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Housing - New Build: air tightness membrane - attaching around a metal chimney (fire sesistance issue)

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  1.  
    • CommentAuthorsplashie
    • CommentTimeApr 18th 2015
     
    Hello

    We have a lovely pellet stove to be fitted in our new build but have hit a snag with the air membrane. We have had advice that it's best, when going round a pipe, to cut a bit of ply or simmilar and put round the pipe and attach the membrane to this. - that way it's more robust and you just foam up the wee space left.

    This is fine except for a chimney as regs state you can't use flammable material right on to the chimney area.

    This must have happened to others sealing their air guard wound a chimney in a wall.

    cheers
    • CommentAuthorjfb
    • CommentTimeApr 18th 2015
     
    Hardibacker board instead of ply?
    • CommentAuthorsplashie
    • CommentTimeApr 18th 2015
     
    we could use cement board but the issue is how to you seal around that? you could tape the membrane round this far enough away, but what about the space between board and flue? usually we'd foam this, but my understanding is this is not fire resistant.
  4.  
    I used 'pink' expanding foam - this is not 'fire proof' but had a high temperature rating 300 deg C or more I think - cost 4 x more as I recall. No idea if this meets regs but it was good enough for me....
    • CommentAuthorjfb
    • CommentTimeApr 19th 2015
     
    you can get very high heat resistant mastics in a tube
  6.  
    You can use fire cement to seal between your cement board and the metal flue.
    • CommentAuthorCerisy
    • CommentTimeApr 19th 2015
     
    I bought some rockwool slabs with a foil face. I'll cut back the membrane, trim the rockwool to the flue and seal that with a fireproof mastic. I'll then tape the membrane to the outer edges of the foil face on the rockwool so the tape is at least 60 cm away from the flue.
    • CommentAuthorBMJT
    • CommentTimeNov 24th 2018
     
    Just reviving this thread as I've just come across a similar headache, and wondered if people had any new ideas?

    I'm replacing the ceiling below the loft in a Victorian house which has a chimney in one of the walls and i'm not sure how to detail this to satisfy both requirementsâ€â€e.g. airtightness and fire regs. I'm leaning towards OSB as the vapour check/airtightness layer as the plastic membranes could melt, but what should I pack the 40mm gap with on all sides without making it a detailing nightmare to retain air tightness?
  9.  
    Posted By: BMJTbut what should I pack the 40mm gap with on all sides without making it a detailing nightmare to retain air tightness?

    Just as a thought - high density rockwool soaked in water glass (sodium or potassium silicate)

    I'm not sure how vapour tight OSB is, but if the plastic membranes melt 40mm+ away from a brick chimney then you have problems other than a melting vapour barrier!
    •  
      CommentAuthordjh
    • CommentTimeNov 24th 2018
     
    Posted By: BMJTwhat should I pack the 40mm gap with on all sides without making it a detailing nightmare to retain air tightness?

    I'd probably look to reduce the gap to a mm or less by scribing some non-flammable sheet such as metal or fermacell or magnesium oxide etc, and then fill the remaining gap with fireclay or similar.
    • CommentAuthorBMJT
    • CommentTimeNov 25th 2018 edited
     
    Thanks for the ideas, will investigate further.

    I'm happy with the vapour tightness of the OSB, it's recommended in a lot of build-ups by different companies and it should be sufficiently more resistive than other materials in the floor (1:5 etc.)
    • CommentAuthorphiledge
    • CommentTimeNov 25th 2018
     
    You could put a sleeve of foil coated industrial pipe insulation round the flue and the seal to the outer foil surface. So long as the inner diameter is a sug fit round your flue youll have a pretty draught proof seal
  13.  
    Posted By: philedgeSo long as the inner diameter is a sug fit round your flue youll have a pretty draught proof seal

    Hmm - draught proof is not the same as air tight or a vapour barrier.
    • CommentAuthorBMJT
    • CommentTimeNov 25th 2018 edited
     
    Ok, might need to make my own thread for this. Wires are getting crossed.
  15.  
    BMJT, I think that might be a good idea. Could we have a pic in the new thread?

    You said: 'I'm replacing the ceiling below the loft in a Victorian house which has a chimney in one of the walls and i'm not sure how to detail this to satisfy both requirementsâ€â€e.g. airtightness and fire regs.''

    I 'see' a brick chimney breast, in which case I cannot see you having enough heat to cause an issue. Please give us a bit more detail. Thanks.
    • CommentAuthorphiledge
    • CommentTimeNov 25th 2018
     
    <blockquote><cite>Posted By: BMJT</cite>Ok, might need to make my own thread for this. Wires are getting crossed.</blockquote> just seen the date of the OP so ignore my post
  17.  
    For airtightness you can get an EPDM / metal chimney fitting that you can glue to the rest of your membrane (the rest of the membrane being sufficient distance from the flue. For insulation, you could use Rockwool or similar heatproof insulation below that membrane.

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