Unvented roof assemblies and roof deck vapor permeance
As a long time visitor and few years member, I have tried to absorb the lessons, opinions, science and conjecture presented here. However, I now find myself confronted with conflicting theory regarding unvented roof profiles.
For my own project, (high altitude, dry, -10 to 90 F) I have regarded the "top" layer of sheathing in an unvented assembly to be essentially in dead space - not exchanging air or moisture - the underside in contact or glued to impermeable insulation, the top side sealed off from all contact with the locally required ice and water shield over the entire roof.
Two current discussions present thinking I am now confused by.
I have already chimed in on the latter, unfortunately before reading the breathable underlayment one. Unfortunately, builders with years of experience seem to be in conflict in both discussions. The thoughts and theories I have derived from this site's expert sources are perhaps incomplete at best.
So, how do I interpret the GBA details of unvented, stacked polyiso sandwiches with mixed ice/water shield and ordinary underlayment with no "venting" under both metal and asphalt roofs?
Posted Jul 24, 2014 6:49 PM ET
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