To Vent or Not to Vent a Low-Slope Roof
Long time reader first time question asker. I’ve really benefited a lot from reading these forums! I’m working in climate zone 5.
Here’s the question. Design calls for a low slope roof with a roof deck above. I’ve read up a lot on this type of assembly (in this forum and elsewhere) and have done my best at mitigating some of the common issues, but can’t help shake the feeling that I’m scoring a bit of an own goal with the way I’m going about this. I understand that a thick layer of closed cell foam on the underside of the deck is a successful approach.
So, the insulation at the 2×12 roof joists will be closed cell to mitigate moisture issues coming from below. Then there will be 2x sleepers to achieve the slope before the roof decking proper and membrane. My question is what to do with the space between these sleepers. My thought is to drill a few holes through the curb to provide some modicum of venting to the cavity between the roofing membrane and the closed cell foam (moisture that get’s in this space won’t dry in on account of the closed cell, and it won’t dry out on account of the membrane) – these holes would lead to the space behind the vertically oriented corrugated metal siding (a siding material that makes it’s own rain gap!) and would be fairly well protected from bulk water entry. Alternatively, I suppose the cavity spaces could be filled with insulation (rigid board would be safest, I would think). Lastly, go back to the drawing board and just slope the joists to eliminate the need for sleepers (although for project specific reasons, I would like to avoid this).
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