Fire and Wood fibre insulation

One common concern when using wood fire insulation is that because it is made of wood it might not be safe from a fire perspective. In this article we show how to use wood fibre insulation in fire safe constructions that comply with all the current UK Building Regulations as of Autumn 2023.

Wood fibre insulation offers many benefits within construction, particularly when we’re addressing climate change and the impact that buildings and building materials have on the environment. However, natural fibre insulations will burn in a fire and so are often, incorrectly, assumed to be much less fire safe than synthetic materials with much better fire ratings.

How are products fire tested?

Fire testing of individual products is very simple and arguably too simple. A high temperature flame (blowtorch) is projected against the vertical surface of the material and the size and spread of any flame that appears is measured. Depending on how much the material burns upwards from this point dictates the fire classification of the product.

Most fires are caused by something within the building burning, rather than the fabric of the building itself burning and so knowing what substances are released by heating a material are very important. This is not measured by the current fire testing.

Synthetic materials, even those with apparently good fire ratings, can release Cyanide gas or Hydrochloric Acid Gas when heated. These gases are extremely incapacitating and asphyxiating and prevent occupants escaping buildings.

Wood fibre insulation burns very slowly, even in intense fires, but does not release asphyxiant gasses. It allows time for occupants to escape fires easily and so is very safe from a fire perspective.

Fire rated timber frame wall constructions – Render finish

For any structural timber framed walls that sit within 1m of a boundary Part B of the UK Building Regulations requires the wall to have a fire resistance of at least 60 minutes from the outside. The below structure has been tested by HFA in Austria and complies with this requirement, giving 60 minutes protection from inside and 60 minutes from outside, called REI 60.

The specification for this wall would be as follows:-

Exterior
8mm Baumit render system
60mm UdiDiffutherm wood fibre boards
160 x 60mm Timber Studs @ 600mm centres, fully filled with SteicoFlex 036 flexible wood fibre insulation
15mm OSB3
Ampatex Sinco vapour control membrane, taped with Ampacoll INT airtightness tape.
50 x 38mm battens, fully filled with SteicoFlex 036 flexible wood fibre insulation
12.5mm Gypsum Type DF Fireline board
12.5mm Fermacell plaster board
Interior

The thickness of the timber studs is absolutely vital as the fire certificate requires them to be no less than 160 x 60mm. You cannot use 38/45 or 50mm studs and still comply with the fire certificates.

The U-values achieved by various stud depths are as follows:-

Fire rated timber frame wall construction – Clad facade

The specification for this wall, as per the Steico documentation, would be as follows:-

Exterior
Timber cladding to fire classification B2
Counter-battens, incorporating intumescent fire stops at regular intervals
Ampacoll RF Black fire rated facade membrane
35mm Steico Universal wood fibre sarking boards
12.5mm Gypsum Type DF Fireline board
160 x 60mm Timber Studs @ 600mm centres, fully filled with SteicoFlex 036 flexible wood fibre insulation
15mm OSB3
Ampatex Sinco vapour control membrane, taped with Ampacoll INT airtightness tape.
50 x 38mm battens, fully filled with SteicoFlex 036 flexible wood fibre insulation
12.5mm Gypsum Type DF Fireline board
12.5mm Fermacell plaster board
Interior

The U-values achieved by various stud depths are as follows:-

Internal wall insulation

When systems are fire rated they achieve a rating of A-E, they receive a smoke emissions rating of s1-s3 (the lower the better) and they receive a rating on the amount of flaming droplets that drip from the structure, d0-d2. Again, the lower the number on the flaming droplets the better, with 0 being none and 2 being unlimited amounts.

Part B of the Building Regulations demands that the surface linings of walls and ceilings in private dwellings should have a fire rating of at least C-s3, d2, which is not particularly onerous, especially given the above regarding the emissions of toxic smoke from products. This rating allows a C fire rating with lots of smoke emission and flaming droplets of product dripping from the surface.

Thankfully, direct plastered wood fibre internal wall insulation (used internally or externally) has a fire rating of B-s1, d0 meaning that it exceeds the requirement, produces very little smoke and does not drip flaming droplets in a fire. The same would apply to the plasterboarded covered version of the wood fibre internal wall insulation as ordinary plasterboard has a fire rating of A2 and if you use fire rated plasterboard then it would be A1.

External wall insulation

For buildings below 11m tall, Part B of the regulations requires a fire rating of B-s3, d2 for external wall insulation systems, including those within 1m of a boundary. When used as an insulation over masonry walls, wood fibre external wall insulation using the Baumit render system has a fire rating of B-s1, d0 which again exceeds the requirements.

However, on buildings with facades above 11m the requirements change to A2-s1, d0, which would mean that rendered wood fibre insulation does not comply.

Roof coverings

The situation for roofs is mostly simpler. On the roof of a detached building you only need to have the roof covered with a non-combustible roof covering (tiles/slates, etc.).

For terraced or semi-detached buildings up to 15m from the ground, you also only need to have the roof covered with a non-combustible roof covering (tiles/slates, etc.) so long as the party wall extends to the underside of the roof covering. Above 15m from the ground you should have a fire separating party wall protruding above the roof line by at least 300mm to prevent fire from moving from one property to another. With this in place, wood fibre insulation is perfectly acceptable as an insulation within the roof structure.

Please be aware that the above information is provided to give an aid to understanding how wood fibre insulation complies with the current building regulations. It is important to check with your building control officer that all of the relevant fire regulations that must be complied with as there may be many more that aren’t covered here.

If you have any questions about the use of wood fibre insulation in your building project, please contact us for your free Project Consultation.