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General: Standing seam mansard roof detail

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    • CommentAuthorShevek
    • CommentTimeJul 4th 2012
     
    This is a standing seam mansard roof detail I'm working on. See any problems? Is it okay if the mineral wool insulation has a higher u-value than the PIR over the rafters? Seem to remember you need to be cautious about this sort of thing.
    • CommentAuthorTimber
    • CommentTimeJul 4th 2012
     
    Yeah, you need enough of the foil faced rigid board on the outside to ensure that there is not a condensation risk on the inner foil face. In addition, you need to think about breathability of the timber structure. It currently had vapour impermiable insulation on both sides, so if it does get wet, or is wet from construction, it might have difficulty drying out.

    Only other real comment is what sort of plywood are you going to use?
    • CommentAuthorCWatters
    • CommentTimeJul 5th 2012
     
    Ditto on the plywood.

    Beware that the quality of WBP is very variable. It's meant to be waterproof but some of the cheap stuff will delaminate if you get a rain shower before you get the zinc on. One of the Builders merchants I used carries three grades. They informally refer to the worst as "cheap Far Eastern" and the best as "Marine ply", however it appears all their stock is made in the far East and their marine plywood wouldn't be considered suitable for boats. Ok for houses though.
    • CommentAuthorTimber
    • CommentTimeJul 5th 2012
     
    Yeah, plywood is going through a rough patch at the moment.

    WBP doesn't actually exist, neither really does marine plywood (p.s. anyone wondering about marine plywood, search for F1 powerboat racing crashes!)

    Look for a class 3 plywood to EN 636. That detials the durability of the glue bond. A Class 3 glue bond is equivilant to a WBP standard. Then you need to consider the durabilty of the timber used in the laminations, but normally a softwood plywood would be ok as long as the glue bond is sufficiently durable.

    If the plywood comes from the far east, I would not touch it with a barge pole (solid timber, not laminated obv!). Pay the money, get the good stuff from a good manufacturer, it is worth every penny.
    • CommentAuthorShevek
    • CommentTimeJul 5th 2012
     
    Thanks for the plywood tip. The other problem with plywood is that it's incompatible with zinc so you end up having to use zinc with a protected layer coating to the underside. The other option is to go with compatible timber boarding (such as Fir, Spruce, Pine or Poplar). Thing is we have some barrel vault roofs which will be a lot simpler to construct with 2 laters of 9 mm plywood.

    Actually that's a thought... how am I going to do the barrel vault areas with the PIR insulation? It's not going to work. :sad:
    • CommentAuthorTimber
    • CommentTimeJul 5th 2012
     
    It might do depending on the diameter of the barrel

    EPS might be a bit more bendy, again depending on radius.
    • CommentAuthorShevek
    • CommentTimeJul 5th 2012
     
    Perhaps mineral wool batts too? We're talking roughly a 3 m radius.
    • CommentAuthorShevek
    • CommentTimeJul 5th 2012 edited
     
    I phoned Kingspan and they can cut slots in the PIR insulation to go around the radius. However this lowers the u-value so you have to use thicker insulation. He also recommended the following build up to ensure the insulation doesn't get compressed when people walk on it:

    1. standing seam
    2. plywood
    3. batten
    4. breather membrane
    5. plywood
    6. board insulation
    7. vapour control layer
    8. plywood
    9. rafters

    Unfortunately this would decrease the amount of insulation we could put in between the rafters too, so we could be looking at 180 PIR over rafters.

    Perhaps a we might be better to go between and under rafters?
    • CommentAuthordjh
    • CommentTimeJul 5th 2012
     
    Posted By: ShevekThe other problem with plywood is that it's incompatible with zinc so you end up having to use zinc with a protected layer coating to the underside. The other option is to go with compatible timber boarding (such as Fir, Spruce, Pine or Poplar).

    What's the problem with plywood and zinc? I'm curious because there's lots of spruce plywood around, for example.

    I phoned Kingspan ... recommended the following build up

    2. plywood
    5. plywood
    8. plywood

    You sure he worked for Kingspan? He wasn't a plywood salesman! :devil:
    • CommentAuthorShevek
    • CommentTimeJul 5th 2012
     
    Yeah I know, a lot of plywood. Going to between and under instead. See attached.

    Not sure what the problem with plywood is. Just says so in manufacturer's literature (pg. 39):
    http://www.vmzinc.co.uk/build-zinc-systems/images/stories/documentation/standing_seam.pdf
    • CommentAuthorTimber
    • CommentTimeJul 5th 2012
     
    Does it? where, becuase I couldn't see it.

    It does say that if you have not provided an air void between the zinc and the substrait, you need to use the fully coated panel. This is because (as suggested in the literature) where there is an air void behind, the movement air in that void helps the zinc take on a patina that protects it. With no void, or chance of air movement, you need the coated zinc, if there is air movement then you don't.

    So it appears that it isn't that the zinc is not compatible with plywood, it is not compatible with a substrait that does not allow air movement over the rear face of the metal.
    • CommentAuthordjh
    • CommentTimeJul 6th 2012
     
    Looking at Appendix 6 in that document (pp80-81) it seems there is an issue with the pH of some wood species and with some types of glue used in engineered products, including those usually used in plywood.

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