Moisture control and permiablity in CZ 4. A client is interested in a SIP roof as a retrofit for an older log home. Currently the plan is to strip the existing roof of asphalt and felt and repair as neede the existing osb. The build out calls for 1.) 30# felt 2.) 4-6″ SIP panel nail board 3.) 30# felt 4.) High albedo standiing seam metal roof. Is the first layer of felt a potential moisture collection issue, would the system be better off without it?
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Q. "Is the first layer of felt a potential moisture collection issue?"
Q. "Would the system be better off without it?"
A. It won't make any difference either way.
Instead of using SIPs, you might investigate nailbase. Nailbase panels have OSB on only one side, while SIPs have OSB on both sides.
Don't forget the Air Control Layer.
It is important to establish a continuous Air Control layer on the warm side of the thermal control layer.
Martin, what do you think about priming and taping the seams of the existing OSB before adding the over-roof?
Taping OSB is iffy at best. Especially if the OSB is old and flaky, it might be hard to tape.
But if the OSB is smooth and the flakes are relatively tight, you could tape the seams with a high-quality European tape after priming. That would certainly be a good idea.
I know that you are worried about convection currents in panel seams (which, like all convection currents, happen in three dimensions). However, I think that if Bruce tapes the seams on the new OSB (the OSB facing on the nailbase), it's highly unlikely that he'll have any problems. With no air exiting the top of the assembly, there won't be any air entering the bottom of the assembly.