Eight Reasons to Use Wood Fibre Insulation

Find out more about how we approach this building scenario.

One of the many reasons that we at Back to Earth focus on insulation so much is that it is the single material which has the most influence over the performance, comfort and therefore enjoyment of a building. Each of our products is carefully selected to perform on the many levels that are required for comfort and not just thermal performance.

The IPCC have recently released their report stating that we have no chance of keeping warming caused by Climate Change to less than 1.5 Celsius and by 2100 it is more likely to be 3 celsius. Between now and then we are going to see an increase in the number of heatwaves, floods and droughts and so we need adapt and improve our resilience to these events.

With this in mind we need to build buildings which do not overheat easily, that require minimal heating and cooling and buildings that are comfortable, healthy spaces. The Global Commission on Adaptation, headed by Bill Gates amongst others, say “Adaptation action is not only the right action to do, it is the smart thing”. ”The costs of adapting are less than the cost of doing business as usual. And the benefits many times larger”.

When considering insulation it is easy to only consider the thickness or cost of the insulation on the assumption that it offers little other benefit than reducing the heating bill. However, here are eight properties to insulation which have a huge impact on a building and deserve greater consideration.

Acoustic Insulation

In new buildings the single biggest cause of complaints is noise, particularly in high density housing. This can be from the exterior, noise reverberating around the interior or from room to room noise. As housing density increases and noise levels rise it is very important to consider the acoustic properties of the insulation materials used in the roof, walls, internal floors and internal walls of buildings.

Acoustic insulation is governed largely by the combination of density and rigidity with high density and low rigidity products absorbing sound well and preventing sound transmission.

Using heavy weight insulation materials such as wood fibre insulation can make an enormous difference to the acoustics of a building. They tend to be between 6 and 12 times the density of synthetic options and better an absorbing vibration, creating quieter, more relaxing spaces.

Fire Protection

It is generally impractical to build low rise buildings entirely with non-flammable materials. Given the toxic nature of them, it is also unhealthy to pack flame retardants into materials which are flammable to avoid fire risk. Almost any insulation material will burn in a fire since even fibre glass insulations have combustible binders and so it makes sense to design with this in mind.

The sensible approach is to design the building around fire safety and then to use materials which burn extremely slowly. Additionally, the materials used should not give off deadly gasses in a fire or leave highly toxic ash products if the worst happens.

Natural fibre insulation materials tend to burn slowly and give off fairly benign levels of smoke. Whilst they will burn in a fire they do not contribute to the fire as they tend not to support a flame. In the case of wood fibre insulations, the rigid boards are dense, contain little oxygen and smoulder slowly, taking many hours to burn through. The best analogy would be burning a book. Putting it page by page in to a fire it will burn quickly however, a whole book in a fire takes many hours to burn through.

The flexible wood fibre wool products also burn but in intense heat will generally resist a flame for much longer than fibre glass or mineral wools. This makes them a better insulation for timber frames as it allows the frame structure to last longer in a fire.


During the construction of a building, contractors are exposed to a wide range of chemicals and particles which can be damaging to health. Contractors who cut and install insulation materials on a daily basis are most at risk of long term health issues from this exposure. Once a building is closed up, these products then affect the internal environment of the building,

Once a new building is occupied the levels of volatile organic compounds (VOC’s) released by the products in the house generally peaks and then slowly drops as the compounds evaporate from materials and are ventilated out of the building. Some insulation products contain compounds which are slowly released over many years in a process known as ‘off-gassing’ and because of the length of exposure can cause health problems in the occupants of the building.

Using natural fibre insulation products, such as wood fibre, wool or hemp fibre can avoid health problems occurring in the installers and significantly reduce the amount of VOC’s in the internal air of buildings, contributing to better indoor air quality. As we now spend around 90% of our time within buildings, this is an area of great importance.


Buildings which stay at an even temperature internally and do not fluctuate wildly are much more comfortable spaces than those that need strenuous control from heating and cooling systems. The noise and drafts from such systems maybe acceptable in some public spaces where people spend little time but they are not acceptable in a domestic environment. Neither is the cost of running them.

Lightweight insulation tends to offer very low thermal conductivity levels and the assumption is that this is better as you need less of it to achieve a particular U-value (heat loss per m2 of the wall at a given temperature difference). However, lightweight insulation also reaches it’s equilibrium state (where heat flow is even across the thickness of the material) very quickly. Consequently lightweight, high performance insulations tend to offer little resistance to summer heat which can quickly build up within a building.

In the UK, heat kills around 2000 people per year currently. This is projected to rise to 7000 per year over the next few decades as summer temperatures stay above 25 Celsius for longer periods. It is therefore vital that we create homes and offices that are adapted to this change in climate and can stay comfortable in-spite of wild temperature changes outside.

Natural fibre insulations, such as wood fibre insulation, are very complex structures that have unusual interactions with water and because of this have extremely useful thermal properties. They are good thermal insulators but also heat moves through them very slowly meaning that the internal environment takes a long time to be affected by the external environment. This effect, known as decrement delay, helps passively regulate the internal environment and reduces the need for heating and cooling.


The term ‘performance gap’ is a term used to describe the difference between the actual performance of a building compared to the performance it’s design should attain. This is largely attributed to the buildability of a building and whether the materials used in it’s construction can actually be installed in such a way as to work as they do on paper.

