Reviewed 13 August 2026. Choosing between Celotex PIR boards and mineral wool is not simply a question of which insulation is “best”. PIR normally provides more thermal resistance for a given thickness, while mineral wool can be the stronger choice where acoustic absorption, a flexible fit or a non-combustible insulation product is the priority. The correct answer depends on the construction, available depth, fire strategy, moisture design and target U-value.
The short answer
- Choose PIR when insulation depth is tight and the design calls for high thermal resistance in a relatively slim layer.
- Choose mineral wool when the specification prioritises sound absorption, easy fitting between irregular framing or an A1/A2-rated mineral-wool product.
- Use a calculated specification rather than selecting thickness by habit. The U-value belongs to the complete roof, wall or floor build-up—not to the insulation alone.
What is Celotex PIR insulation?
Celotex is a well-known name for rigid polyisocyanurate (PIR) insulation boards. The current Celotex GA4000 technical data sheet describes a rigid PIR foam board faced on both sides with low-emissivity aluminium composite foil. It is intended mainly for roof, wall and floor insulation.
That data sheet declares a thermal conductivity of 0.022 W/mK for GA4000 in thicknesses from 50mm to 100mm. Lower conductivity means greater thermal resistance at the same thickness, all else being equal. This is why PIR is frequently specified where the available build-up is limited—for example beneath floors, within roof constructions or in internal linings where every millimetre affects room space.
PIR is rigid, so accurate cutting and continuity matter. Poorly fitted pieces, open joints and unaddressed junctions can create air-leakage paths or thermal bridges that undermine the calculated result. Foil facings and taped joints may form part of an airtightness or vapour-control strategy only where the chosen system and manufacturer’s detailing permit it.
What is mineral wool insulation?
Mineral wool is a family of fibrous insulation products manufactured from glass or stone. It is supplied in rolls, slabs and application-specific products for roofs, walls, floors, partitions and building services. Its open fibre structure allows it to fit between studs and joists and makes many products useful in acoustic constructions.
Performance varies by product. For example, Isover Acoustic Partition Roll is designed for internal wall and floor systems and is classified Euroclass A1 for reaction to fire. ROCKWOOL Sound Insulation Slab is a stone-wool product with declared thermal conductivity of 0.038 W/mK and Euroclass A1 reaction-to-fire classification. These examples demonstrate why “mineral wool” should never be treated as a single universal specification: always use the exact product data sheet and Declaration of Performance.
Celotex PIR vs mineral wool: performance comparison
| Decision factor | Celotex PIR board | Mineral wool |
|---|---|---|
| Thermal performance per millimetre | Typically the stronger option. GA4000 declares 0.022 W/mK. | Product-dependent and generally requires more thickness to provide the same thermal resistance as a low-lambda PIR board. |
| Space | Useful where the build-up depth is restricted. | More depth may be needed, but rolls and slabs can fill framing cavities efficiently. |
| Acoustics | Not normally the first choice where sound absorption is the main requirement. | Many application-specific products are designed to contribute to acoustic wall and floor systems. |
| Reaction to fire | Check the exact board and application. The current GA4000 data sheet declares Euroclass F for the product. | Many glass- and stone-wool products achieve Euroclass A1 or A2, but verify the exact product. |
| Cutting and fitting | Needs accurate measurements, close joints and correct treatment of junctions. | Flexible or friction-fitting products can be easier around studs, joists, cables and pipes. |
| Moisture strategy | Foil facings may assist a designed vapour-control or airtightness layer, but the whole construction must be assessed. | Must be specified within a build-up that controls rain, water vapour and condensation risk. |
| Cost | Often costs more per square metre, but can reduce the depth needed. | Can be economical over large areas, though the required thickness and complete system cost should be compared. |
Which gives the better U-value?
If two insulation layers have the same thickness, a product with lower declared thermal conductivity normally provides greater thermal resistance. On that narrow comparison, GA4000 PIR at 0.022 W/mK will outperform mineral-wool products with a higher declared lambda.
However, a U-value is calculated for the complete element. Board joints, timber or metal framing, fixings, air layers, plasterboard, masonry and repeating thermal bridges all influence the result. The required target also depends on whether the project is a new build, an extension, a renovation or a change to an existing thermal element.
For work in England, consult the version of Approved Document L that applies to the project and its transitional arrangements. A designer, architect, energy assessor or building-control professional should confirm the calculated build-up rather than relying on a generic thickness chart.
Which is better for sound insulation?
