Part B fire compliant vapour-permeable wall & roof buildup

[Ano]

Hello all,

 

I'm a long-time fan of this forum, and first-time poster, so thank you all for your insights so-far.

 

I've designed and just received planning for a Mansard-inspired upward extension, with vertical 'roof' walls turning to a double pitched roof, all slated like a giant pointy hat. Our priority is to keep the build-up vapour permeable, so using wood fibre and maybe even vapour-open racking board as the internal finish (ie Panelvent / Elka). I've attached a draft build-up below (internal racking / frame & bio-insulation / solid wood fibre / battens & slates).

 

The only issue I can foresee, however, is that we are within 1m of our boundary on both sides, so fire rating is obviously going to be a BC issue.

 

My question to you, the well-informed congregation, is — does anyone know of a proprietary / tested system similar to my drawing, or have any insight? Am I right in thinking the external cladding needs to be 1hr, and the internal 30mins—therefore this should work if I treat the internal for 30 mins (but is the external void behind the slates an issue)? Hoping to iron this all out before it becomes a problem on site!

 

Any thoughts would be most appreciated. We're trying to keep the build-up as simple as possible, with woodfibre etc for the high decrement factor.

 

 

Thanks for your time!

 

Ano

 

 

 

wall_buildup.pdf

Edited September 6 by Ano

[ADLIan]

Check with your BCO - may need to be one hour from both sides. If so I greatly doubt you will find a wood fibre manufacturer with the correct test data. Remember any test will be very specific and only applicable to the actual materials/products tested. 

[Ano]

Thanks ADLIan, that's a helpful point.

 

We're considered blown cellulose for layer 2 also, but I guess that doesn't make much difference...

 

Is the wall finish ext & int to FR60 enough, or a whole tested system necessary (therefore a pricey proprietary system no doubt 😵‍💫)?

 

Thanks for your time