Mineral Wool vs PIR

Understanding the materials

What is mineral wool?

Mineral wool, also known as stone wool, is a material made by melting volcanic rock (basalt and recycled slag) and spinning it into fine, dense fibres. This creates a naturally fire-resistant material used to create thick, dense insulation, which is ideal for soundproofing, moisture resistance and thermal applications.

What is PIR?

PIR (polyisocyanurate) is made by reacting polyols with methylene diphenyl diisocyanate to create a thermoset plastic core, which is then mixed with additives and facings, such as foil. Once cured, a closed-cell structure is formed, trapping gas within the foam and making it highly effective for thermal applications.

PIR is also significantly thinner than mineral wool, making it ideal for applications where space is limited. 

Moisture resistance

Mineral wool is more suitable than PIR for moisture protection.

This is because it’s essentially volcanic rock, which is naturally hydrophobic, doesn’t absorb water, sag, or lose performance even if it’s exposed to a leak. It’s also inherently porous, meaning it’s breathable and good at keeping heat in, water out and allows vapour to escape.

On the other hand, PIR’s rigid foam structure is highly resistant to water vapour and if there’s no ventilation, it can lead to moisture becoming trapped within the wall or roof, which increases the chance of condensation, mould and long-term damage.

This doesn’t mean PIR boards are not suitable for flat-roof applications – in fact, manufacturers such as EcoTherm, Kingspan and Recticel produce PIR boards specifically for flat roof applications. These are designed to eliminate the need of upper timber decks which removes the issues surrounding moisture build up in a warm roof construction.

Acoustic properties

If you require soundproofing, mineral wool is the only material you should consider.

This is because it has a naturally dense and fibrous structure, which is effective for absorbing sound waves, reducing airborne noise (like voices and music) and reverberation within spaces. Mineral wool is often used in partition walls, floors, ceilings, and anywhere else where noise reduction is essential for this reason.

PIR, on the other hand, should not be considered for applications requiring soundproofing. This is because it’s incredibly thin and features a rigid foam structure, which doesn’t lend itself to noise reduction between rooms.

Which is cheaper?

Mineral wool and PIR serve two very different purposes – mineral wool is used for fire protection and soundproofing, whereas PIR is used solely for thermal performance. This is why cost should not be a deciding factor.

If your main goal is to achieve high thermal performance, you should opt for PIR insulation. It’s the most cost-effective and space-efficient option, offering both excellent thermal efficiency and an incredibly thin profile that fits into virtually every cavity space, allowing you to achieve target U-values using less material and space.

If your main goal is either fire resistance or soundproofing, you should opt for mineral wool. Whilst it is more expensive upfront, the material composition will allow you to achieve the highest fire safety and effectively reduce noise transfer between walls.

If you want to achieve high thermal performance, fire resistance, and soundproofing, you should opt for both and use them in conjunction with each other (assuming you have enough space for them).

If you need help with your insulation project, please don’t hesitate to reach out to us, and we’ll be more than happy to assist. UKFR is a family-run business with 50+ years of experience across flat roofing & insulation, ready to help you find exactly what you need.