Wood fibre insulation: Why use anything else?

Wood Based Insulation

Don’t just reach for the mineral wool. Wood fibre insulation has many advantages over its synthetic rivals.

Insulation is about much more than cost and thickness. While mineral wool is an industry standard, wood fibre insulation is on the rise. In Europe about 10% of the entire insulation market is natural fibres; and around 6% is wood fibre.

Chris Brookman is Founder of Back to Earth, which supplies sustainable building materials and provides consultancy services on using them. He talks us through the many reasons we should all be considering natural materials to insulate our buildings.

A heavyweight when it comes to thermal mass

Wood fibre insulation has more thermal mass than mineral wool. It prevents heat escaping the house in cold weather, like any insulation should. But its thermal mass means it can prevent overheating in hot summers. It is heavy and slow to change temperature, unlike lightweight synthetic insulation.

“It takes about 15 hours for summer heat to get through and start warming up the interior with wood fibre insulation,” Chris points out. “But it only takes about 3.5 hours with glass fibre insulation. Because it takes so long with wood fibre, heat doesn’t penetrate the building.“

This is “diffusivity” at work: the speed at which heat moves through material. Denser materials, such as wood fibre insulation, have a much higher heat capacity, and their diffusivity is much slower.

“Keeping buildings at tolerable temperatures will become more and more of an issue, as heatwaves become more common,” Chris points out. Last summer, when it was 40 degrees outside, his own office building, which uses natural fibre insulation, peaked at a temperature of 25 degrees.

Your builders’ lungs will thank you

Another plus, according to Chris, is the benefits for the health of those who install it. “Breathing in mineral fibres is bad for your lungs,” he suggests. “True, the fibres dissolve after 2 or 3 weeks, but if you’re a tradesperson installing rock fibre day after day then they are constantly being topped up. Natural fibres, on the other hand, don’t persist in your lungs.”

Anti-moisture superpowers

Durability is another big plus for natural fibre insulation, in particular its natural ability to dry out more quickly.

“Fibres in natural materials – wood, hemp, sheep’s wool – are designed to transport moisture,” Chris explains. “And they continue to do that when you put them inside a timber frame panel.”

“If any water were to enter a panel filled with natural fibre insulation, it will dry out in two weeks. But if you use glass or mineral fibre it will take much longer – it may be vapour permeable but it has no ability to distribute the moisture within the panel. And if you use PIR it may not dry out at all.”

“Using carbon-intensive materials has an immediate bad impact, irrespective of any operational energy efficiencies that the building might possess”

In the event of a fire…

There is a popular misconception that wood fibre insulation is more of a fire hazard than other options. Fire Resistance of Wood Fibre Insulation is rated at Class E, which is very similar to other insulating materials.

“Wood fibre burns very slowly. It will char and smoulder away, but it’s highly unlikely to catch fire and ignite into a big blaze,” Chris states. “It would work its way through very slowly.”

It’s worth bearing in mind that mineral wool is typically bound using a formaldehyde binder. If it burns it releases smoke which can contain hydrochloric acid gas. PIR may not burn but, when heated, it can release hydrogen cyanide. The toxicity of such materials in the event of a fire is a consideration as they release incredibly toxic materials which in some cases could be worse than the fire.

timber Installation of insulation

For a timber-framed building, ‘standard’ insulation such as PIR can be problematic. “If you use materials that aren’t compatible with a timber frame, such as rigid PIR boards, it can lead to problems,” Chris warns. “You can get gaps in between insulation and boards, with additional heat loss.”

Installing wood fibre insulation is no harder than installing its synthetic counterpart. Chris makes the point that all insulation – of any kind – needs to be fitted conscientiously. He points out that PIR insulation has a tendency to shrink, so that if it is merely cut tight and pushed between the studs it will eventually fall out.

Perimeters should be taped, and any air gaps sealed with expanding foam. But in practice, many installers aren’t quite that thorough. Wood fibre does not have any problems with shrinking.

A win for embodied carbon

For those considering using wood fibre insulation, the biggest incentive is probably the lower carbon footprint. “If you’re using carbon intensive materials, that has an immediate impact irrespective of any operational energy efficiencies that the building might possess,” Chris states. “It makes sense to use plant fibres: you’re locking carbon into the building. You’re removing CO2 from the atmosphere, not pumping more into it.”


Read Designing Timber issue No 2 here