SPECIFICATION GENERATOR
Find out which low impact materials are right for your building project.
What Are The Differences Between PIR and Wood Fibre Insulation? – Can I Just Ask? – Ep.10
When specifiers look at insulation options, PIR often gets the nod for its low thermal conductivity and slim build-ups. But performance on paper rarely tells the full story. At Back to Earth, we find it far more useful to evaluate insulation materials using a broader framework, one that looks at the many roles insulation plays in a building. Here, we compare wood fibre and PIR through the lens of the eight roles of insulation: acoustic performance, fire safety, health, comfort, buildability, durability, sustainability, and thermal performance.
If you’d rather hear Andy and Chris talk through the ideas in this article, you can watch Episode 10 of Can I Just Ask? below. 👇
1. Acoustic Performance
PIR is a lightweight, rigid foam. That rigidity means it does little to absorb or dampen sound, in fact, it tends to conduct it. In contrast, wood fibre insulation is significantly denser and more flexible, allowing it to absorb airborne noise far more effectively. The result: better acoustic separation between rooms and from external sources.
2. Fire Behaviour
Fire performance is often misunderstood. PIR boards may carry a high fire rating and appear non-combustible, but this doesn’t make them safe. Under fire, PIR can off-gas highly toxic fumes that incapacitate occupants before flames are even an issue. Wood fibre, being organic, does burn, but it does so slowly and predictably, without producing toxic smoke. This predictability can buy valuable time in an emergency.
3. Health & Handling
Working with PIR can be unpleasant. Cutting it generates persistent dust, and the process releases gases like pentane. The material also tends to cling to tools and clothing. Mineral wools pose their own hazards, particularly through fibrous dust that can damage lung tissue.
Wood fibre, by contrast, is relatively benign. Its dust is still something to manage, but it doesn’t persist in the lungs the way mineral fibres do. It’s safer and more comfortable to work with, especially for those on site day in and day out.
4. Comfort & Thermal Mass
PIR delivers high thermal resistance per unit thickness, which makes it appealing in tight build-ups. But it lacks thermal mass, meaning it does little to moderate temperature swings. Buildings insulated with PIR can overheat in summer, even if they perform well in winter.
Wood fibre has significant thermal mass. It absorbs and slowly releases heat, helping to stabilise indoor temperatures throughout the day and across seasons. This makes for a more comfortable, resilient building envelope.
5. Buildability
Installing wood fibre is straightforward. Flexible batts compress easily into stud cavities, while tongue and groove boards connect neatly together with minimal reliance on tapes, foams, or fiddly seals. This simplicity reduces installation errors and speeds up the build.
PIR, in comparison, requires precision cutting and airtight taping to perform well. Any gaps or compression can dramatically reduce effectiveness, particularly in complex details.
6. Durability on Site
There’s a common misconception that wood fibre can’t handle moisture. While it will absorb water, it also dries rapidly and doesn’t lose performance in the process. It can even help adjacent timbers dry out more safely. In a typical UK build, where exposure to rain is likely, that resilience is a real asset.
PIR is more sensitive to moisture ingress. Water can degrade its thermal performance and compromise structural integrity, especially if the foil facing is damaged.
7. Sustainability
PIR is manufactured using petrochemicals and involves significant embodied carbon. The production process is energy-intensive and contributes to greenhouse gas emissions before the product ever reaches site.
Wood fibre, on the other hand, locks carbon into the building. It’s often made from timber industry by-products and uses comparatively low-energy processes. Choosing wood fibre can be a meaningful step toward reducing a project’s carbon footprint.
8. Thermal Performance in Practice
On paper, PIR boasts a lower λ-value (thermal conductivity) than wood fibre. But thermal performance isn’t just about numbers. Wood fibre’s high thermal mass and low thermal diffusivity allow it to slow down heat transfer dramatically. That means more stable internal conditions and better comfort over time.
In hot weather especially, wood fibre outperforms many synthetic insulations by reducing peak temperatures and delaying heat ingress, key benefits in a warming climate.
Conclusion: Choosing the Right Insulation for the Job
While PIR has its place in certain build-ups, a broader lens reveals the clear advantages of wood fibre across a building’s lifecycle. From acoustic comfort and fire safety to buildability and sustainability, it offers a depth of performance that PIR can’t match.
So next time you’re specifying insulation, look beyond U-values. Think about what that material will really be doing for the building, and for the people inside it.