The single most asked question by specifiers and contractors comparing insulation systems in the UK is some version of:
“How thick does the spray foam need to be to hit U = 0.18?”
The answer depends on three things: the type of foam (closed-cell or open-cell), the substrate beneath it, and what’s already in the build-up. This article gives the table that decides those tradeoffs in 30 seconds, then explains the assumptions behind each figure — so you can defend the calculation at Building Control.
All thicknesses are calculated to BR 443: Conventions for U-value calculations, using declared λ values to BS EN 12667. They cover the U-value targets you actually meet on UK projects in 2026:
- 0.26 W/m²K — current Approved Document L Volume 2 limiting wall U-value
- 0.18 W/m²K — current ADL2 limiting roof and floor U-value
- 0.15 W/m²K — typical “design U-value” used to pass the Building Emission Rate (BER) calculation on a non-domestic project
- 0.13 W/m²K — indicative Future Buildings Standard pitched-roof target (subject to final draft)
For the regulatory context behind these targets, see our Approved Document L for non-domestic insulation guide.
Quick reference: spray foam thickness vs U-value
The table below gives the bare minimum spray foam thickness required to hit each common U-value target, assuming the spray foam is the only thermal layer (no PIR underlay, no mineral-wool fill, no concrete contribution). Real projects almost always have additional layers, so these figures are the conservative case — your final spec will usually be the same or slightly thinner.
| Target U-value | Closed-cell spray foam (λ = 0.022 W/m·K) | Open-cell spray foam (λ = 0.038 W/m·K) | PIR rigid board (λ = 0.022 W/m·K) | Mineral wool (λ = 0.035 W/m·K) |
|---|---|---|---|---|
| 0.26 W/m²K (walls) | ~80 mm | ~140 mm | ~80 mm | ~130 mm |
| 0.18 W/m²K (roofs / floors) | ~115 mm | ~205 mm | ~115 mm | ~185 mm |
| 0.15 W/m²K (BER tightening) | ~140 mm | ~245 mm | ~140 mm | ~225 mm |
| 0.13 W/m²K (FBS pitched roof) | ~165 mm | ~285 mm | ~165 mm | ~260 mm |
The figures round to the nearest 5 mm. They include the conventional 0.13 m²K/W internal surface resistance and 0.04 external for a roof, no allowance for substrate or air layers.
Two facts the table makes obvious:
- Closed-cell spray foam and PIR rigid board are within rounding distance of each other at every target — both run at λ ≈ 0.022, so the thickness needed is essentially identical. The differences between them are practical (board needs cutting and sealing every joint; spray foam bonds and seals every junction in a single application), not thermal.
- Open-cell spray foam needs roughly 70 % more thickness to hit the same U-value as closed-cell — λ 0.038 vs 0.022 — and on a non-domestic warehouse roof that often makes open-cell impractical because of soffit clearance.
How the lambda figures are derived
Every U-value calculation in the UK starts with the declared thermal conductivity (λ — lambda) of each layer, in W/m·K. The lower the λ, the better the insulation per millimetre.
Closed-cell rigid polyurethane spray foam (the system Duratite installs for commercial UK projects, supplied as Enverge): typical declared λ at commercial application densities 0.022 W/m·K. Achieved by the closed-cell structure trapping low-conductivity gas inside each cell.
Open-cell flexible polyurethane spray foam: typical declared λ 0.038 W/m·K. Cells are open and air-filled, so the conductivity is closer to still air. Cheaper per litre but needs significantly more thickness for the same U-value.
PIR rigid board: typical declared λ 0.022 W/m·K (modern facer-bonded PIR — Kingspan TR26, Recticel Eurothane GP, Celotex GA4000 etc.). Same thermal conductivity as closed-cell foam at peak performance. Slightly worse in real-world installations because of joint thermal bridging.
Mineral wool (rock or glass): typical declared λ 0.034–0.038 W/m·K. Used as 0.035 in this table as a representative working figure. Cheap, fire-resistant, but bulky.
EPS expanded polystyrene: typical declared λ 0.034–0.038 W/m·K (Class A1 grades higher, modern grey EPS lower). Similar working thickness to mineral wool.
Vacuum insulated panels (VIP): λ 0.005–0.008 W/m·K as long as the vacuum is intact. Specialist applications only — too fragile for most warehouse / industrial roofs.
