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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: So - why can't I put a warm roof on its side?

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    • CommentAuthorlineweight
    • CommentTimeOct 20th 2015
     
    Look up celotex/kingspan standard details/tech advice and they are happy for you to do a timber framed warm roof - EPDM on PIR insulation on ply deck on joists (with VCL at deck level).

    But turn that construction on its side, and what was the water and vapour tight EPDM layer now has to be a breather membrane. You can have your PIR insulation sheathing the outside of the timber frame, but you have to have breather membrane outside of that, and then a ventilated cavity before your final wall finish, whether that's a brickwork leaf or something like vertical tiling.

    Is there any logic to this?

    If there are concerns about errant moisture in the timber frame being able to dry out, why are they a worry in the wall situation but not in the roof situation?
    • CommentAuthorDarylP
    • CommentTimeOct 20th 2015
     
    .... err not quite. You can have the PIR on the o/s of the timber frame, with the (redundant) breather membrane between the 'external' PIR and the OSB sheathing.

    However you are correct, this is an anomaly with the VCL requirements of the TF industry... :devil:
    • CommentAuthorlineweight
    • CommentTimeOct 20th 2015
     
    Kingspan standard detail I'm looking at asks for BM between OSB/PIR *and also* on outside face of PIR, before battens to support vertical tiling.

    But I see that the one with brickwork outer leaf doesn't have BM outside PIR.
    • CommentAuthorringi
    • CommentTimeOct 20th 2015
     
    The VCL at deck level can be a single bit with no joints. On a wall there tend to be windows etc.

    Is it that they don't trust the VCL on walls?
    • CommentAuthorlineweight
    • CommentTimeOct 21st 2015
     
    Roofs often have windows in them too.
    • CommentAuthorTimber
    • CommentTimeOct 21st 2015
     
    Interesting question lineweight.

    There are two points really - technically yes I guess you could argue that a warm flat roof on its side as a wall would be fine. Not sure that I can think of a proper reason why it would be any different to a flat or mono pitched roof of the same construction.

    The counter point is that Approved Document C does state that a timber frame wall should have cladding, cavity and breather membrane in front of the structure/outer insulation layer.
    • CommentAuthorCWatters
    • CommentTimeOct 21st 2015
     
    Posted By: lineweightKingspan standard detail I'm looking at asks for BM between OSB/PIR *and also* on outside face of PIR, before battens to support vertical tiling.

    But I see that the one with brickwork outer leaf doesn't have BM outside PIR.


    Tiles aren't considered totally water tight, snow, fog rain can blow in so some sort of secondary protection is required.
    • CommentAuthorlineweight
    • CommentTimeOct 21st 2015
     
    Posted By: Timber

    The counter point is that Approved Document C does state that a timber frame wall should have cladding, cavity and breather membrane in front of the structure/outer insulation layer.


    True, the wording is

    "5.17 Any framed external wall will meet the requirement if the cladding is separated from the insulation or sheathing by a vented and drained cavity with a membrane that is vapour open, but resists the passage of liquid water, on the inside of the cavity (see Diagram 11)."

    But as generally with the approved documents that is a statement as to how the requirement can be met, rather than prescribing the only way it can be met.

    Maybe there aren't many people who want to put EPDM on a timber framed vertical wall, so the insulation manufacturers haven't considered it worthwhile proving such a system.
    • CommentAuthorringi
    • CommentTimeOct 21st 2015
     
    Maybe it is considered less of an issue if a roof deck needs replacing then if a wall does.

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