ZIP sheathing + blown insulation vs osb sheathing + foam
I have a client that is questioning which is a better route…going with ZIP sheathing on the exterior and blown insulation for air tightness, or going with a standard 7/16″ osb and open cell foam on the interior for air tightness. Exterior foam would be installed in either situation. Zone 6 for construction.
I was really not sure what is the best recommendation on air sealing. Cost wise, it appears it will be almost the same.
I think I am leaning towards ZIP since I can continue the air barrier down the wall and over the sills and caulk it to the mud sill. Plus it seems to be able to be left exposed for a lot longer without the risk of building wraps ripping, like they always seem to do.
Anyone have any strong thoughts on the matter?
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Replies
For new construction foam insulation in stud cavities should not be considered an appropriate air barrier layer strategy. In a couple of field tests I've been witness to with a blower door running using smoke to find air leaks air can find ways through intersections of foam and framing, particularly where foam in applied between the window frame and rough opening. (I would recommend the practice of taping the window frame to the R.O. with a double sided butyl tape). Open cell spray foam also expands quickly and I have opened up walls to find good sized air pockets within the foam. I've also seen with my own eyes how open cell foam can shrink and pull away from the studs. In short foam doesn't guarantee a consistent air seal.
A project in my area recently achieved excellent air tightness results at an in progress stage due to in part using the exterior sheathing and taping all the joints. I'm not personally familiar with the ZIP system so can't speak to its quality.
Why use exterior foam(petroleum) when you could use more wood(sustainable) to increase blown insulation thickness? I like the zip sheathing over standard osb or plywood.
How about using a polyurethane injected SIP that has ZIP as the exterior skin? I used Eco-Panels 4.5" R-26 SIPS on my prototype home with ZIP as the exterior skin and my blower door test was .48 ach 50. There are no splines or studs and Eco-Panels has patented fully insulated and structural corners and panels that use a cam-lock system etc.
Craig