External Comfortboard: vapour/air barriers
Hello! I live on Vancouver Island in Marine climate zone 4. I am building a shop 20′ by 50′ with a simple shed roof oriented south that will carry 40 solar panels. There will be no windows. Trusses with a 3 in 12 slope have been designed.
If we do 2 by 6 construction with external Comfortboard 80, what would you recommend as far as placement and type of air/vapour barrier? I plan on 3/4 inch plywood on the interior walls and rockwool bats. I am also planning on running wiring through conduit on the internal surface of the wall to prevent any unnecessary intrusions through the internal surface.
The Rockwool installation manual recommends: (Exterior to interior)
• Cladding • Drained/vented cavity • Wood strapping, screwed through insulation • ROCKWOOL COMFORTBOARD™ 80 rigid insulation (thickness to meet R-value requirement) • Vapour-permeable sheathing membrane (air-barrier) • Sheathing (plywood or OSB) • 2×4 or 2×6 wood framing with ROCKWOOL COMFORTBATT® insulation • Polyethylene sheet vapour barrier (cold climates only) • Gypsum board and interior finish
It seems strange that they want a poly sheet inside; this goes against everything I’ve read about external insulation. Is it different for this material because it is vapour permeable? I guess these walls would dry to the outside?
I was thinking a combined air/vapour barrier on the sheathing under the Comfortboard would be best instead of a vapour permeable sheathing membrane – perhaps a paintable product?
Thanks. I’m a home owner, not an experienced builder and will be getting help with the contruction from an experienced builder but I wanted to try to get these questions answered before we submit drawings.
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