Mineral wool is generally the more useful material when airborne-sound absorption within a partition, separating floor or ceiling void is the main objective. Its fibres can absorb sound energy within the cavity, and manufacturers publish complete tested or assessed systems using specific wool products, framing and board layers.
Insulation alone does not determine acoustic performance. Mass, cavity depth, resilient separation, flanking paths, sockets, service penetrations and workmanship can all change the result. Where Building Regulations apply, use the current Approved Document E guidance and a tested wall or floor system rather than assuming any roll of wool will achieve the desired sound reduction.
Which is better for fire performance?
Do not compare insulation materials using broad labels alone. Reaction-to-fire classification belongs to the exact product, while the fire resistance of a wall, roof or floor belongs to the complete tested construction.
The current GA4000 technical data sheet declares reaction to fire as Euroclass F. By contrast, the Isover APR and ROCKWOOL Sound Insulation Slab examples linked above are Euroclass A1. That does not automatically make one complete construction compliant and another non-compliant: lining boards, cavities, fixings, barriers, substrates, height and building use can all affect what is permitted.
Check the applicable edition and amendments of Approved Document B, the manufacturer’s Declaration of Performance and the project fire strategy. Higher-risk, external-wall and fire-separating applications require particular care and professional specification.
Where does each insulation work best?
Situations that often favour PIR boards
- Roofs, walls and floors where thermal performance must be achieved in a restricted depth.
- Continuous insulation layers designed to reduce thermal bridging through framing.
- Projects needing a rigid board that can be cut to repeatable dimensions.
- Build-ups designed around a foil-faced board and properly detailed joints.
Situations that often favour mineral wool
- Internal partitions and intermediate floors where acoustic absorption is important.
- Stud and joist cavities containing services or small variations in spacing.
- Applications that specifically require an A1 or A2 reaction-to-fire insulation product.
- Large loft or framing areas where available depth is less restrictive.
When a hybrid build-up makes sense
Many projects combine insulation types. A roof might use mineral wool between rafters and a continuous PIR layer beneath them; a house might use PIR in an external thermal element and acoustic mineral wool in internal partitions. A hybrid design is sensible only when the layers, ventilation, vapour control, interfaces and fire performance have been assessed as one system.
Installation mistakes to avoid
- Buying from thickness alone. Compare declared lambda, thermal resistance and the calculated U-value of the complete element.
- Leaving gaps around PIR. Measure carefully, cut cleanly and follow the specified joint and perimeter treatment.
- Compressing mineral wool without design evidence. Use the product at its intended thickness and fit it without avoidable voids.
- Ignoring thermal bridges. Joists, studs, rafters, lintels and junctions can bypass the insulation layer.
- Assuming foil equals a complete vapour-control solution. Moisture safety depends on the entire construction and climate conditions.
- Substituting an unverified product. Similar-looking boards or rolls can have different thermal, fire and application declarations.
How to choose in five steps
- Identify the element: roof, external wall, partition, floor, loft or service application.
- Confirm the available insulation depth and required regulatory or project performance.
- Prioritise thermal efficiency, acoustics, reaction to fire, handling and cost for that element.
- Select an exact product and obtain its current data sheet and Declaration of Performance.
- Have the full build-up checked where U-values, condensation risk, fire resistance or acoustic compliance matter.
Frequently asked questions
Is 100mm PIR equivalent to 100mm mineral wool?
No. Equal thickness does not mean equal thermal resistance. Use each product’s declared thermal conductivity and the complete element calculation. GA4000, for example, declares 0.022 W/mK, while the ROCKWOOL sound slab example declares 0.038 W/mK.
Can mineral wool replace Celotex between rafters?
Sometimes, but it may require a different thickness and construction. Roof ventilation, condensation risk, thermal bridging and the position of control layers must be recalculated; it is not a like-for-like substitution.
Is Celotex good for soundproofing?
PIR is principally selected for thermal performance. For partitions and intermediate floors where sound absorption is the priority, an application-specific acoustic mineral-wool product is normally the more appropriate starting point.
Can PIR and mineral wool be used together?
Yes. Hybrid systems are common, but the full build-up must be designed so that heat, moisture, fire and acoustic requirements work together.
Shop the right insulation for your project
Browse our PIR insulation range or compare products in our mineral wool collection. For a board-and-cutting route, see Celotex PIR insulation boards, our bespoke PIR cutting service and the insulation knife bundle.
If you are still unsure, contact the BuyInsulation team with the application, dimensions, available depth and required performance. We can help you narrow the product choice, but final regulatory and construction design should be confirmed by the appropriate project professional.
