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Heating and cooling: Insulation round flue through a pitched ceiling

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    • CommentAuthornprior
    • CommentTimeJul 12th 2018
     
    We are having a new stove and flue installed in a room with a full height pitched ceiling ie the flue exits from the room directly to the outside through the roof. Clearly we have to have suitable clearance all round the flue to anything combustible. This then leaves us with a minimum 50mm gap around the flue pipe with only the roof flashing between us and all the cold air outside.

    Having spent much time in our original build preventing our builder from punching holes in the envelope at will, I'm reluctant to end up with such a large hole in the roof (from a thermal point of view anyway).

    Stove installers I have spoken to have been fairly insistent that they would not normally fill the resulting cavity in the roof with anything, not even nonflammable insulation. Reasons vary from "never done it" to "you'll see the insulation poking through the trim plate". My guess is they haven't had to think too hard about air tightness but I may be maligning them.....

    So: can we just stuff a load of nonflammable insulation wool round the flue in the hole through the roof? If so what would you recommend we use? If not, why not?

    (Exiting through the wall is no better option - timber frame)
    • CommentAuthornprior
    • CommentTimeJul 12th 2018
     
    Should have searched more diligently: this thread seems pertinent: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8302&page=1#Item_8
  3.  
    The thread you found could be relevant - it depends upon the type of flue you are using. When asking questions detail like this is important.
    • CommentAuthorowlman
    • CommentTimeJul 13th 2018
     
    I had a similar situation in my sunroom. As Peter said what type of flue is it, mine's twinwall insulated. Also, what is the ceiling underdrawn with?
    • CommentAuthornprior
    • CommentTimeJul 13th 2018
     
    The flue is Docherty Group's Excelsior Plus, a twin wall insulated flue. I can't find anything in their documentation about sealing exit holes in solid roofs.

    Not sure what underdrawn means. The roof is insulated with 6" foam board between rafters with a further layer of insulated plasterboard giving another inch of foam board insulation. Then being in Scotland the roofs is sarked and is finished with a breathable membrane.
    • CommentAuthorowlman
    • CommentTimeJul 13th 2018
     
    I'd kind of guessed plasterboard finish.
    My sunroom has large fairly deep polished wood rafters with a painted T&G effect ( actually machined MDF) recessed/set back, boarding between. Where my twinwall exits I omitted the boarding at an appropriated distance from the flue and replaced it with suitable FR sheet material close fitting around the flue and then painted it to match the T&G. You could do similar by cutting out a section of PB and replacing it with some FR material. Skim it if necessary and paint into the rest of the ceiling. You could also put a SS collar round the flue to finish it off, if needed.

    PS. To finish the job access the flue from the top and pour vermiculite into the void, or stuff insulation from below before sealing.
    PPS. The heat from the twinwall could melt the insulation on the plasterboard so the above solution would solve that also. Stove installers generally as you say, may not be interested in fitting niceties as I describe, either from the aesthetic or the practical side.
    • CommentAuthortony
    • CommentTimeJul 13th 2018
     
    In my book , no wood within 50mm of the flue pipe, no sheet insulation within 300mm of the flue pipe (or down lighters) and don’t use fire retarded insulation rather non combustible insulation instead.
  8.  
    I am not au fait with Scottish regs, but the installation surely has to comply. What do Building Control say?
    • CommentAuthorowlman
    • CommentTimeJul 17th 2018
     
    Usually Nick, AFAIK, only in relation to combustible surface distance.

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