Vapor control for metal roofing over SIP roof

Guest User
6/20/2009
I have a new structure with 12 1/4" structural insulated panel roof at a 2:12 pitch. (The low pitch is due to code which requires me to retain the snow load.) There is no attic space to ventilate and the roofing must be applied to the top surface of the SIP. Even though the roof has an R value of 50 and there is 11 1/4" of EPS foam between the panels, I have heard war stories from people who suffered moisture damage to the top skin (1/2" OSB) of the SIP after a few years. Ice dams are a common problem in this climate. What I know is that I will have to put some Ice and Water barrier to deal with ice dams and I need to have some way to prevent condensation on the inside. These seem like contradictory requirements. How do you suggest I handle this situation? Thanks
Todd Miller
Isaiah Industries, Inc.
6/21/2009
I think your only safe way is to add a vented airspace on top of the SIPs, followed by decking, underlayment and roofing. Make certain that the roof you install is approved for that pitch. I would suggest a mechanically seamed standing seam.
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