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    • CommentAuthorCWatters
    • CommentTimeAug 27th 2017
    The 6th combination (Rock wool and ACM with a limited combustibility filler) passed..



    So overall its looking as expected. Rockwool is safer than PIR but some combinations of PIR and cladding are ok.
    • CommentAuthorCWatters
    • CommentTimeAug 27th 2017 edited
    Updated table
    • CommentAuthorTimSmall
    • CommentTimeSep 1st 2017 edited
    This has made me think about the 2nd/3rd floor junction on my house (30s semi, retrofitted).

    Loft conversion (second floor) dormer is (in-to-out):

    Firecheck board
    100mm PIR between studs
    18mm OSB3
    100mm PIR
    Breather membrane (PE based?)
    44x44 batten (bolted through to OSB with 6mm stainless stud and 40mm washers at either end)
    Limited combustibility render carrier board ("Reaction to fire: Class '0'") - Euroform Rendaboard.
    Weber render

    Below the dormer is a little 200mm stub of pitched roof (effectively the build-up above is continued, but on the pitch instead, with fibre-cement slates instead of the render), like this:


    The windows (uPVC) of the floor below are tight up against the roof.

    So this is a bit like the 14th of Aug "passed" test except with:

    breather membrane, timber battens, no intumescent fire stop. BCO seemed OK with it at the time, but it was not on the original plans.

    Mains interlinked smoke alarms in all rooms (in excess of the minimum requirement of just hallways).

    This isn't a high rise, but it is second floor. I'd imagine there are lots of similar dormers (but perhaps the external PIR is a bit unusual, but would OSB inside the ventilated cavity fare any better in a fire?).

    What can I change (without re rendering the dormer)? Nothing much. I suppose I can try and take out and refit the first floor windows, and replace the 20cm of PIR on the pitch with rockwool (but the slates tuck under the rendaboard, so that's going to be tricky)? Perhaps add intumescent ventilated cavity closers here too?

    I could Improve fire lining around the PVC windows too at the same time, but only really useful if I also get rid of the ventilated cavity intakes at the top of the window too on the PVC soffit board, and make that solid and fire resisting too?

    Thoughts? The whole protected exit way seems like a charade to me, since there's no requirement for door closers etc. The 1st floor ceiling (timber joists) is designed for 30 minutes fire protection. How would that compare with time for the fire to spread externally, and break into the dormer, probably not a lot of difference?

    • CommentAuthortony
    • CommentTimeSep 1st 2017
    Make sure the smoke detectors are tested regularly (bimonthly) and you will be ok
    • CommentAuthorlineweight
    • CommentTimeSep 18th 2017
    I don't think it's a comparable situation with a Grenfell scenario unless external fire spread would rapidly fill your escape route (rather than the room) with smoke.

    Also if it's rendered outer layer it's more a cavity situation than rainscreen gap.
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