Unvented roof underlayment choice
My questions are related to part of the reply Mr. Holladay made to the “Proper Underlayment for Unvented Asphalt Roof” question posted on 11-14-14.
The response noted the manufacturer’s requirement to use a vapor permeable barrier on sheathing for unvented roofs – in that case with open cell foam under the sheathing. I thought a vapor permeable barrier could put the sheathing at risk for moisture adsorption during cold weather from either side. Even if asphalt shingles are indeed a low permeance barrier, will the potential vapor driven moisture passed by either the underlayment or the open cell foam build up in the sheathing and be trapped far too long to ever escape during the seasonal drying period? Shouldn’t both sides be impermeable to prevent this?
I am very concerned given my current roof plans in zone 6, dry alpine, 7500HDD.
The unvented roof profile planned is truss or frame, sheathing, cover with impermeable underlayment, then 3″ Polyiso, 6″ EPS nailbase, HT impermeable water shield, possible furring sleepers, metal roofing. The overall design intent (like REMOTE) is for the entire structure to be secured inside a WRB with out-sulation providing 2/3 of the total wall/roof values. Unfortunately, by code in my building location, I am required to use the equivalent of high temp ice and water shield from eave to ridge under metal (for insurance in fire area) roofing.
I have considered Grace Triflex and equivalents for the primary underlayment to save costs, but the lack of a seal at penetrations by the numerous long screws for the foam build up, leaves me queasy. Many of the discussions about vented and unvented roofs with outsulation frequently do not even note the presence of a WRB under the insulation sheets. This would seem to invite moisture transport from inside the house to the inevitable voids in the sheathing and foam stack up. Very few, if any, of the many offered diagrams for unvented roofs, here and elsewhere, make specific statements of WRB vapor permeance.
I have searched the extensive range of related prior discussions again and I must say I am now re-confused about the permeable vs. impermeable question. The REMOTE manual itself mentions use of bituthene sheet, poly sheet and Tyvek with generally okay results on walls, but I am unaware of double WRB layers occurring on roofs beyond the inherent properties of the foam stacks themselves.
Are all the superinsulated roofs going to get soggy? Will the foam get soggy? Is air likely to drive outward from living space into the top side foam?