30 Aug '13, 8pm

What are your recomendations regarding rock wool, dead air space, reflective polyisocyanurate and vapor... #green

With out a vented roof deck, with Ice & Water shield on the exterior of the roof (or even #30 felt + shingles) there is no such thing as exterior drying. The I & W is 0.05 perms max, a felt and shingle stackup is about twice that, or ~0.1 perms. To have any chance at all of drying it needs to be more than 0.5 perms, but that's ignoring the fact that during at least 30% of all annual hours the surface of the shingles are wet with liquid water from snow/ice/rain/dew, which would prevent drying toward the exterior at any vapor permeance of those materials. Installing either polyethylene sheeting OR foil faced goods in an unvented assembly with Ice& Water Shield would create a severe moisture trap, whether there's an (unvented) air gap between the foil & fiber or not. Either of your solutions would all but guarantee moisture build up in an rot in the roof deck, probably sooner...

Full article: http://www.greenbuildingadvisor.com/community/forum/energ...

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