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Housing - New Build: Airtightness - How to make pitched roof airtight at eaves

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    • CommentAuthorPingy
    • CommentTimeSep 23rd 2008 edited
     
    I'm planning on using cellulose fibre insulation in a vaulted roof. To contain the insulation I'm proposing a roof construction as follows:

    Slates/lathes/counterbattens/breathable membrane/sarking board (either OSB or ply)/rafters/purlins/sub-rafters/OSB or ply boarding/battens/plasterboard.

    I'm using Keith's technique to increase insulation depth and reducing cold bridging by using sub-rafters below the purlins.

    The problem I'm having is trying to find an effective way of making the resulting 'box' for the insulation airtight at eaves level. The rafters will be left exposed externally for aesthetic reasons. The only thing I can think of is use pieces of ply between each rafter to close the gaps (sounds a tedious task).

    Anyone got any better ideas?
    • CommentAuthortony
    • CommentTimeSep 23rd 2008
     
    How about a noggin put in as you build approximately on top of the wall plate, so go noggin - rafter - noggin - rafter -etc keeping it all tight and square.

    I would be more concerned about keeping the building air tight than that box -- where can the air go if does get into the box?
    •  
      CommentAuthorrichy
    • CommentTimeSep 23rd 2008
     
    Use a vapour barrier just above plasterboard sealed into one on the walls?
    • CommentAuthorPingy
    • CommentTimeSep 25th 2008
     
    Tony, thanks for your suggestion, it sounds easier than trying to fill the gaps after the rafters are installed. My main concern was to contain the cellulose fibre within the roof as it will be installed using the pumped technique. I'm planning on using an air barrier within the service void created on the inside of the insulation. Should I be using polythene or a breathable membrane? Instinct tells me to avoid polythene but I could be wrong.
    • CommentAuthortony
    • CommentTimeSep 25th 2008
     
    I dont think that you can do it without a vapour control layer on the warm side of the insulation as the potential problems are too horrendous to contemplate.
    • CommentAuthorPingy
    • CommentTimeSep 25th 2008
     
    Is there an alternative to polythene?
    • CommentAuthortony
    • CommentTimeSep 25th 2008
     
    yes anything that is resistant to water vapour -- polythene is cheapest and easiest.

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