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Permeance of fiber-faced Polyiso panels?

dan7210329 | Posted in General Questions on

I just had installed a new metal standing seam roof over Dupont Tyvek Protec 160, Perm <.05, over plywood decking.  House is in Mixed humid, zone 4, moisture regime A. 
Below the previous asphalt roof, attached to the bottom of roof rafters, was 3/4″ fiber-faced poly-isocyanurate panels, installed about 1995.   Anyone want to hazard a guess at the permeance of such a Poly-iso product?  I know my new roof needs to be vapor permeable from below, given the low permeability of the new metal/tyvek/plywood roof.  Rather than throw out the Poly-ico board, I’d like to keep it installed, but need to know it won’t be locking up moisture in the rafter bays, which are closed and can’t be opened.

Researching non-foil-faced PolyIso products I see product perms about 1.0 perm.  Should I risk keeping the PolyIso in place?  Ugh, I’d hate to throw out this stuff into the trash.

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  1. Expert Member
    Akos | | #1

    This is a hard number to find as most manufactuers only provide a max, no minimum. I know the the stuff I can easily get locally that has glass facers is 1 perm. If it is coated with anything it would be less but sill permeable.

    It is hard to say that you'll be locking moisture into your rafter bays, the assembly you have is considered risky in colder climates. In your climate, I think the BSC folks recommend either open cell SPF with vapor retarder or closed cell spf for unvented roofs.

    You can search through their site to see if there is an assembly that better fits your roof.

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