A 100mm void is not automatically a 100mm insulation specification. Whether the job is a warm flat roof, timber-frame extension, masonry cavity wall or suspended timber floor, the board or quilt you choose has to meet the target U-value and suit the build-up around it. That is where PIR vs mineral wool insulation becomes a practical site decision, not just a price comparison.
PIR generally delivers more thermal resistance in less thickness. Mineral wool is often easier to fit around irregular details, performs well acoustically and is commonly selected where non-combustibility is a key requirement. Neither is the right answer for every location. The best choice depends on the available depth, required thermal performance, fire strategy, moisture control and how reliably the material can be installed.
PIR vs mineral wool insulation in practice
PIR insulation is a rigid polyisocyanurate foam board, normally faced with foil. It is used in floors, roofs, internal wall linings and other areas where space is tight. Standard PIR boards commonly achieve a declared thermal conductivity around 0.022 W/mK, although always check the exact product data.
Mineral wool is a fibrous insulation made from rock or glass fibres. It is available as slabs, rolls and specialist batts, with different densities and thermal values for different jobs. Typical thermal conductivity figures are often around 0.032 to 0.040 W/mK. That means mineral wool usually needs more thickness than PIR to achieve the same thermal resistance.
The gap matters on a refurbishment. If you only have 50mm to build out an internal wall, a PIR board may help you reach a meaningful improvement without losing excessive room space. In a deeper timber stud wall or loft, mineral wool may be the more sensible, economical material, especially when sound reduction and close fitting are important.
Thermal performance and thickness
For a like-for-like thickness, PIR normally insulates better. A 100mm PIR board can offer a higher thermal resistance than 100mm of standard mineral wool. This makes PIR a regular choice for insulated plasterboard systems, warm roof constructions and concrete floors where the finished floor level cannot rise too far.
That does not mean thinner is always better. The real question is whether the complete element meets its designed U-value. Timbers, rafters, joists, cavity ties, blockwork and air gaps all affect the result. A well-detailed mineral wool build-up with enough depth can outperform a poorly fitted PIR installation with open joints and unsealed edges.
For any regulated work, work from the project specification or a proper U-value calculation. Do not select insulation solely from a thickness quoted on another job. A solid wall upgrade, for example, needs a different approach from a new-build timber frame.
Where PIR is usually the stronger option
PIR earns its place where high insulation performance must fit into a limited depth. On ground floors, rigid boards can sit over a suitable slab and below screed or chipboard decking, provided the product has the required compressive strength and the floor build-up is designed correctly. It is also commonly used between and below rafters in warm roof and loft conversions.
Foil-faced PIR can assist with vapour control when joints are tightly taped, but it is not a substitute for checking the full condensation-risk strategy. Roofs and walls need the right layers in the right position. Depending on the construction, that can mean a separate vapour control layer, a breather membrane, ventilation or all of these.
PIR boards are also useful for internal dry-lining where a customer wants to retain as much floor area as possible. Rigid boards create a flat substrate and are straightforward to cut to regular bays. However, they need accurate cutting. Gaps at board edges, around sockets, at window reveals and beside joists reduce thermal performance and can create unwanted air paths.
PIR is not naturally the best material for awkward, uneven voids. Forcing rigid offcuts around services often leaves gaps. If the installation involves plenty of pipes, cables, irregular stonework or uneven stud spacing, mineral wool can be quicker to fit well.
Where mineral wool comes into its own
Mineral wool is highly adaptable. It compresses slightly, springs into a timber stud bay and can be packed neatly around noggins, pipes and cables without relying on lots of small foam-board pieces. That makes it a practical choice for partition walls, timber-frame cavities, lofts, suspended floors and between joists.
Acoustic performance is another major advantage. The open fibre structure helps absorb airborne sound, so mineral wool is widely used in internal stud partitions and separating floors. A home office, flat conversion or refurbishment next to a busy road may need more than a good U-value. The wall or floor also needs the correct board layers, cavity depth and sealing, but mineral wool is a useful part of that system.
