Guide
PIR boards vs mineral wool insulation
PIR boards insulate more per millimetre, so they suit tight spaces such as rafters, floors and walls with limited depth; mineral wool is non-combustible, better for sound and usually cheaper per square metre, so it suits lofts, partitions and wherever fire performance matters.
Written by the SK Trade team. Last reviewed 1 November 2026.
Side by side
| PIR boards | Mineral wool | |
|---|---|---|
| Insulation per mm | Higher (around 0.022 W/mK) | Lower (around 0.032 to 0.044 W/mK) |
| Fire | Combustible | Mostly non-combustible (A1 or A2) |
| Sound | Limited | Good |
| Best for | Rafters, floors, flat roofs, walls with limited depth | Lofts, partitions, cavities, fire stopping |
Figures are typical; always use the manufacturer's declared values.
Building regulations
The thickness you need depends on the U-value required for the element and the rest of the build-up. In England that is set in Approved Document L; Wales, Scotland and Northern Ireland have their own standards. Combustible insulation is restricted in the external walls of some buildings.
Lofts
For a loft insulated at joist level, the common recommendation is 270 mm of mineral wool in total, usually as 100 mm between joists and 170 mm across them.
Related products
Related questions
Is PIR better than mineral wool?
PIR insulates more per millimetre, so it suits tight spaces. Mineral wool is non-combustible, cheaper per square metre and better for sound.
How thick should loft insulation be?
270 mm in total is the common recommendation, usually as 100 mm between the joists and 170 mm across them.
Sources
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