PIR (polyisocyanurate) rigid boards and mineral wool (rock wool and glass wool) products are the two workhorses of UK new-build and retrofit insulation. They are rarely interchangeable: each suits a different set of conditions, and choosing the wrong one costs money or creates problems down the line. Here is a direct engineering comparison.
What Each Product Is
PIR (often mislabelled as PUR/polyurethane) is a closed-cell rigid foam board, typically foil-faced on both sides. The closed-cell structure gives it a very low thermal conductivity and virtually zero vapour permeability, making it an effective vapour control layer as well as an insulator. Thickness range: 12.5 mm to 200 mm in standard stock; 250 mm+ on request.
Mineral wool covers two distinct products:
- Glass wool (silica-based fibre): lambda 0.032–0.044 W/mK; lower density; suited to lofts and partition walls
- Rock wool / stone wool (basalt or slag-based): lambda 0.033–0.040 W/mK; higher density; better acoustic and fire performance; used in walls, floors and industrial applications
Both are open-cell, vapour-permeable, and non-combustible. They are sold as rolls (typically 0.6 m wide), rigid slabs, or semi-rigid batts.
Lambda Values and Thickness for Target U-values
Lambda (λ) is the thermal conductivity of the insulation itself; a lower number is better. The table below compares lambda and the insulation thickness required to achieve a 0.18 W/m²K U-value (typical Part L 2021 requirement for a new-build wall) in a simplified single-layer calculation ignoring surface resistances and structural elements.
| Product type | Typical λ (W/mK) | Thickness for 0.18 W/m²K (mm) | Thickness for 0.11 W/m²K (mm) |
|---|---|---|---|
| PIR foil-faced board (premium) | 0.022 | ~120 | ~190 |
| PIR foil-faced board (standard) | 0.025 | ~135 | ~215 |
| Rock wool rigid slab | 0.036 | ~195 | ~310 |
| Rock wool semi-rigid batt | 0.040 | ~215 | ~345 |
| Glass wool roll (standard) | 0.044 | ~240 | ~380 |
| Glass wool roll (high-performance) | 0.032 | ~175 | ~275 |
These figures are illustrative; whole-element U-value calculations must account for structural members (thermal bridging via studs, joists or blocks), airgaps, and surface resistances. Use BRE BR 443 or a DEAP/SAP calculation tool for design purposes.
The space advantage of PIR is significant in retrofit projects where wall thickness cannot increase much: a 100 mm PIR board on a solid brick wall can achieve 0.28–0.30 W/m²K; achieving the same with glass wool would require 140–160 mm.
Fire Performance
This is the most consequential difference between the two material families.
Mineral wool is classified as Euroclass A1 (non-combustible) to BS EN 13501-1. It does not contribute to fire spread, produces no significant smoke, and is often used as a fire barrier within constructions that include combustible materials. This makes it the default choice for:
- External wall insulation (EWI) on buildings over 11 m (mandated by Building Regulations Approved Document B and the Building Safety Act 2022 provisions for higher-risk buildings)
- Party wall fire-stopping
- Cavity barriers
PIR is typically classified Euroclass E or F — combustible, and will ignite and burn if exposed to flame. When installed within a building element (e.g. between studs, on a roof deck with a membrane above), it is protected and fire risk is managed through construction detailing. However, PIR is not permitted in the external wall systems of residential buildings over 11 m under current UK Building Regulations (Circular Letter 06/2023 reiteration of AD B 2019 amendment); the exception is where it is fully encapsulated within a non-combustible cladding system tested to BS 8414.
For domestic single-occupancy houses below 18 m, PIR can be used in external walls provided it is appropriately protected by masonry, plasterboard or other facing material.
Moisture and Vapour Behaviour
PIR’s closed-cell structure gives it a water vapour resistance factor (μ) of 60–200, making it an effective vapour check. When foil-faced joints are taped, it can function as a combined insulation and vapour control layer in a warm-side application.
Mineral wool has μ of 1–2 — effectively vapour-transparent. This is a benefit in applications where drying ability is needed (timber frame inner leaves, breathable construction systems) but means a separate vapour control layer (VCL) is almost always required on the warm side.
Cost Per m²
| Product | Typical supply cost (£/m²) | Performance basis |
|---|---|---|
| PIR 60 mm (λ 0.022) | £8–£12 | ~0.35 W/m²K on own |
| PIR 100 mm (λ 0.022) | £12–£18 | ~0.22 W/m²K on own |
| PIR 150 mm (λ 0.022) | £16–£24 | ~0.15 W/m²K on own |
| Rock wool 100 mm slab (λ 0.036) | £4–£7 | ~0.35 W/m²K on own |
| Rock wool 150 mm slab (λ 0.036) | £6–£10 | ~0.23 W/m²K on own |
| Glass wool 200 mm roll (λ 0.044) | £3–£6 | ~0.22 W/m²K on own |
On a like-for-like U-value basis, PIR typically costs 1.5–2.5× more per m² than mineral wool but requires roughly half the thickness. When space is constrained, PIR’s premium is often justified.
Typical Application Matching
| Application | Preferred choice | Reason |
|---|---|---|
| Loft (cold roof, between joists) | Glass wool roll | Cheap, deep, no space constraint |
| Warm roof (flat/pitched) | PIR board | Thin, closed-cell, moisture-resistant |
| Solid wall IWI — plasterboard on dabs | PIR | Space-critical; combined VCL |
| Timber frame stud infill | Rock wool batt | Breathable; friction-fit; acoustic |
| External wall EWI (>11 m) | Rock wool slab | Non-combustible; AD B compliant |
| Underfloor (between joists) | Rock wool slab or PIR | Depends on access depth |
| Cavity wall partial fill | Rock wool or EPS | PIR boards can be used but uncommon |
Standards and Approvals
All insulation used in UK construction must carry a declaration of performance (DoP) under the Construction Products Regulation. Key standards:
- BS EN 13165 — factory-made rigid PUR/PIR thermal insulation
- BS EN 13162 — factory-made mineral wool thermal insulation
- BS EN 13501-1 — fire classification
- Approved Document L (2021) — U-value targets (new dwellings: walls 0.18, roofs 0.11, floors 0.13 W/m²K)
- Approved Document B (2019 + 2023 amendments) — fire safety including EWI restrictions
- BS 9991:2021 — fire safety in residential buildings (replaces earlier guidance for high-rise)
Third-party certification (BBA, KIWA, or similar) provides additional assurance of long-term thermal performance — particularly important for PIR, where lambda can degrade slightly over 25+ years as blowing agents diffuse from the foam.
