Chris Brookman from Back to Earth explains what wood fibre insulation is, how it could be used on a project and why careful specification is required.
Interview with Chris Brookman
Chris Brookman, founder and owner of Back to Earth explains how his chemistry degree has been put to good use within the construction industry. His experience ranges from working with natural cob to examining the properties of different building materials to create specifications for particular needs. He has built his own Passivhaus and here shares his technical understanding of wood fibre and the many properties it offers for buildings.
Chris working on a cob building
Traditional cob building is strenuous
Straight from university Chris worked as a cob builder. He explains that traditionally cob was a combination of clay, sub soil and straw mixed together by cattle treading over it to make a plasticine kind of mix, that was stacked on top of a low stone wall. These days sands, grits or aggregates might be added to adjust the properties of the mixture. One of the reasons Chris eventually moved on from cob building was because the nature of the work was extremely physical.
Traditional materials can play a part in modern construction
Chris then came across wood fibre, which worked well with cob and that got him interested in learning more about different materials. He was particularly interested in finding materials which have the properties of traditional materials, but are in a modern format, so are easy to use by a modern builder. He eventually moved away from the building business to create Back to Earth, which deals with the specification and supply of materials.
Wood fibre is not just about thermal insulation
For Chris, thermal insulation is only one of the reasons for using wood fibre products. Alongside this it offers acoustic insulation, thermal mass to help reduce summer overheating, and good fire resistance. It is also locking up carbon for the duration of its lifetime, which is really important.
Wood fibre products start with waste timber
As a by-product of the timber industry, the production process for wood fibre starts by grinding up the waste timber to fiberise it, which is then treated in one of two ways.
The wet process uses the natural lignin to bond the product
The more traditional process is to mix it with water and detergents to create a slurry, which is then put on a conveyer belt to strain all the water out, before it is compressed and steamed. Through steaming, the natural lignin glues the fibres together, producing a board around 20mm thick.
The dry process uses a small about of glue
The more modern way is the dry process, which uses a polyurethane-based glue to stick the fibres together. You blow the fiberised wood with the glue, compress it to the right thickness and density before curing it with warm humid air. This creates an homogeneous product, from 20mm up to 200 mm thick.
The dry process makes a lower density product, which is a bit more rigid and has a lower thermal conductivity. However, it massively reduces the amount of energy used to make the product and is slightly cheaper to make. As it only uses a small amount of glue, maybe 3-4%, it has a minimal effect on moisture retention. Most boards are made in this way now and whilst it can be better to use wet processed boards for internal wall insulation they can generally be used interchangeably.
Check out our live training with Chris Brookman on the roles of insulation.
The density of wood fibre can be a benefit over other natural products
Chris explains that he chooses materials based on their properties but, in most scenarios, natural fibre products tend to work better than synthetic materials. Common uses include insulating timber frames, roofs or internal walls.
The density of wood fibre is also a useful characteristic when it comes to adding thermal mass to a building, which in turn absorbs the heat more slowly and can make a big difference to summer comfort. This is one key difference to most other natural fibres.
Building new made it possible to create an accessible home
Chris’ first son was born disabled and it quickly became apparent that the small rooms of their old cob house were not going to work. The house was over three floors with no circulation space for a wheelchair, so they started thinking about building a new house.
A Passivhaus would provide a good environment for his disabled son to stay warm
Chris recalls that John Williamson, in Wales, had just built one of the early Passivhaus projects. He was also aware that when you’re sitting in a wheelchair, your environment needs to be warm.
Bricks from the bungalow are reused as hardcore for the new house
A specialist architect provided input on the designs
In 2011 they found a suitable site, with a 1950s bungalow they planned to demolish and use as hardcore for the new build. Chris explains that they hired a specialist architect to look at their designs and make suggestions about circulation space, door widths and accessibility over the whole site. The whole plot was levelled as a consequence, with just a slight fall to help water drain away, all the thresholds are level.
The office was built before the main house
The office was built first to test their approach
Chris worked on the detailing for the build, but it was a friend who carried out the construction. The office was their test case, and they made a few minor mistakes which meant they were just over the 0.6 air changes per hour at 50 Pascals required to meet the Passivhaus standard. This meant that when it came to the main house they really focused on airtightness.
