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Housing - New Build: Celotex underneath OSB in roof, how to fix?

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    • CommentAuthorHairlocks
    • CommentTimeFeb 12th 2012
     
    My Architect designed my house to use warmcel and I joists for the timber frame. When this cam to the roof the builder and timber frame company claimed you can't make a roof out of I joists. So instead of 300 mm think I joist with warmcel they decided to go with 220mm rafters, and 60mm of celotex They have now stopped trading and left me with lots of problems.

    It is a warm roof with a pitch of 45 degrees. The raffers have OSB on the inside and have the warmcel installed. How can I fix the celotex and plasterboard to the underneath. I am tempted to increase the celotex to say 90mm, but and I going to end up sizing the celotx based onthe wooden battens that are avaliable? 4x2 battens is starting to feel like overkill. Could I just fix the plaster board to the osb through the celotex?

    Thanks

    Hairlocks
    • CommentAuthortony
    • CommentTimeFeb 12th 2012
     
    Yes, aim for the joists for all joins and consider a vapour barrier on the warm side.

    what centres are the rafters?
    • CommentAuthorCav8andrew
    • CommentTimeFeb 12th 2012
     
    Another option might be to fix smaller battens through the celotex into the joists and then screw plasterboard to battens using standard plasterboard screws.
    • CommentAuthorHairlocks
    • CommentTimeFeb 12th 2012
     
    According to the warmcel people (execl fibre or whatever they are called) the Vapour barrier can go on the cold side of the celotex. I was going to put it there to protect it against damage later. There is not going to be a serivce cavity in the pitched roof bit.

    How big would the smaller battens need to be? Hitting the joists could be fun!
    • CommentAuthorHairlocks
    • CommentTimeFeb 12th 2012
     
    I would like to avoid cold brigdes as well if possible. So if I batten it I was going to counter batten it.
    • CommentAuthorMike George
    • CommentTimeFeb 12th 2012 edited
     
    • CommentAuthorCav8andrew
    • CommentTimeFeb 12th 2012
     
    Was just thinking about that! I was wondering if 25 x 50 (face) would be o.k. Maybe initially fixed to the celotex with a bead of high tack mastic then mechanically fixed. Batten needs to be deep enough to take a plasterboard screw so 25 mm should be enough I would have thought. Locating your joists I will leave to you!
    • CommentAuthorCav8andrew
    • CommentTimeFeb 12th 2012
     
    Sorry, I am wondering if I have misunderstood the roof buildup as I am not sure where the cold bridging could occur. My understanding is 200 mm deep joists fully filled with Warmcel, this is retained with OSB to the underside of the joists (not sure of thickness). There is then, or will be, a layer of celotex of 60 to 90 mm. I am then suggesting you fix a batten through the celotex into the joists. The plasterboard would then be screwed to the batten as per usual spec. You could in theory infill the 25 mm gap between the battens with more insulation or leave as a void, although you have said it is not required. If I am correct in describing your construction then I am unsure where a significant cold bridge would occur
    • CommentAuthortony
    • CommentTimeFeb 12th 2012
     
    through the 220 rafters, now somewhat mitigated by the sheet insulation.
    • CommentAuthorCav8andrew
    • CommentTimeFeb 13th 2012
     
    Exactly but it not really now an absolute cold bridge in the sense that it has a layer of insulation in the form of the celotex. It also seems that given that it is already constructed using solid timbers and not I beams, which with their additional depth and form of construction would have mitigated this effect, there is the option of increasing the depth of celotex to improve the situation. My confusion was how counter battens would help counter (sorry) this effect other than lift the batten off the insulation. Would not the effect of this be of minimal benefit and that it would be a better option to increase the thickness of celotex if necessary.
    • CommentAuthortony
    • CommentTimeFeb 13th 2012
     
    counter battens are a waste of materials, space and potentially and very likely will cause thermal bypass and greatly increase air leakage.
    • CommentAuthorHairlocks
    • CommentTimeFeb 15th 2012
     
    The cold bridging I was worried about was from lining up the battens with the raffers ending up with on thick piece of wood (in effect anyway) through both sets of insulation.

    Currently going to go for no battens and just use long drywall screws, can buy 100mm ones. The internet tell me only 5/8 inch, 15mm ish needs to go into the joist for a good fix (the OSB is 9mm). the plaster board will be 12.5 mm, so I should be fine with 60mm cellotex, anything more is probably pushing it.

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