Mineral wool is also commonly specified for its reaction-to-fire properties. Many stone wool products are classified Euroclass A1 or A2-s1,d0, but classifications vary by product and facing. Check the declaration of performance rather than assuming every mineral wool roll or slab has the same rating.
For cavity and external wall applications, use a product designed for that location. A general loft roll is not a substitute for a water-repellent cavity bat, external wall slab or fire-rated façade insulation. Density, water resistance and fixing method all change with the application.
Fire performance is about the full build-up
Fire performance is often discussed as if one insulation material gives a complete answer. It does not. The lining, cavity barriers, membranes, fixings, cladding, roof covering and junction details all form part of the fire strategy.
Mineral wool is frequently chosen where a non-combustible insulation product is required or preferred, particularly within certain wall, partition and fire-stopping details. PIR is combustible and must be used within a tested or specified construction that provides suitable protection. In many internal applications it is covered by plasterboard or another lining, but the exact arrangement matters.
Do not swap PIR for mineral wool, or the other way around, on a fire-rated detail without checking the specification. A substitution can alter cavity dimensions, fixing lengths, acoustic behaviour and the tested fire performance of the system. On commercial, multi-occupancy or higher-risk work, that check is essential.
Moisture, condensation and site handling
Neither product should be treated as a cure for damp. If a wall has penetrating damp, defective pointing, failed render, leaking gutters or a bridged damp-proof course, fix the building defect before insulating internally. Covering a wet wall can store up a larger problem behind the finish.
PIR is relatively resistant to moisture uptake, while mineral wool products are often treated to repel water. Even so, both need protecting from prolonged wetting during storage and installation. Keep packs raised off the ground, covered from rain and brought inside when possible. Wet or damaged insulation should not simply be built in and forgotten.
With mineral wool, avoid crushing the material. Compression reduces the designed thickness and can reduce thermal performance. With PIR, keep cuts neat and support board edges properly where the floor or roof system requires it. In both cases, continuity matters more than a few pounds saved on the wrong product.
Cost: compare the finished thermal result
Mineral wool usually costs less per square metre at the same thickness. PIR often costs more upfront, but its higher thermal performance can reduce the thickness needed. That can save space, avoid deeper battens, reduce threshold alterations or make a roof conversion viable where headroom is limited.
Labour is part of the calculation as well. PIR can be fast across large, regular areas, but complicated details increase cutting and taping time. Mineral wool is quick in repetitive stud bays and joist spaces, but may need deeper framing or more material to achieve the target U-value.
When pricing the job, compare the whole build-up: insulation thickness, membranes, tapes, battens, fixings, plasterboard, floor deck, trims and labour. The cheapest pack price is rarely the cheapest installed solution.
Choosing the right insulation for common jobs
For a warm flat roof or shallow pitched roof, PIR is often the practical choice because thickness is restricted and high thermal performance is needed. For a loft floor with generous depth, mineral wool roll is usually a cost-effective option and is easy to lay in layers with joints staggered.
For timber stud partitions, mineral wool is generally the natural fit where acoustic control is wanted. For external walls lined internally, PIR can preserve room dimensions, provided the wall is dry and the condensation detail is properly designed. For suspended timber floors, mineral wool supported between joists can work well, while PIR may be better where a rigid insulated layer is being formed above a solid floor.
For new extensions, do not choose by habit. Check whether the design needs a partial-fill cavity board, full-fill cavity bat, insulated plasterboard, between-stud insulation, continuous external insulation or a combination of layers. Each approach has different detailing requirements.
A good insulation job is measured by more than the label on the pack. Start with the space available and the required U-value, then match the material to the fire, moisture and acoustic demands of the build-up. If the detail is unusual or the specification is unclear, getting the insulation schedule checked before materials arrive on site is cheaper than reopening a wall or roof later.
