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General: Cold Bridging/Condensation Risk Advice.

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  1.  
    Sometime ago I posted on the Green Building Forum about insulating my dormer windows front/back & trying to stop the draughts coming in through the floor void at the upstairs floor junction. http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8395&page=1#Item_30

    After going through all the different options of how to do the work from internal, external, removing the soffits, getting prices from trades people for quotes, we've pretty much come up with a bit of a plan that I would like some advice on.

    We were reluctant to do the work internally at first as we had made the mistake of decorating/fitted furniture a part of the upstairs before we realised how cold & poorly insulated the dormers were. We then decided not to do no more until we had rectified this problem. After getting quotes from trades people to do the work externally (some over £10k) we've now decided to try to do the work internally after speaking to a plasterer friend & try to work around where we've had the fitted furniture if possible.

    Our plan is to remove the internal plasterboard from the stud walls/sloping ceiling/dormer cheeks & the flat roof part of the upstairs ceilings from the three bedrooms & bathroom. Seal & insulate the upstairs floor junction to stop the draughts. Insulate the stud walls/sloping ceiling/dormer cheeks & the flat roof part of the ceilings & then use foil backed plasterboard.

    My concern is as we haven't used insulated plasterboard over the studs/ceilings etc are we at risk of cold bridging/condensation in these areas??

    Thanks.
  2.  
    Hi,

    You say ''My concern is as we haven't used insulated plasterboard ...'' but before that you say ''Our plan is to ..... Insulate the stud walls/sloping ceiling/dormer cheeks & the flat roof part of the ceilings & then use foil backed plasterboard.''

    From that I took you to mean that you *are* proposing to insulate everything. Are you asking ''is insulated plasterboard better than insulation on its own with plasterboard on top?''?
    I would not often use insulated plasterboard now, though I did a lot in the past. Even if you make sure you use a board wwith a vapour Control Layer (VCL), the VCL does not exist at board junctions. Ditto with foil-backed pl'bd as a VCL. Better to use foil-faced Pu board and tape all joints and perimeters or, if, say, you are using quilt insuation, use a plastic VCL and ordinary pl'bd over.

    And when you say '' Insulate the stud walls/sloping ceiling/dormer cheeks & the flat roof part of the ceilings..'' how much insuation do you mean? On the sloping ceilings it should be a min of 125 Pu, so if you have a roof with non-breathable felt, that's a 50mm air-gap behind the tiles, 25mm beetween rafters and 100 under.
  3.  
    Hi Nick.

    Thanks for your reply.

    Sorry for the confusion but what I meant to say was that we were hoping to insulate between the studs/rafters only with foil faced insulation boards & then put foil backed plasterboard onto this. Due to fitted wardrobes & windows next to the dormer cheeks we were hoping that we could get away with no additional insulation on top of the studs/rafter etc. As for the thickness we would use on the sloping ceiling we wouldnt know till the plasterboard was removed & then then measure the rafters minus 50mm.

    Do you think this would work ok??
  4.  
    Hello again,
    If, as is probable, you have 75mm rafters, and you have a non-breathable felt, that leaves you only 25mm for Pu, and an unmitigated thermal bridge through every rafter. You will about halve your heat loss (U = c 0.9 adjusted for thermal bridging against 2.0 uninsulated), but Bldg Regs is 0.18 and the ideal is lower still. See the current 'scout hut' thread for discussion of condensation despite 25mm phenolic insulation (performance equal to or slightly better than Pu). Your humidity levels are prob much lower, but I'd still like you to get better U value if at all poss. Could you squeeze 50mm in under rafters? 25mm?

    Particularly if you can get nothing under rafters, I would use a plastic VCL, and not rely on foil-backed pl'bd.

    Edit: Sorry this is brief. I have written it 3 times, and lost it 2ce! Gets shorter each time!!
    • CommentAuthorlineweight
    • CommentTimeMay 28th 2012
     
    I'd agree that you should try and get some insulation under the rafters if at all possible.

    If you could post some photos it would make it easier for people to suggest ways of doing this without messing up your fitted furniture etc too much.
  6.  
    Yeah! Pictures please!
    • CommentAuthorOnTheEdgeOf
    • CommentTimeJun 2nd 2012 edited
     
    Sorry for the delay in posting back but I can't seem to upload any photos from my iPhone?? :cry:
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 3rd 2012
     
    Posted By: OnTheEdgeOfupload any photos from my iPhone

    Best to avoid Apple products, sorry, could not resist :bigsmile:
    • CommentAuthorOnTheEdgeOf
    • CommentTimeJun 13th 2012 edited
     
    .
    • CommentAuthortony
    • CommentTimeJun 13th 2012
     
    You can add insulation inside the stud walls to the dormer cheeks and then overlay with insulated plasterboard or sheet insulation then membrane then lining boards (plasterboard) coming out over a part of the window frame, I would.

    Taking the tile hanging would work and is not difficult nor is taking off roof tiles, then infill between studs and overlay with insulation, counter batten, batten and re hang tiles. I would do it from inside or fill all the voids with eps using a blower fan.

    this could be of use http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8395

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