Mineral Wool vs Foam Insulation — UK Guide
Comparing mineral wool (glass/rock wool) and rigid foam (PIR/PUR/EPS) insulation — lambda values, cost, fire rating, breathability and Part L compliance.
Side-by-Side Comparison
| Property | Mineral Wool | PIR Foam | EPS (Expanded Polystyrene) |
|---|---|---|---|
| Lambda value (W/mK) | 0.032-0.044 | 0.022-0.023 | 0.030-0.038 |
| Fire rating | Euroclass A1 (non-combustible) | Euroclass B-C (combustible) | Euroclass E (combustible) |
| Cost per m² (100mm) | £5-10 | £12-20 | £4-8 |
| Breathable | Yes — allows moisture transfer | No — vapour barrier | Slightly |
| Acoustic performance | Excellent | Poor | Poor |
| Compressive strength | Low (not for floors) | High (120-150 kPa) | Moderate (70-100 kPa) |
| Water resistance | Poor if saturated | Good — closed cell | Good |
| Thickness for 0.26 U-value | 90-110mm | 70-80mm | 85-100mm |
| Common brands (UK) | Knauf, Rockwool, Isover | Celotex, Kingspan, Recticel | Jablite, Kay-Metzeler |
| Typical applications | Walls, lofts, partitions | Walls, roofs, floors | Floors, cavity walls, EWI |
Pros and Cons
Mineral Wool (Glass/Rock Wool)
Pros
- + Non-combustible (Euroclass A1)
- + Excellent acoustic performance
- + Breathable — reduces condensation
- + Cheaper per m²
- + Easy to install around pipes/cables
Cons
- - Thicker for same thermal performance
- - Poor compressive strength (not for floors)
- - Loses performance when wet
- - Irritating fibres during installation
- - Can sag in vertical applications
PIR Foam Board
Pros
- + Best lambda value — thinnest option
- + High compressive strength (for floors)
- + Closed-cell — resists moisture
- + Clean to handle, no fibres
- + Foil facing acts as vapour barrier
Cons
- - Combustible — needs fire protection
- - More expensive per m²
- - Poor acoustic performance
- - Not breathable — condensation risk
- - Difficult to fit around services
UK Cost Comparison (2026 Prices)
For insulating a 50 m² loft floor to Part L standard (0.16 W/m²K):
| Cost Element | Mineral Wool (270mm) | PIR Board (150mm) |
|---|---|---|
| Insulation material | £350-550 | £750-1,200 |
| Tape, sealant, fixings | £20-40 | £40-60 |
| Labour (if professional) | £200-350 | £300-500 |
| Total | £570-940 | £1,090-1,760 |
When to Choose Each
Choose Mineral Wool When
- Loft floor insulation (between joists)
- Timber frame walls (breathability)
- Acoustic partitions (Part E)
- Budget is the priority
- Fire safety is critical
Choose PIR Foam When
- Floor insulation (needs compressive strength)
- Limited cavity/depth (thinnest option)
- Flat roofs and warm roofs
- Masonry cavity wall insulation boards
- Below DPC/ground level (moisture resistant)
Environmental Considerations
Mineral wool is made from recycite glass or volcanic rock with high recycled content (up to 80% for glass wool). It has moderate embodied energy. PIR/PUR foam uses blowing agents and petrochemical raw materials with higher embodied carbon, though this is offset by its thinner profile (less material needed). EPS can be recycled but rarely is in practice. From a whole-life carbon perspective, any insulation saves far more carbon in heating reduction than it costs to manufacture.
Related Calculators
- Insulation Thickness Calculator — calculate required thickness for target U-value
- U-Value Calculator — calculate U-values for wall, floor and roof build-ups
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.