Fire is a complicated topic, mainly because of the differences between the fire testing and what actually kills people in a fire.

We generally assume that products should not burn at all and so Halogenated Flame Retardants are added to avoid the risk of fire. Products are then tested by directing a flame (blow torch) against the surface to see how the surface burns. However, studies by the University of Central Lancashire's Fire Research department show that in spite of the flame retardants used, many products, like PIR (foil faced foam boards), continue to burn in a fire but in addition produce high levels of highly toxic gases like Carbon Monoxide and Hydrogen Cyanide (Cyanide).

These gases are highly effective asphyxiants and can incapacitate people in a matter of seconds, meaning that although they have a much better seeming fire rating than products rated as E, they are far more likely to mean that occupants will die in a fire due to incapacitation. Whilst materials like mineral wool and glass wool do not burn as such, the insulation contains binders that release the above gases in a fire. Materials like wood and wood fibre do burn in fires but as you say, wood fibre boards smoulder slowly but without producing toxic incapacitating gases. This may seem counter intuitive but it allows you to escape the building and lets the fire be extinguished in as safe a way as possible.

The building regulations currently ignores the production of toxic gases and a product's ability to incapacitate occupants and focusses solely on a products ability to burn. Additionally, the products that remain after a fire involving these kinds of synthetic insulation are highly carcinogenic and are implicated in the 4 fold increase in rates of cancer amongst fire officers in the UK.

With regards to the composition of boards, we are keen to point out the differences between the manufacturing processes of all of our boards and include it at the top of the page in the product description. We have also produced an article on the differences between wet and dry processed boards (those that use water based glues and those that use polyurethane based glues) here - https://www.backtoearth.co.uk/wet-process-vs-dry-process-wood-fibre/  You can also see more information on each product's data sheet.

Finally, on the use of wood fibre internally, the wood fibre is protected from a fire for quite some time by the surface plaster. This is a non-combustible mineral plaster (lime) and prevents oxygen reaching the boards behind and so gives the system an overall fire rating of B-s1-d0. This means it has a fire rating of B, there is the lower level of smoke production (s1) and there are no droplets formed in a fire (d0). This is well in excess of what is required under the building regulations.

So in short, if you do catch your wood fibre insulation on fire (you really do need a blow torch for this, not just an electrical fault) you do need to extinguish it properly but if you're unlucky enough to have a fire in your house (built using wood fibre insulation) the chances of escaping are very high and what is left is not going to cause cancer in fire officers who come and extinguish it.