They used a timber frame filled with wood fibre insulation, which was relatively quick way to build. An airtight OSB was used internally as the airtight layer.
Clay boards were added to the internal walls to improve the acoustics
In a Passivhaus you need to have a little gap under the internal doors to help the air flow, but that can cause issues with sound transfer. So Chris used clay boards on all the internal partitions to improve the acoustics.
Where structural, the internal partitions have Fermacell on one side and clay board on the other. The non-structural partitions have clay board on both sides with flexible wood fibre in between.
The clay boards have given the added benefit of increasing the thermal mass of the building, so help to reduce summertime overheating.
All new builds have a few niggles
Chris and his family are really happy with their home but there were few things aspects they were keen to address. The biggest mistake they made was with the front door, where they cut a metre wide opening into the frame, but hadn’t realised the thickness of an external Passivhaus door. So they ended up with an opening width of 740mm which made it difficult to get in and out with a wheelchair. This was, however, changed a few years ago so now they have a 1300mm wide opening.
They also swapped two of their clerestory windows to ones with electric openers to make it easier to purge ventilate in the summer. Chris had initially thought the MVHR system would be able to deal with this but it wasn’t enough on its own.
Something that couldn’t be changed without substantial work was the liquid screed floor finish. They had intended to use underfloor heating tiles, but they were too expensive, so they used liquid screed instead. Chris regrets this choice as this floor transmits sound right across the house.
One other regret was connecting the house to the gas main. Not only was it expensive to install but at the time grid electricity was about 500 grams of CO2 per kilowatt while gas was 180 grams of CO2 per kilowatt, so using gas to heat the home seemed like a good option. Chris may well switch his heating source in the coming years.
Back to Earth’s specifications ensure their materials will give the right results
Chris sees the primary role of Back to Earth as creating specifications for materials to ensure they achieve the best results on each construction project. Most of their work is on retrofit projects and they use WUFI to model the specifications they are producing.
A WUFI analysis of a specification
Chris sees them as problem solvers, looking at specifications to provide the right material for a particular function. His own construction experience also means they can add in practical details alongside the specifications.
The Fibres Academy training platform
Chris believes a lot more people would use natural materials if they understood them better. To that end, Back to Earth have just revised and relaunched their Fibres Academy training platform. This is free to use and gives you access to courses, 3D model and installation guides to help you use the materials with confidence.
Chris tells us that Europe is a long way ahead in the amount of natural fibre materials that they use. Figures from a few years ago showed Europe’s use of natural fibres at around 10%, whereas in the UK we’re struggling to reach 1% of the market share. There is a fundamental difference in perception and accessibility between the UK and Europe. Back to Earth’s training platform is trying to improve people’s understanding of the products to increase use and push forward this agenda.
Generally as a share of the market, Europe is using about 10 times more natural fibre than we are.
Natural fibres are generally a bit more expensive but worth it
Natural fibre products may be slightly more expensive, but for Chris it is like comparing apples with rotten eggs. People might say why use wood fibre instead of PIR? But often the installation of PIR isn’t good. It may just be wedged between timbers, resulting in air gaps opening up later and reducing the performance.
So often the perception is that modern materials are cheap, but this can be because the time is not taken to install them correctly. Back to Earth give detailed guides, going through all the airtightness layers and the whole build up. This ensures correct installation so the product works.
It’s also important to recognise the value and added benefits that you get with these materials. It’s not just about cost, there’s the environmental impact, the health of the installers and the health of the occupants years down the line, along with the longevity of the building and the finishes.
A change in attitude and better regulation is needed in the UK
Chris feels we have a different attitude to our housing than the rest of Europe. We’ve commodified housing, so we buy and sell with the expectation that we will make money from our properties. If people actually proiritised comfort and health, they would probably steer clear of major developers and make different choices.
The building regulations also have a role to play in increasing the use of natural materials and better legislation from the Government would have a big impact. Government should not be a passive overseer, but be there to guide people’s activities, which could make a big difference in the emissions from the construction industry. This could include an assessment of the embodied carbon of products and the biogenic stored carbon. The locked-up carbon in wood fibre, may only be for the lifetime of the product, but surely that’s better than it not being locked up now.
Find out more
The Alliance for Sustainable Building Products
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