Can you put exterior insulation over Zip System plywood?
Building a house and have zip plywood on the exterior (not zip-r which is insulated) & want to add 1" rigid foam to the exterior with open cell on the interior. Or 2" closed cell with r13 bath insulation. Which combination above would be best? Also would there be an issue with installing one inch foam directly to zip? Planning on attaching until siding directly to the foam.
The Zip System sheathing is a combination product with a weather barrier integrated right into it, and it is already a bit less permeable to moisture than typical house wrap weather barriers. Adding an inch of foam will make it even less permeable. There is a lot of good about the Zip System, but vapor permeability is not one of them and it is a bit of a concern.
Depending on where you are, building codes have in some regions improved thermal requirements in the last several years and now you often see sheathing with foam attached. This helps get a wall system up to minimum requirements of code, but it can in effect mean you are adding a second, and exterior vapor barrier, which is not a good thing in cold climates as it prevents walls from drying out properly. Here are a few pages that cover it in more detail:
OSB or Plywood sheathing for Roofs, Walls Which is Better?
Choosing a House wrap, Which is Best?
The thing is, in northern cold climates, hybrid sheathing with weather barriers and /or insulation attached is quite common place now, so its not like you are doing something out of the ordinary. It just leaves a few things unanswered for me, and living in a very cold climate I’m not a huge fan of products that increase the risk of trapping moisture in walls. As an alternative, what about using rigid mineral wool insulation? It lets moisture pass right through it, so it wouldn’t change the vapor profile of the wall at all.