Module 12: Choosing the right systems


There are many different manufacturers of wood fibre insulation but they all supply one or all of three types of wood fibre insulation. The first and most time tested form is Wet Processed wood fibre board.

Wet processed boards

As the name suggests, water is used to process the wood fibres. Basically the wood fibres are ground up and then boiled up in water (and other additives) to release many compounds from the wood. Importantly the sugars which it contains are also removed. The fibres are then strained off in to 20mm thick slabs and then heated. The heat softens the lignin (natural glue that is around the wood fibres) which bonds the fibres together.

The 20mm layers are then laminated up into thicker layers by glueing each layer to the next with water based glues. The densities of the layers alternate between high and low to ensure that there is no warping in the board, for additional stability. By removing the wood sugars from the fibres you also remove the food source for many insects and moulds or fungi and so make the board more resistant to insect attack.

Wet processed boards have been around for decades and have been used as a render carrier for nearly 30 years in the form of Diffutherm. They have also been used for roof and other internal insulation for many years before that.

Dry processed boards

The second, much newer type of wood fibre board is the Dry Processed board. This involves mixing a glue in with what is essentially saw dust and then gently forming the board. This is a much less energy intensive way to produce the insulation and so these boards are usually cheaper than wet processed boards.

Whilst this type of wood fibre is still very breathable and has the same thermal mass as the wet processed boards it does not manage moisture in the same way. Because each fibre has a thin layer of glue around it it has much lower capillarity and cannot move moisture in the same way as a wet processed board. Therefore, these boards need more care on site to keep them dry and will not help dry masonry out in the way that wet processed boards do. Whilst there are renderable dry fibre boards available, they are not as time tested as the wet processed boards.

However, dry processed boards tend to be a little more rigid and have higher compressive strength than wet processed boards and can be formed in thicknesses up to 300mm. This makes them ideal for use on the outside of timber framed walls/roof where very low U-values are required. They can be protected with a breather membrane or covered by another board suitable for carrying render which makes achieving Passivhaus or near standards much more economical.

Essentially, dry fibre boards are very useful in new timber framed construction but where you’re insulating solid walls you should really be using the wet processed boards. When sourcing your insulation it is important to know what type of product you’re buying.

Suppliers of wood fibre insulation

Flexible wood fibre

The third type of wood fibre is the flexible wood fibre batt which is usually only used between rafters, joists or studs. This is another type of dry fibre product but much lower density and, as the name suggests, it is flexible. Basically the wood is processed in to fairly long fibres and then blown with a polyester fibre into slabs of the required thickness. The slab is then heated and the polyester melts and sticks the whole mass together, creating the flexible wood fibre batts.


The main suppliers of wood fibre insulation systems we have in the UK are Gutex, Homatherm, Pavatex, Steico and Unger-Diffutherm. Each supplier sells a mixture of the three types of boards and has their own portfolio of products for use in different locations for different uses. The advice we’ve given above about which type of board is best for what scenario is based on our own experience which may be different from that given by these suppliers. Please feel free to contact us to discuss the specifics of your requirements as board use can vary.

Product approvals, Guarantees and Warranties

A product approval is a document that shows that an independent third party organisation has tested a product or system and deemed that it complies with the required standards dictated by either their national certification body or by the EU. Most standards are set by the EU as a way of harmonising production standards across the union, enabling products to be freely traded and used anywhere within the EU.

Each country has various private companies that test products and in the UK we have BBA, Trada and various others, all approved by the UK accreditation Service. It is useful to note that the testing standards across the EU are the same and so a product approval from one country is no more or less appropriate than another. All EU product or system approvals are acceptable under Building Control as proof of the intended use.

Most wood fibre products come in from Europe and so have European technical or product approvals. However, some invested in UK based approvals from companies such as BBA to make it easer to get a new building warranty when using their systems.

A product or system guarantee is the standard manufacturers guarantee that states that the system is suitable for it’s intended purpose and that the products are produced to the standard required to achieve that. This only covers the system components and not the installation, which will need to be covered by the installer.

A warranty is an insurance backed policy that usually covers the cost of repair or replacement of any part of the system that fails. It should cover the system against failure, so long as it has been maintained as required and not damaged in any way, for a period of 10 - 25 years, depending on what is specified.

Not all wood fibre systems can get warranties. As warranties are provided by an insurance company it is up to them to decide what products and construction methods they will insure. It is therefore very important that if you need an insurance backed warranty you should consult your provider, letting them know the system and construction build-up of your building.

System approvals from UK companies such as BBA do not guarantee a warranty can be provided and equally, an EU based technical approval does not mean you cannot get a warranty. There are many factors that require consideration such as exposure rating of the location and what the construction build-up is. For more information speak to your Building Inspector and your warranty company and call us if you need any help.