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I am working on the design details for my house in the the Pacific Northwest. The roof assembly will be 12 1/4" SIP (OSB inside cellulose filled cavity, fibreboard outside with a waterproof vapour permeable membrane).
I am wondering if I need to add a vented detail made of 2x4 or 1x4 and then attach the standing seam panels or apply secondary fibre mat (creating a small air gap) membrane and attach the standing seam panels onto that.
Maybe somebody could comment on the attachment of metal panels against fibreboard as well. Thanks for your insight
Overall, this sounds good. Make sure that the seams between the SIP panels are done properly to prevent moisture migration. I'd also suggest some sort of vapor barrier, even an oil base paint, on the underside of the SIP panels. As far as the fibreboard outer decking ... help me understand for certain what that is. Is it a cement fibreboard or are you referring to an engineered wood particle board? On the latter, the key is keeping moisture / condensation out of it. The moisture would be from inside the living space trying to drive its way out. The insulation will help with the potential temperature differential while proper sealing prevents moisture migration. If all of that is done well, ventilation should not be necessary. That said, the secondary mat you mentioned before applying the standing seam can be a "belt and suspenders" approach by creating a thermal break and helping prevent the outer decking from getting cold enough to create condensation with any moisture that is driving out from the living space.
Cellulose in sealed systems is not usually the choice because of it sensitivity to moisture.
I would absolutely recommend some sort of additional and continuous vapor control layer on the inside of the home.
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