I am wanting to use rockwool insulation (R-23) for insulating the exterior walls of a home that has 2×6 wall framing. Zip system sheating was used on the exterior of the home. I am getting mixed reviews on if i need a vapor barrier on the interior side of the wall. House is located in climate zone 5.
I’ve been building in zone 5 for 20+ years. If you install plastic sheeting or foil you will have major condensation problems. All the drywallers in my area will not warranty their work if installing over those barriers. You have to install a breathable barrier, so moisture can dry out. There are quite few breathable barriers available on the market that you will need to order from a building supply house or insulating company (do not have it at big box stores). I truthfully would not install any since you are doing rockwool and zip sheathing. Both excellent products with good moisture handling. The wool will not absorb moisture, but any moisture in wall cavity needs to be drawn back out into the interior to be absorbed in atmosphere of the home and removed with forced air. It’s critical that you air seal the outside as much as possible and install zip tape properly. Also, you don’t see it a lot in zone 5, but if you install 1x3 strapping after insulation install it will help create an air space for wall cavity to dry out. You also help solve the problem of thermal bridging. Another big thing is to make sure your hvac person is installing a properly sized unit to handle load on your home. If they install too large of a unit, it will not run long enough to remove moisture.
Thanks for the helpful info, I was leaning toward not installing any since we are using the zip sheathing on the exterior of the home, which itself is a very good air barrier itself when taped properly. I had the same thoughts on the rockwool with the moisture handling. The rockwool is mainly being installed on the north and west walls of the home where most of the wind occurs with the colder weather.
Most interior paint will get you the code-required vapor retardation.