taken at λ 50 like any mild steel (BR 443); its thickness makes no difference to the U-value. The battened service void gets BR 443’s batten bridge (11.8% timber) automatically.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
50mm closed-cell foam≈ 0.42
75mm closed-cell foam≈ 0.30
100mm closed-cell foam≈ 0.23As pre-filled: meets the 0.26 limit
125mm closed-cell foam≈ 0.19A self-build reported 0.2 for 125mm of the same foam
100mm foam + a polythene VCL behind the service void≈ 0.23Same U-value; far less condensate (below)
75mm PIR between 75mm timber studs on a 25mm gap, VCL≈ 0.36The studs bridge the boards through their full depth
50mm foam + 50mm PIR between studs, VCL≈ 0.26Just misses 0.26 (0.263)
Insulated outside: 100mm PIR under ventilated timber cladding≈ 0.22The steel stays warm: the condensation check passes it

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 corrugations and the frame are the conventions to settle. Foam sprayed onto corrugated steel is thickest in the troughs; U-Monkey takes it at its thinnest point, as BR 443 does where profile ribs compress insulation (§4.9.2, “at its thinnest point”, ignoring what spills into the ribs), and counts nothing for the air in the profile (as BR 443 §4.7.7 does for the troughs of a metal deck). The corner posts and the top and bottom rails are heavy steel sections that bridge the insulation in lines: they are linear thermal bridges, outside the U-value of the panel, and BS EN ISO 6946 leaves insulation penetrated by metal outside its method altogether (BR 443 §2) — so line a container with timber studs or battens, not steel ones. Thickness then does the work: 50mm of foam gives 0.42, 100mm 0.23, 125mm 0.19. Claimed figures are worth checking against that: a self-build container house sprayed with 125mm of the same foam reported about 0.2, which U-Monkey reproduces, while a container hotel was reported at 0.2 with 50mm, which it would take far more foam to achieve. To be lived in, a container has the figures of a new dwelling to meet — Approved Document L sets 0.26 for walls (Table 4.1), and the notional dwelling 0.18; a small detached garden office of up to 30m² with no sleeping accommodation can be exempt from most of the Building Regulations (Schedule 2, Class 6), though Part P still applies to electrics fed from the house (regulation 9(3)) — see the garden room pages.

Condensation in an insulated shipping container

U-Monkey’s free BS EN ISO 13788 check fails every row on this page that is insulated on the inside, because the steel skin is a vapour barrier on the cold side. With 100mm of closed-cell foam and nothing else, 271 g/m² of condensate is still in the wall at the end of the year (peak 307 g/m²); 125mm of foam leaves 219 g/m²; a sealed polythene vapour control layer behind the service void brings it down to about 30 g/m² a year, and PIR boards between timber studs behind a vapour control layer to about 6.5 g/m² — smaller, but still building up year on year, which the standard counts as a fail.

Two arrangements pass. Insulation outside the steel — 100mm of PIR under ventilated timber cladding passes with every assumed vapour value tested across its range, the steel taken with its laps unsealed. And inside, a foil vapour control layer about as tight as the steel: behind 100mm of foam, one rated at about sd 5,300 m or more passes even at a third of its rating, because almost no vapour then reaches the foam (U-Monkey does not count the trace the arithmetic leaves between two layers that tight). Whatever the insulation, seal the vapour control layer and every service penetration, and expect the steel frame members at the corners to stay colder than the panels: they are where surface condensation and mould show first.

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 shipping container wall?

A shipping container’s side wall lined with 100mm of closed-cell spray foam (BASF Walltite, λ 0.025), taken at its thinnest point, then a battened service void and plasterboard, has a U-value of about 0.23 W/m²K — inside the 0.26 limiting value for a new dwelling; 125mm gives 0.19 and 50mm only 0.42. The steel adds almost nothing to the U-value: its part in the calculation is to be vapour-tight on the cold side, and that is why U-Monkey’s condensation check fails every version on this page insulated on the inside — 271 g/m² of condensate is still in the wall at the end of the year with the foam alone, about 30 g/m² with a polythene vapour control layer added — and passes the same container insulated on the outside, or inside behind a sealed foil vapour control layer about as tight as the steel.

Does spray foam stop condensation in a shipping container?

It reduces it, but by the standard screening calculation it does not stop it. Closed-cell foam bonded to the steel leaves no air gap behind it and resists vapour (around μ 60), but the steel outside is completely vapour-tight, so any vapour that reaches it has nowhere to go: U-Monkey’s BS EN ISO 13788 check leaves 271 g/m² in a wall lined with 100mm of foam at the end of the year, and about 30 g/m² with a sealed polythene vapour control layer behind the service void as well — still a fail, because it builds up year on year. Two versions pass: insulation outside the container, under ventilated cladding, which keeps the steel warm; or a foil vapour control layer behind the foam rated at about sd 5,300 m or more and sealed at every lap and penetration, which seals the foam between two vapour-tight layers.

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.

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