Trying to determine best assembly to insulate an old church roof being converted to office
We are looking to keep the existing roof decking and rafters exposed in a church in Atlanta, Climate Zone 3. We have read Martin Holladay’s FHB Article about unvented, Super Insulated Roofs, and Joe LUtiburek’s article BSI-036 Complex 3D Air Flows talking about the subject. One of the big differences (really somewhat minor) I see between the two recommended details is the best place to locate the fully adhered air barrier. Martin shows placing it at the outermost layer right on top of the new sheathing, right below the roofing material. In this case asphalt shingles. Joe showed it layed first, directly applied to the existing roof deck with multiple layers of rigid insulation and new sheathing for nailing surface above. Curious what the best apporoach is for our climate zone.
We are shooting for R-20, expecting to get to that with two layers of rigid. We will definitely stagger the seams. I saw that Martin also called out to tape and seal the seams at the top layer of the rigid insulation. Is that an insurance policy, or a very critical part of the assembly. I’m wondering about the ability of the top layer of OSB to ever breathe and dry out should it get wet. It would seem that it couldn’t dry downward toward the rigid, and the fully adhered membrane would keep moisture from going upward.
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