For exterior rock wool continuous insulation, you must control the vapor flow
This Q&A is from a Building Science Fundamentals class taught by Dr. Joseph Lstiburek, released in the form of a "Building Science Brief."
Tim has a question: Is it okay to use Rockwool on the outside of the wall in lieu of foam board, given that Rockwooll is more air permeable than foam board?
Yes, it is if the rock wool is on something like plywood or OSB. If it's on the gypsum board, the water vapor is going to blow through the board like shit through a goose (and through the Rockwool). And you're going to have a problem on your cladding, so what we need to have with Rockwool is a vapor throttle, and so I wrote this as a building science Insight. It's called “Throttling the Flow.”
And by the way, I love the Rockwool folks, okay? And there are stories that I can tell you, some of which are true, so I go out of my way to make sure that something like Rockwool can actually work in roofs and walls, but it's so Vapor open I need a vapor throttle. Now, plywood and OSB are great vapor throttles; gypsum board is not. I might have to coat the gypsum board to reduce the Vaper transmission to slow it down so it doesn't blow through the Rockwool too quickly.