The quick idea
Both PIR (polyisocyanurate) rigid boards and mineral wool are proven choices — they just solve different problems. If you need high thermal performance in a tight space, PIR shines. If you want cost-effective coverage, acoustic control, and non-combustible materials, mineral wool is hard to beat.
What are PIR boards?
Rigid foam panels with a high thermal resistance per millimetre. Commonly used in roofs, linings, and floors where depth is limited.
Advantages
- Excellent thermal performance in slim build-ups
- Space-saving; helps achieve target U-values with less thickness
- Lightweight and straightforward to handle
- Good moisture resistance when detailed correctly
Considerations
- Usually higher upfront cost than wool products
- Needs accurate cutting and tight joints to avoid thermal bridges
- Combustible; follow fire-safety detailing and manufacturer guidance
What is mineral wool?
Insulation made from rock or glass fibres supplied as rolls or slabs. Versatile and easy to fit between joists and studs.
Advantages
- Cost-effective and widely available
- Strong acoustic absorption for quieter rooms
- Non-combustible (typically Euroclass A1/A2)
- Forgiving to install; easy to shape around services
Considerations
- Lower thermal resistance per mm than PIR (requires greater thickness)
- Must be kept dry in use; add vapour control where specified
- Can feel bulkier where space is limited
Performance at a glance
| Aspect | PIR Boards | Mineral Wool |
|---|---|---|
| Thermal efficiency (per mm) | Very high; slim build-ups achieve low U-values | Moderate; needs more depth to match PIR |
| Acoustic control | Moderate | Strong sound absorption |
| Fire behaviour | Combustible; detail to regs | Non-combustible (A1/A2) |
| Moisture handling | Good when joints are sealed | Keep dry; use VCL where specified |
| Installation & detailing | Precise cuts; tight, taped/foamed joints | Flexible fit; easy around services |
| Typical cost | Higher upfront | Budget-friendly |
Best use cases
Choose PIR boards if you need:
- High thermal performance where depth is tight (rafters, dormers, floors)
- Clean, flat linings ready for plasterboard
- Build-ups that benefit from integrated foil facings
Choose mineral wool if you need:
- Cost-effective upgrades across large areas (lofts, stud walls, intermediate floors)
- Better sound reduction between rooms/floors
- Non-combustible materials for fire-safety design
Hybrid tip: Many projects combine them — e.g., PIR at rafters for a slim roof build-up plus mineral wool in partitions for acoustic control.
Making the call
- Space constrained & chasing U-values? Go PIR.
- Big areas on a budget & want quieter rooms? Go mineral wool.
- Mixed priorities? Use a hybrid and detail vapour control/ventilation correctly.
