Spray Foam vs Mineral Wool vs Rigid Board — A UK Specifier’s 3-Way Comparison (2026)

Specifiers and contractors picking insulation for a UK commercial roof, wall or soffit usually shortlist the same three options:

1. Closed-cell spray foam — applied wet, expands, bonds to the substrate
2. Mineral wool (rock or glass) — fibrous batts or rolls, friction-fit or mechanically fixed
3. Rigid PIR board — pre-formed, tongue-and-grooved, mechanically fixed

All three pass UK Building Regulations when correctly specified. None is universally “best” — they win on different jobs. This article is a side-by-side comparison aimed at a specifier or builder who needs to pick one, fast, and defend the choice at Building Control.

For the regulatory context — what U-values you have to hit and why — see our Approved Document L for non-domestic insulation guide. For the exact thicknesses each material needs to hit common U-value targets, see our spray foam thickness reference table.

Side-by-side specifier table

The thermal figures are typical declared values to BS EN 12667 — your project’s specific datasheet figure should always be used in the actual U-value calculation.

How to read each row

Thermal conductivity (λ) and required thickness

Closed-cell spray foam and PIR rigid board are essentially thermally identical at λ ≈ 0.022 — the same thickness gives the same U-value. Mineral wool runs ~60 % higher conductivity, so it needs ~60 % more thickness for the same target.

On a UK commercial roof with limited soffit clearance, that thickness difference often decides the spec. A warehouse needing U = 0.15 in a 145 mm void can take 140 mm of closed-cell foam — but only ~80 mm of mineral wool, which gets nowhere near the target.

Fire rating (BS EN 13501-1)

This is where mineral wool is unambiguously the strongest of the three:

• Mineral wool is naturally non-combustible — A1 (rock) or A2 (glass) under BS EN 13501-1. No fire upgrade needed; it can be left exposed in any non-domestic building.
• Closed-cell spray foam classifies to Euroclass E as bare polyurethane. With a IFTI DC315 intumescent topcoat at the manufacturer’s published wet film thickness, the system reaches Euroclass B (BS EN 13501-1) and Class 1 / 0 (BS 476 Part 7 / Part 6) — sufficient for warehouse, hangar, industrial, retail, education and most other non-domestic spaces.
• PIR rigid board also Euroclass E. Most commercial PIR roof specs use a non-combustible facer plus a Class 0 plasterboard ceiling to manage fire performance, rather than upgrading the foam itself.

If the spec calls for non-combustible insulation throughout (e.g. high-rise, certain healthcare, certain specialist plant rooms), mineral wool is often the only option that meets the brief without auxiliary protection.

For the full fire-compliance route on spray foam systems, see our Spray Foam Fire Protection: UK Guide.

Airtightness

Approved Document L Volume 2 limiting air permeability for new non-domestic is 8 m³/(h·m²) at 50 Pa. To pass the Building Emission Rate (BER) calculation, design figures often need to be 3–5 m³/(h·m²).

This is the row where closed-cell spray foam pulls ahead of the other two. Because it’s applied as a liquid that expands and bonds, it forms a continuous monolithic seal — every junction, penetration and overlap is sealed in the same operation. Mineral wool and rigid board both rely on a separate vapour control layer (VCL) and air barrier membrane to hit the same airtightness — which is fine on a clean newbuild, but very hard to retrofit on a complex existing roof with multiple penetrations.

On a refurbishment of an existing warehouse roof with multiple roof lights, smoke vents, plant penetrations and gable details, closed-cell spray foam is usually the only material that hits a tested 3 m³/(h·m²) without an exterior membrane reinstall.

Substrate adhesion and cold bridging

Mineral wool and rigid board both fix mechanically — typically helical fasteners through to a structural deck. Every fixing is a thermal bridge through the insulation, and every fixing penetration is a potential air leakage path that has to be sealed separately.

Closed-cell spray foam bonds chemically to the substrate. No fixings, no bridges, no penetrations to seal. On a profiled steel commercial roof, that eliminates the linear thermal bridge that ACDs (Approved Construction Details) usually penalise at junctions.

Conformability — curved, irregular and complex substrates

Spray foam wins decisively on:

• Curved roofs (Quonset huts, Nissen huts, hangars)
• Profiled metal roofs with deep ribs
• Concrete soffits with as-cast irregularities
• Piling walls with sheet-pile interlock corrugations
• Around services, ducts, conduits and structure

Boards and rolls don’t conform — they need either bespoke cutting (slow, wasteful, leaves gaps), or a sub-frame that introduces its own thermal bridges and cost.

Installation speed and application weather window

Mineral wool and rigid board can be installed in almost any weather. Closed-cell spray foam needs:

• Substrate temperature ≥ 5 °C (typically ambient ≥ 10 °C with morning warm-up)
• Substrate dry (no surface water, no condensation)
• Wind ≤ 30 km/h for external applications (overspray control)

In practice, this means UK spray foam projects are most efficient March–November, with cold-snap windows possible if the building is heated overnight. A typical commercial warehouse roof of ~2,000 m² is sprayed in 2–3 working days by a 2-person crew — significantly faster than the equivalent in mineral wool (5–7 days with separate VCL trade) or PIR (5–8 days with cutting and taping every joint).

