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Can Rockwool be installed over Type II EPS?
Brian Bonnell
March 30, 2020Updated: Sept. 10, 2020
I am replacing the windows and siding on my house built in 1994. The wall construction from interior to exterior is 1/2 " drywall, 2x3 strapping filled with fibergalss insulation, 6 mil vapor barrier, 2x6 wall with fiberglass insulation, 5/8 plywood, 2 1/2 " Type II EPS. Tyvek, siding. I intend to remove the EPS and install a self-adhering WRB on the plywood. Since I have the EPS will I create any problems if I re-install it and add 1 1/2" Rockwool Comfortboard 80 over the EPS?
When insulating the exterior of a house whether that’s a new home or adding insulation to an existing exterior wall, it is important not to trap moisture. Rockwool insulation is one of our preferred methods for adding insulation to the outside of a house because it is very resistant to moisture damage, and moisture moves through it easily. This page better explains the properties of each type of insulation –
The difference between Polyiso, EPS & XPS Foam Insulation & Styrofoam
Sometimes adding XPS or EPS rigid foam to the outside of a house can cause problems since it can trap moisture. That said, it is more of a problem when it is only a thin layer of foam. So I think the idea of putting an additional layer of Rockwool insulation on the outside of the EPS rigid foam is actually a really good idea, it will keep the EPS warmer, and effectively move it further into the wall assembly which will reduce the negative impacts of how it can stop moisture movement.