What Are The Differences Between PIR and Wood Fibre Insulation? – Can I Just Ask? – Ep.10

PIR and wood fibre are both widely used insulation materials, but they differ in almost every meaningful respect beyond their basic function of reducing heat loss. On acoustics, the contrast is stark: PIR is lightweight and rigid, which makes it an effective conductor of sound, while wood fibre’s density and relative flexibility give it strong sound insulation properties. On fire behaviour, the comparison is more nuanced than most people expect. Wood fibre does burn, but slowly, predictably, and without producing toxic gases — giving occupants time to leave safely. PIR may not ignite as readily, but when it does it releases gases that can rapidly incapacitate, and some mineral wool products using high-binder formulations can release hydrochloric acid. A non-combustible rating is not the same as being safe in a fire.

The health profile during installation is another area where the two materials diverge significantly. Wood fibre fibres, if inhaled, do not persist in the lungs and cause no lasting damage. PIR produces fine dust when cut, releases pentane during cutting, and is generally unpleasant to work with. In-use comfort is shaped largely by thermal mass: wood fibre’s high specific heat capacity and low thermal diffusivity mean it transfers heat slowly, keeping buildings stable in both winter and summer. Lightweight insulations like PIR achieve their U-values on paper but offer little resistance to the rapid temperature swings that make buildings uncomfortable in warm weather.

On durability and site handling, wood fibre’s natural moisture tolerance is often underestimated. It will absorb water if exposed to rain during construction, but dries out quickly and helps the surrounding timber structure do the same. And from a sustainability perspective, the materials are in entirely different categories: wood fibre is manufactured from waste timber, actively sequesters carbon within the building fabric, and carries a low embodied energy footprint. PIR manufacturing is carbon-intensive before a single board reaches site. Taken across all these dimensions — acoustic, fire, health, comfort, durability, and environmental impact — the differences between the two materials are substantial.