Embodied carbon

Mineral wool — particularly rock wool — has the lowest embodied carbon per kg of the three, by a meaningful margin. PUR/PIR-based products (whether sprayed or board) carry similar embodied figures because the chemistry is the same. For BREEAM-credited projects where Mat 01 (responsible sourcing of materials) is in scope, mineral wool often scores best on this single line — even though spray foam’s airtightness contribution typically wins more total credits across Mat 01, Ene 01 (energy efficiency) and Hea 04 (thermal comfort).

Cost band

Specific £/m² figures depend on project scale, location, substrate prep and access — quoting a single figure here would mislead. As a relative band:

• Mineral wool: lowest installed cost on simple new-build (cheap material, fast batt install). Higher than headline if a separate VCL/airtightness trade is needed.
• PIR rigid board: mid band. Material more expensive than mineral wool, install labour similar.
• Closed-cell spray foam: mid–high band as a unit cost, but often the lowest total cost on commercial refurbishments where mineral wool / PIR would need a separate VCL, additional labour for cutting around penetrations, longer programme, or extra structural reinforcement to carry weight.

For a project-specific cost comparison, request a free site survey — Duratite’s surveyors quote spray foam against the other two on like-for-like specifications.

When each one wins

Mineral wool wins when

• Non-combustible insulation is mandated (high-rise residential over 11 m, certain healthcare, certain plant rooms, certain assembly buildings)
• The spec needs sound absorption as well as thermal — internal partitions, acoustic ceilings
• Embodied carbon is the dominant LCA driver and BREEAM Mat 01 is in scope
• The substrate is flat, simple, and dry — no curves, no penetrations, no weather pressure
• Programme has plenty of time and the structure can carry the additional weight (rock wool is dense — ~30–80 kg/m³ vs ~30–35 kg/m³ for closed-cell foam)

PIR rigid board wins when

• The substrate is flat and clean (newbuild flat roof, screed deck)
• The build-up is sandwich-panel style — clean joints, no service penetrations
• The application is below-ground (waterproof PIR variants, e.g. for piling walls in non-aggressive ground)
• The job is small (single-room loft, partition wall) and a spray crew callout isn’t economic

Closed-cell spray foam wins when

• The U-value target is tight (≤ 0.18) and clearance is limited
• The substrate is complex — curved, profiled, irregular, full of penetrations
• Airtightness has to be demonstrated by ATTMA pressure test, not designed-in by membrane
• The project is a refurbishment where stripping the existing roof isn’t an option
• The building stays operational during the works (warehouse, hangar, soffit)
• The substrate is steel or concrete, where chemical bonding eliminates thermal-bridging fixings
• A 20-year manufacturer-backed product warranty is required at single-system level

Decision matrix

A simple way to pick:

If the matrix points at two answers (often spray foam vs mineral wool on commercial projects), the deciding question is usually fire performance vs airtightness: pick mineral wool when the brief mandates non-combustible; pick closed-cell spray foam when airtightness has to be tested.

What about open-cell spray foam?

Open-cell spray foam is roughly λ = 0.038 — same range as mineral wool — and runs at about half the price per litre of closed-cell. It’s a domestic acoustic / loft specialty, not a commercial product. For UK non-domestic projects, the choice is essentially closed-cell vs mineral wool vs PIR; open-cell is rarely shortlisted. See our closed-cell vs open-cell guide for the full breakdown.

Frequently asked questions

Q: Can I mix materials in the same build-up?
A: Yes — and on refurbishment projects this is common. A typical 1990s industrial roof refurbishment might keep an existing 50 mm PIR board between rafters and overspray with closed-cell foam to top up to the modern U-value. The U-value calculation simply adds R-values from each layer.

Q: Does mineral wool sag in roof voids?
A: Glass wool can settle slightly in unsupported overhead applications over 20+ years. Rock wool holds shape longer because of its higher density. Both are typically supported by mesh or netting in roof voids to prevent settlement. Closed-cell spray foam doesn’t sag at all — once cured, it’s rigid and structurally self-supporting (no fixings, no fasteners, no support mesh required).

Q: What’s the thinnest insulation that meets U = 0.18 in any material?
A: Vacuum insulated panels (VIP, λ ≈ 0.005 W/m·K) reach U = 0.18 in ~25 mm. They’re fragile, expensive, and used in specialist applications — not generally cost-effective for warehouses or industrial buildings. Among mainstream commercial insulants, closed-cell spray foam and PIR are tied at ~115 mm.

Q: Which has the longest service life?
A: All three, correctly installed and protected, run 30+ years easily. Closed-cell spray foam has been deployed in commercial roofs since the 1970s with documented retention of thermal performance. Mineral wool that hasn’t been wetted holds performance indefinitely. PIR can show some thermal drift if facers are damaged. Service life on any UK commercial project is typically driven by the outer layer (membrane / coating / cladding) rather than the insulation itself.

Q: How does each material handle water ingress?
A: Closed-cell spray foam is hydrophobic and unaffected by short-term water exposure. Mineral wool absorbs water and loses thermal performance until it dries — a wet quilt is essentially useless until it dries out, which can take weeks in a closed roof void. PIR’s foil facers protect it from water if intact, but a damaged facer lets water into the foam where it stays. On any commercial roof with potential for leaks during the works, closed-cell spray foam tolerates the conditions best.