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How do I attach 6″ of rigid foam insulation to a roof deck?

George Champlin | Posted in Green Building Techniques on

July 2011 Fine Homebuilding’s article about roof venting included a method for insulating cathedral ceilinged roofs with 6″ of rigid foam topped with plywood sheathing and conventional shingles. I have a roof deck of 2×6 T&G on a post and beam constructed roof. If I want to use three 2″ sheets of XPS covered with plywood, how do I attach the foam/plywood sandwich to the roof deck? Code requires a 6″ edge and 12″ field fastener pattern.

FHB example:
http://www.finehomebuilding.com/CMS/uploadedimages/Fine_Homebuilding/Articles/221/021221071-3.jpg

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Replies

  1. TJ Elder | | #1

    George,

    Use long screws to attach the plywood. Here's a value priced screw that would work: http://www.bestmaterials.com/detail.aspx?ID=16720

    Polyiso would be a better choice than XPS due to its higher R-value and much lower global warming potential.

  2. George Champlin | | #2

    Your link is for the #14 x 9" screw. I think an 8" would be the right length. What do you think of using a #12 screw. This is a low-pitched roof, (maybe 4/12) so there isn't a lot of shear force.
    Any tips on keeping the sandwich from slipping before it's screwed together. Maybe tack each layer in place with a few screws?

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