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What is wood fibre insulation and how does it compare to manmade insulation materials? Manmade insulations are the norm in UK construction, for obvious reasons: they’re designed to deliver great performance at optimum prices. But many of these are oil-based, plastic products such as PIR, which have a relatively high embodied energy (meaning they’re pretty carbon-hungry to manufacture).
If you want to put more focus onto the environmental impact of your project, there are some great natural and recycled options to consider. Perhaps the best known of these is wood fibre insulation. But what are the benefits of wood fibre insulation and could it be the right fit for your project? Here I’m taking a look at what you need to know about wood fibre insulation.
Wood fibre insulation is made from the residues of timber processing, such as sawdust and wood shavings. It comes in two basic forms – boards and batts – but is also available in a blown-in format.
Wood fibre insulation boards are rigid and available anything from 25mm to 200mm thick. They can be square-edged or interlocking (to improve airtightness). Batts are spongy semi-rigid blocks of insulation, which can be compressed slightly to fit snugly in between timber studs, for instance, so they are commonly used in timber frame construction. Frequently, the two products are used together, ie batts between studs and boards over them, which addresses any thermal bridging through timber wall and roof panels.
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Available from Ecological Building Systems, Gutex Multitherm is a breathable insulating sarking board made of recycled wood chips. It can be used for a variety of applications, including as external insulation for timber frame walls and pitched roofs. The close-fitting tongue-and-groove profile helps eliminate thermal bridges and ensures a windtight barrier
There are several ways in which wood fibre insulation scores well when compared to conventional insulation products. Most of these relate to its sustainability credentials, but there are other practical benefits, too:
We need to learn to treat waste as an asset rather than a problem. Wood fibre is a byproduct of the timber processing industry, so it’s a great example of making intelligent use of something that might otherwise be classed as waste.
Timber is grown, and trees should be planted everywhere, so it is by definition a renewable resource. This compares well to products such as rigid foam insulation, which are derived from oil – a non-renewable, depleting resource.
Wood fibre insulation is used in a form that’s pretty close to its natural state, requiring minimal processing. This is important because, as we get better at reducing energy use in buildings (for heating, lighting etc), the embodied energy of the materials that we use to construct them becomes a much more important component of the overall environmental impact of construction.
Organic materials such as timber and other plant-based materials effectively lock up carbon during the life of the product. As the tree or plant grows, CO2 is transformed into carbohydrates by photosynthesis. It is then stored in all derived materials when used in construction.
You may have heard aspiring eco-builders use the term carbon-negative when describing their dream home plans. This means the CO2 emitted manufacturing the building materials that go into constructing the house is more than offset by the carbon stored in the materials themselves.
One big technical benefit of wood fibre insulation is that it’s vapour permeable. This means that it allows moisture (in vapour form) to pass through, enabling the building fabric to breathe. This reduces the risk of moisture build-up, condensation and mould growth – issues that have plagued older buildings in the UK’s damp climate (particularly when homeowners look to upgrade their performance with inappropriate non-breathable insulations).
To maintain a vapour-open construction, the other wall or roof materials must also be permeable. This applies to boards, membranes, plasters and paint.
At first, it might seem contradictory that a breathable material that allow vapour to pass through can also absorb and store a limited amount of moisture. This buffering effect helps to even out natural fluctuations in internal moisture levels as we occupy our buildings.
Wood fibre insulation is very good at storing moisture (up to a point). By contrast, although mineral wool is vapour permeable, it has no moisture-bearing capacity and becomes ineffective as an insulant if it gets too wet.
Other types of wood waste-based insulationIt is possible to simply use the source material – waste products such as wood shavings and sawdust – as insulation. This is cheaper and less processed than wood fibre boards and batts. However, the R-value of these raw products is much lower, so they’re not as effective as insulants. Some Nordic countries have carried out trials using wood shavings as insulation, including coating the shavings with clay to improve fire resistance, thermal performance and moisture capacity.
Baufritz’s high-performance walling panels are factory-fitted with its own Hoiz spruce wood shaving insulation. The company treats the shavings with whey for fire resistance, soda to protect against fungal attack, and compresses the product to achieve good thermal resistance In Germany, some established building companies are beginning to use less-processed, wood-based insulation – typically within timber frame panels – with their own approaches to achieving the requisite standards.
As with all natural options, wood shaving insulation must be combined with vapour-open membranes, boards and finishes to achieve a breathable structure and deliver optimal air quality. Always ask to see certifications that the product is suitable for the application, and meets UK fire safety criteria. |
Most insulation materials have very little ability to store heat. Sometimes, modern eco buildings can overheat because they lack thermal mass – somewhere for heat to be stored – and are instead too lightweight.
Thermal mass helps to even out fluctuations in internal temperature caused by solar gains and by occupation, so should always be part of the thermal strategy for new buildings. Wood fibre insulation does have thermal mass, and while it’s true that it’s not as good at storing heat as dense masonry, it does at least offer some heat-bearing capacity to help with internal temperature management.
One of the big advantages of wood fibre boards is that they can be used as a substrate for plaster – so internal finishes can be applied directly to the insulation with no need for any other layers. Lime or clay plasters are the best options for use with wood fibre boards as they are vapour-open, like the boards themselves.
As a self builder, you have the power to choose your construction systems. If you’re aiming to build sustainably you may well choose timber frame, which is ideally suited to sustainable and natural insulations. Timber frame wall panels insulated with wood fibre should be very similar in thickness to a conventional cavity wall construction for the same performance level, albeit slightly deeper than timber panels using foam-based insulations.
Oakwrights’ WrightWall Natural system – which provides the insulating, airtight encapsulating wrap for its oak frames – is now available with Steico Zell air-blown wood fibre insulation, delivering excellent thermal performance, enhanced noise reduction, a healthy living environment and sustainable, low embodied carbon construction
Well-publicised failures in eco retrofit projects are frequently linked to the use of impermeable, plastic insulation products that trap moisture, leading to mould growth and resulting health issues. Wood fibre insulation does the opposite and actively promotes occupant health via good moisture management and breathability. It can even help to absorb toxins to improve air quality.
Internal wall insulation (IWI) in older buildings (ie properties of traditional construction) is a technically tricky subject and fraught with risks due to condensation and thermal bridging problems. Wood fibre insulation, applied correctly and at limited thickness, can help to resolve many of these issues and is one of the very few materials that can be used safely in this application. It ought to become the go-to material for IWI in future.
When comparing insulation costs, you have to allow for the thickness of insulation needed to achieve the same U-value. The thickness required would be little different from mineral wool, but you’ll need a greater thickness than with plastic-based insulations to achieve the same U-value.
Wood fibre insulation is currently more expensive than mineral wool. However, as a proportion of the overall build cost, or even the cost of retrofitting when you take the labour element into account, the difference is not that significant. For the modest extra uplift, you get all the benefits of using a renewable healthy product with low embodied energy. If your aim is to reduce CO2 emissions, then it makes sense to start with a product which is low in embodied emissions and actually locks up carbon.
Definitely. This is exactly the sort of material we should be using as it’s renewable, low impact, healthy and can be locally sourced – with economic, social and environmental benefits. [Ed’s note: it seems the big players in construction agree: in early 2024, Kingspan paid €250m for a majority stake in leading wood fibre insulation manufacture, Steico.] We need to get away from our dependence on oil-based materials and start to use products that actually benefit us all, and do not deplete limited resources.
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