What fire-rated board do you recommend we use to the outside face of our dormer cheeks, which are insulated with Icynene? We need a 30 minute wall.

Will fire-resistance boards adversely effect breathability of the wall?

Out Approved Inspector said it could be on the outside only but I'll double check because I don't think that makes sense in terms of fire rating the wall in both directions.

Our Approved Inspector never mentioned it but what about 30 minutes of fire insulation as well (i.e. Table A2 in Approved Doc B)?

The boards will only provide 30 mins fire integrity right? To provide the fire insulation we need mineral wool or something. The Icynene's not going to provide that.

(starting to get a bit worried my Approved Inspector doesn't know what he's talking about with regard to fire protection )

Joe, we're using vertical roof tiles for the cladding. But the backing board that needs to be fire resistant too.

My reading of Part B (Table A1 5.a. and Table A2) is that we need to provide 30 mins fire integrity and insulation. The boards will provide integrity but I think we need to add mineral wool between studs (in addition to the Icynene), which would be a pain but not the end of the world.

P.S. I've changed title of thread to be a bit broader

So despite what my insulation man is telling me and what my Approved Inspector is telling me I think we need to do something like the attached.

Paul, good point, thanks. Do you think the calcium silicate board would provide the same structural properties as OSB in terms of racking and holding onto screws? We'd also have to take it all the way down to the sole plate, whereas placing it over top of the OSB means we can stop it at roof level.

Why do you suggest Blucad? I think that may be dearer than Promatect 250.

The studs should actually say 400 crs, so thanks for spotting that. Thanks for the tip on increasing counter batten size too. Although I notice I have the counter battens and battens labelled the wrong way round. Do you mean increase the vertical battens to 50 x 25?

Edit:
By the way the Promat tech help was saying that the problem they've found with this type of construction is that the heat from a fire can melt, or cause to combust, the foam insulation and then the wall lining can peel away and up goes your structure. So to avoid this you need to provide fire insulation resistance in the linings, hence the thickening and doubling up of the fire resistant sheets.

Posted By: TimSmallAny reason not to use mineral wool instead of Icynene? It's cheaper and similar U value.

You could full-fill with rockwool flexi, or use 50mm rockwool flexi, and your choice of PIR or EPS inside that I think?

I originally designed it all using EPS + mineral wool, however as I detailed it up it became increasingly obvious that it was going to be a huge headache in terms of airtightness, workmanship and time, especially at the existing eaves and party wall junctions.

By wrapping the existing pitched roof, the new dormer walls and flat roof internally with Icynene we deal really effectively with airtightness and it's all done in one day.

Alternatively, could you do this?

Studs with PIR infill (or Icynene or whatever).
OSB
50mm x 50mm batten with 50mm rockwool flexi infil
A fire resistant board of your choice
tiling battens
hanging tiles

I thought about this but 2 layers of fire resistant board would be much less hassle I think. Plus, to prevent the problem that Promat identified to me, we'd need to provide this stone wool layer to both sides of the wall.

Promatect 250 seems to be reasonably priced:
http://www.insulationdirect2u.co.uk/insulation-products/147-promatect-250.aspx

Posted By: PaulJDo just enough for b regs.

Given that my Approved Inspector didn't seem to have much of a grasp of tables A1 and A2, nor the risks of using combustible insulation within the wall, I don't know that this is the wisest move.

If somebody was injured or killed because of my design decisions, particularly after taking advice from a specialist like Promat (notwithstanding they have a product to move), I doubt it would matter much in terms of my liability if the Approved Inspector was wrong, and it certainly wouldn't help my conscious.

Thanks Graeme, nice timing. Just about to order! Is "resistant" a brand name?

Cripes, it's a hell of a lot cheaper too! Thanks Graeme. In fact it's even cheaper than the OSB, let alone the Calcium Silicate.

£6.50 for 9 mm sheets, £7.50 for 12 mm. Is there a downside to this stuff? Closer to the price of plasterboard.

Edit:
Found a few downsides: no BBA certificate, reputation for variable quality and imported from China.

Posted By: Shevekno BBA certificate, reputation for variable quality

Are those just in general or for specific products? i.e. have you asked particular vendors?

Posted By: djhAre those just in general or for specific products? i.e. have you asked particular vendors?

Dragonboard sound like one of the more reputable suppliers. They've had their board tested by BRE but that's all it says on their website. They don't have a BBA.

Euroform have the most reputable info: CE Marked, tested to BS EN 1716:2002 and 1182:2002, etc. No BBA. Just waiting on a price back from them.

Apparently variable quality was/is a big issue in the U.S.

I believe all of it's sourced from China because they have major deposits of Maganesium Oxide, although smaller ore deposits exist in Europe and Canada. I think I read somewhere that Sweden had a factory but it's closed down.

What the Dragonboard person told me was that theirs is sourced from a single factory, which their competitors don't. But no CE marking....

Would I be right in saying I could exchange the Icynene for Warmcel in my detail?
http://www.greenbuildingforum.co.uk/forum114/?PostBackAction=Download&AttachmentID=4211

I wouldn't have this fire resistance problem with Warmcel.

Posted By: ShevekHe also tells me their product is CE marked

Isnt CE a self cert marking, but I did hear that it could stand for 'China Export'.
https://en.wikipedia.org/wiki/CE_marking