On an exterior walls sheathed with 1/2″ ply, asphalt felt, and ribbed steel exterior siding – Where will condensation form?
I’m in the northern part of climate zone 6B, approx 80 miles south of climate zone 7B.
I’m in the process of finishing my insulated and heated workshop. The walls are 2×8 framed with 2×4 cross strapping, so the insulation thickness of the walls will be 8.75″ at the full depth areas. The exterior sheathing is 1/2″ cdx with Zip tape at all seams, followed by astm #15 asphalt felt, with screw down vertical rib exterior steel siding. Windows are triple pane with a U=.19 rating. The interior finished wall surface is 7/16″ acx plywood that would be only nailed to the framing members. No caulk or tape to air seal.
I’ve been interviewing insulation companies and have finally reached a company that really seems to understand energy efficient construction methods and insulating methods. His company has a track record of several homes testing at a .65 ACH or better without spray foam, and his previous clients seem extremely satisfied. He is proposing to install a blown fiberglass bib system with an approximate full depth cavity r-value of R-34.
However, he is adamantly recommending that an interior vapor retarder being installed. His recommendation is to install continuous poly from bottom plate to top plate, sealed at the edges with Tremco acoustic sealant. The poly rolls would be wide enough to cover the entire wall without a horizontal seam. Any vertical seams in the field would be taped. His concern is moisture from the interior reaching the cold exterior sheathing and condensing. I was planning on no interior vapor retarder, under the assumption that the interior plywood will function as a smart vapor retarder. It concerns me that the exterior steel siding is likely a vapor barrier. I’ve been trying to figure out how much benefit (if any) that the ribs help the siding function as a vented rainscreen. Google searches have been fruitless, but I don’t think I can count on much drying to the exterior.
Where do you think that moisture would condense in this assembly? Does it condense on the nearest cold surface (i.e. interior side of the exterior sheathing) or do you think it would condense on the interior side of the steel siding? Condensation on the siding would probably be harmless, as it would be able to follow the asphalt felt and flow out the bottom of the ribbed siding. If it condenses on the backside of the plywood sheathing, I could potentially have rotten, wet sheathing.
Because the space is a shop (no cooking, no showers, etc) there are few sources of interior moisture. I still want my walls to be as resilient as possible. Thoughts?
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