The figures above are typical declared values to BS EN 12667. The product datasheet’s specific declared λ should always be used in the actual calculation — and the commercial spray foam sector is one of the few where the declared λ has been improving rather than holding flat (the better 0.022 figure is now common where 0.024 was typical five years ago).
Worked example 1: warehouse roof, U = 0.15
A typical UK commercial warehouse re-roof, soffit-applied closed-cell spray foam between profiled steel rafters:
External surface resistance Rse 0.04 m²K/W
Profiled steel sheet (negligible R) 0.00 m²K/W
Closed-cell spray foam, 140 mm at λ = 0.022 6.36 m²K/W
Internal surface resistance Rsi 0.13 m²K/W
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Total R 6.53 m²K/W
U = 1 / R = 0.153 W/m²K
That hits the BER tightening target with a single 140 mm layer. The same target in mineral wool needs ~225 mm — meaning a deeper rafter cavity or external over-deck insulation, which on most existing warehouse roofs isn’t an option.
Worked example 2: pitched roof refurbishment, U = 0.18 (existing-building target)
A 1990s industrial pitched roof being refurbished, with the existing 50 mm PIR board still in place between rafters and adequate to keep:
External surface resistance Rse 0.04 m²K/W
Tile + batten + felt 0.06 m²K/W
Existing 50 mm PIR board, λ = 0.022 2.27 m²K/W
NEW closed-cell spray foam topping, 65 mm 2.95 m²K/W
Internal plasterboard 12.5 mm 0.06 m²K/W
Internal surface resistance Rsi 0.13 m²K/W
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Total R 5.51 m²K/W
U = 1 / R = 0.182 W/m²K
The existing PIR contributes 2.27 m²K/W “for free”, so the spray foam topping only has to deliver another ~2.95 m²K/W. 65 mm is enough. That’s typical of a refurbishment economic case — overspraying existing insulation is much cheaper than stripping and re-doing it from scratch.
Worked example 3: FBS-ready pitched roof, U = 0.13
For new commercial buildings that need to be Future Buildings Standard ready:
External surface resistance Rse 0.04 m²K/W
Profiled steel + air gap 0.21 m²K/W
Closed-cell spray foam, 165 mm at λ = 0.022 7.50 m²K/W
Internal surface resistance Rsi 0.13 m²K/W
-----------
Total R 7.88 m²K/W
U = 1 / R = 0.127 W/m²K
165 mm of closed-cell spray foam takes a 2026 commercial pitched roof from build today, FBS-ready before the regulation kicks in. The same target in 100 mm PIR + 65 mm spray foam combination saves roughly nothing at material cost, and adds two trades — most projects choose the single-layer spray application for that reason.
Why “λ = 0.022 vs 0.024” matters more than it sounds
A 9 % shift in declared lambda — say 0.022 vs 0.024 — translates directly to a 9 % shift in thickness for the same U-value. On a U = 0.15 target:
- λ 0.022 → 140 mm spray foam
- λ 0.024 → 153 mm spray foam (+13 mm, or +9 %)
13 mm doesn’t sound like much in isolation, but on a profiled roof with limited soffit clearance, a difference of 13 mm can be the difference between fitting under existing services and requiring a redesign. Always specify the declared λ value (BS EN 12667), not the manufacturer’s “best-case” or aged-90-day figure, when calculating against Building Regulations.
Substrate effects: when the build-up changes the numbers
The bare-foam table above ignores everything except the foam. Most real warehouse, hangar and industrial projects sit on one of these substrates, and the substrate does contribute:
| Substrate | Typical R contribution (m²K/W) | Effect on spray foam thickness for U = 0.18 |
|---|---|---|
| Profiled steel (plain) | 0.00 (negligible) | No reduction |
| Profiled steel + 25 mm internal air gap | 0.18 | Spray foam thickness drops by ~10 mm |
| Existing 50 mm fibreglass quilt (kept) | 1.50 | Spray foam thickness drops by ~35 mm |
| Existing 25 mm PIR board (kept) | 1.14 | Spray foam thickness drops by ~25 mm |
| 150 mm reinforced concrete soffit | 0.10 | Spray foam thickness drops by ~3 mm |
The biggest free-R-value gains come from existing fibrous quilt or PIR layers that can be retained. Any existing layer in good condition (not contaminated, not collapsed, not water-damaged) should be left in place — its R-value comes for free, and removing it costs labour without saving regulation.