Small air gaps in and around rigid insulation boards can massively increase heat loss. A 3mm gap between insulation and surface to be insulated can increase it by 150% and a 10mm gap by up to 400%. It is small wonder that on building sites where time is of the essence, the weather is variable and two timbers are rarely exactly parallel, you’ll find gaps around rigid insulation boards installed in timber frame structures.

Similarly, when constructing cavity walls, it is virtually impossible to install a rigid insulation board up against a block with with absolutely no gaps. The mortar pressing out of the mortar joints and the slight deviation from straight of the blocks ensures small gaps are present everywhere.

Using extremely simple but highly effective construction methods with materials which allow some tolerance in the shape of the walls or roof means that you can achieve the targets you’re aiming for. Our wood fibre insulation systems use firm and flexible insulation batts between studs which do not slump and completely fill the voids that they occupy, ensuring no air movement around them.

The wood fibre boards used over the frames are flexible enough and allow some compression so that a tight fit with the rafters or stud walls can be achieved, preventing unwanted air leakage. Our wood fibre insulation boards for masonry accommodate uneven surfaces, preventing any voids forming between the insulation and the substrate.

These are extremely simple but highly effective systems that close the performance gap and in some cases out perform the original designs.


During the construction phase of a building, particularly ones that contain timber structures, whether as a roof or the walls, rain inevitably makes it’s way in to the building fabric. Once the building is water-tight this moisture needs to be removed as quickly as possible to avoid damaging the timber and creating conditions suitable for rot and mould growth.

Rigid impervious insulation products can seriously impede this drying by being highly vapour resistant. Whilst mineral and glass wools are vapour permeable they do not actively move moisture and can still allow moisture levels to stay very high for many months after construction is complete.

Natural fibre insulation, such as wood fibre insulation, is made from plant fibres that were designed to transport moisture, against the pull of gravity, in plants. They are capable of moving significant amounts of moisture quickly and so actively dry wet structures through dispersing moisture which is then allowed to evaporate out of the building. Due to their density, these insulation materials do not slump or disintegrate during heavy downpours and so form a very durable, low risk insulation system.

Externally, surfaces need to be able to cope with more extreme weather and also more extreme variability. Sudden changes in temperature and humidity have damaging effects on rendered and clad surfaces.

Wood fibre insulation boards can be used as a render carrier and the mass of the wood fibre material acts as a heat sink for the render. This reduces the thermal stress on the render, caused by the heat of the sun, preventing buckling in render systems. The flexibility of the wood fibre material also allows the render to move, increasing it’s lifespan and lowering the maintenance costs of the building.


Currently, most commercial buildings are designed for a 40 year service life, after which time they are often gutted and refurbished or completely demolished. With such a short lifespan the materials used must be recyclable to avoid waste to landfill and to reduce the impact of the building as a whole. Insulation materials also make up a large part of new buildings and so the embodied energy in them is a significant part of the overall impact of the new building.

Wood fibre insulation is made with waste wood, is completely recyclable and sequesters carbon during the growth of the trees from which it is made. At the end of it’s life it can be either recycled in to the fibre stream for other fibre products, used for fuel or even composted. It’s embodied energy varies depending on which type of wood fibre insulation is used but all are either carbon neutral or carbon negative in their production.

Using carbon neutral or carbon negative materials is a way of preventing what is known as the ‘Carbon Burp’, the large amount of CO2 released by the construction or refurbishment of buildings.


Thermal performance, or rather thermal conductivity, is a measure of the amount of heat flow over a given thickness of a material at a given temperature difference. Generally, the lower the conductivity the less material is required to achieve a given level of thermal insulation. However, this is rather like saying the best car on the road is the fastest one and ignores all of the other properties of the insulation.

The thermal conductivity is measured once the material reaches an equilibrium state. This means it reaches the point at which the heat flow in to the insulation and the heat flow out of the insulation are at a constant level. It does not comment on how long it takes to reach the equilibrium state or what impact that may have on the building.

If heat moves more slowly through a material it means that it takes longer for the interior to heat up and cool down. This property of insulation is called the decrement delay and it has a big impact on comfort in hot weather and can also affect overheating in buildings. Generally, the longer the decrement delay, the cooler the interior of a building stays and the more stable the internal temperatures are. 

Very lightweight insulations may perform well on thermal conductivity but they tend to perform poorly on decrement delay. This tends to be most noticeable in buildings with rooms within the roof structure or loft conversions, which generally get very warm during the day and cool down quickly over night, even though they appear to have very good thermal performance on paper.

Heavyweight insulation materials, such as wood fibre insulation, may require a slightly thicker build-up to achieve a certain thermal performance but they also provide very long decrement delays. These can be more than twice that of lighter weight materials and mean that although the thermal performance is the same on paper the amount of heat entering the building is significantly less with the wood fibre insulation.

This results in buildings which are more likely to maintain stable temperatures, much less likely to overheat in hot weather and also buildings that require less heating in winter.


To build buildings of the future we need to design, build and assess buildings much more thoroughly. All of the above properties of insulation should be considered carefully by building designers to ensure that the buildings they create are robust, durable and resilient. We have the technology, the skills and the materials available to achieve this already but we have still to add in the will to do so.

In the near future the climate will be warmer and much more variable so accepting this change is coming and building with this in mind now will save considerable costs, and potentially lives, tomorrow. 

If you’d like to discuss any of the topics in this article or see how we can help you with your project, please feel free to contact us.

The topics above are originally from Steico’s Protexion campaign. See https://www.ecomerchant.co.uk/protexion/

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