Above Sheathing Ventilation

Eric Thompson
4/24/2017
Hi I'm an architect considering a standing seam metal roof for a project I'm working on here in Columbus, Ohio. Our contractor has suggested that we need to use an above sheathing ventilation system if we go with a metal roof. My question is is this necessary in our climate? I would prefer not to include it, though I do understand that it might improve energy efficiency. I have designed metal roofs in the past, in more extreme climates (New Hampshire) and have *not* included this, so my sense is it is not required. I should also say that we are planning a few inches of SPF insulation on the underside of the sheathing and then batts to make an insulation value of at least R-30 or more, so the assembly will be well insulated. We do plan to check that the dew-point will not occur on the inboard side of the SPF. Thanks
Todd Miller
Isaiah Industries, Inc.
4/24/2017
Thanks for your post. This does not sound like an application that would need ASV. It sounds like there is no ventilation but there is good insulation and a vapor barrier That should suffice in your climate, assuming the building does not have extremely high moisture levels for some reason.
Eric Novotny
An informed customer is the Best Customer!
4/25/2017
+1 Based on the insulation, climate, and roof construction (i.e. sealed/hot roof), I do not see any necessity for above deck venting. Does it make the roof work a bit better, sure. Is it overkill here, probably.
Eric Thompson
4/25/2017
Thanks! --Eric

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