When to choose closed-cell over open-cell
Open-cell spray foam at λ = 0.038 is roughly half the price per litre of closed-cell at λ = 0.022. The cost per square metre installed is closer than that, because the open-cell needs ~70 % more thickness. So when does open-cell win?
Open-cell makes sense when:
– The application is internal acoustic / domestic (lower thermal demand, sound absorption matters)
– Ceiling void depth is generous (no clearance constraint)
– The spec doesn’t include vapour control (open-cell isn’t a vapour barrier)
– Budget pressure is severe and U-value target is forgiving (e.g. an old domestic loft)
Closed-cell is the standard non-domestic spec when:
– The U-value target is tight (0.18 or below)
– Soffit clearance is limited
– Airtightness has to be demonstrated by ATTMA pressure test
– The substrate is steel, concrete, or asbestos-cement — closed-cell bonds and seals; open-cell can sag under its own weight on overhead substrates over time
– The roof is exposed to occasional water ingress before completion — closed-cell is hydrophobic; open-cell holds water
For UK commercial roofs, hangars, warehouses, industrial buildings, soffits and piling walls, closed-cell is the default. Open-cell is a domestic / acoustic specialty.
Fire performance — does thickness affect it?
Reaction-to-fire classification under BS EN 13501-1 is a property of the system, not the thickness alone. Bare polyurethane spray foam classifies to Euroclass E at any installed thickness — which means it cannot be left exposed in most non-domestic occupied spaces.
The compliance route is to apply an intumescent topcoat — IFTI DC315 (International Fireproof Technology Inc) — at the manufacturer’s published wet film thickness. The system then achieves Euroclass B (BS EN 13501-1) and Class 1 / 0 (BS 476 Part 7 / Part 6 — the older system still cited by some Building Control bodies). At that point, the system is acceptable for warehouse, hangar, industrial, retail, education and most other non-domestic occupied spaces.
The intumescent topcoat is typically 0.5–0.8 mm dry film. It does not change the U-value calculation in any practical way.
For deeper coverage of the fire compliance route, see our Spray Foam Fire Protection: UK Guide.
Frequently asked questions
Q: Can I rely on the manufacturer’s lab λ value for Building Regulations?
A: Use the declared λ value to BS EN 12667 — that’s the figure on the product datasheet that’s been independently tested. “Lab best” or “aged-90-day” figures are marketing and Building Control will reject calculations that use them.
Q: Does spray foam shrink over time and lose U-value?
A: Closed-cell spray foam in a properly-applied installation does not measurably shrink under standard UK climate conditions. The aged-180-day declared λ is the figure used in Building Regulations calculations and represents the long-term performance. Open-cell foam can compact slightly in overhead applications over many years — another reason it’s not specified for non-domestic roofs.
Q: How does spray foam thickness compare to PIR rigid board for the same U-value?
A: Within rounding distance — both run at λ ≈ 0.022, so the millimetre count is essentially identical for the same target. The choice is on practical grounds: PIR has cut joints (every joint is a thermal bridge unless taped), spray foam doesn’t.
Q: Is there a maximum thickness of spray foam in a single pass?
A: For closed-cell rigid polyurethane spray foam, typical maximum lift per pass is 50 mm. Thicker installations are built up in multiple passes, allowing the previous lift to cure. A 165 mm Future Buildings Standard pitched-roof spec installs in 4 passes over the same day in normal conditions.
Q: What about cold-bridging at fixings?
A: This is the practical reason specifiers prefer spray foam over board on profiled commercial roofs. Boards are mechanically fixed — every fixing is a thermal bridge through the insulation layer. Spray foam bonds chemically to the substrate, eliminating fixings completely. ψ-values at junctions are dramatically lower as a result, which improves the BER calculation against TER.
Q: Where can I get the U-value calculation signed off by a competent person?
A: Duratite’s surveyors complete the U-value calculation as part of every commercial spray foam survey, with a competent person’s certification suitable for Building Control submission. Request a free site survey and the calculation comes back with the quote.
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