2x4 wall buildups with polyiso/air barriers/vapor retarders
I am building a 16x40 / 2 storey residential addition in climate zone 7 (Columbia Valley, BC Canada)
My wall section that I am currently specing is as such:
2x4 framing, c/w roxul R14 batt insulation
1/2" plywood sheathing
3/4" rain strapping
My question is mostly concerned with the right polyiso product to use and how this will affect whether or not we have to install an interior vapor retarder, and if so which class - 1, 2 or 3?
A lot of this will depend I imagine on the type of polyiso and their respective perm ratings ie foil back (atlas energy shield), fiberglass (Atlas rboard) or basic unfaced, cardboard backed.
Also whether or not we then tape and caulk all seams of the polyiso to make it air tight.
If the energy shield or R-board polysio is taped and detailed properly it can also be used as the WRB and Air barrier, otherwise I would need to employ a housewrap of some description between the polyiso and the rain screen.
My choice would be to use either the energy shield or Rboard, taped, flashed and detailed to be the effective air and water barrier and eliminate both the exterior house wrap and interior vapor retarder, although class 3 could be used.
The local inspector and code still prescribe a VR of some sort but I think we could remove it and we would lessen the risk of "double bagging" the wall assembly which would not allow it to breathe.
I know this is long winded but I hope it makes sense to someone out there who can lend me their advice on the best way to proceed. I am almost done framing and will soon need to decide on the polyiso in order to start getting my windows in.
Posted Sep 3, 2014 5:12 PM ET
Edited Sep 5, 2014 1:11 PM ET
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