Quick answer: PIR and mineral wool solve different insulation problems. Choose PIR when you need high thermal resistance in a relatively thin rigid layer. Choose mineral wool where loft insulation, acoustic performance or a flexible fit between framing is the main requirement.
If the project has a specified U-value, acoustic target or fire requirement, choose the insulation as part of the complete construction rather than treating the two materials as direct substitutes.
PIR is useful when insulation depth is limited
PIR is a rigid insulation board. Its main advantage is strong thermal performance for the thickness, which makes it useful where the wall, roof or floor build-up has limited depth.
That is particularly useful in designed roof, wall and floor systems where a rigid board is required. Shop our PIR insulation board in 25mm, 50mm, 75mm, 100mm and 150mm thicknesses.
Mineral wool is practical for lofts and acoustic partitions
Mineral wool is supplied as rolls or slabs rather than rigid foam board. It can fit around timber framing and irregular spaces more easily, and products designed for acoustic partitions are commonly used where sound reduction is the main goal.
For open loft-floor insulation, a purpose-made loft insulation roll is normally the more practical product. For internal partitions, see our stud wall insulation guide.
For thermal performance, compare the complete build-up
PIR generally provides more thermal resistance per millimetre than mineral wool, but thickness alone does not tell you the finished U-value.
The roof, wall or floor construction, timber or masonry, cavities, boards and other layers all affect the result. If the project has a thermal target, read our PIR U-value and thickness guide.
For fire performance, use the specified product and system
Do not choose between PIR and mineral wool from the material name alone where fire performance matters. Fire classification and system performance depend on the exact product and complete construction.
If the project has a defined fire requirement, follow the relevant tested build-up and product specification. For the current PIR-specific question, read Is PIR Insulation Banned? Fire Safety Rules Explained.
PIR needs accurate cutting; mineral wool is more flexible
Rigid PIR needs to be cut accurately so boards fit neatly without avoidable gaps. Mineral wool is more forgiving around uneven framing because it can flex to the space.
If you are using PIR, see How to Cut PIR Insulation Board.
Which should you use?
- Limited insulation depth: PIR is usually the more practical choice.
- Loft-floor insulation: use a purpose-made mineral wool loft roll.
- Acoustic stud partition: choose an insulation product designed for acoustic partition use.
- Specified roof, wall or floor system: follow the required build-up rather than swapping insulation types by thickness alone.
To compare the products we stock, browse our building insulation range or PIR insulation range.
