Exterior Rockwool between WRB and polyiso – GreenBuildingAdvisor

Hello All:

We are building our house with all exterior continuous insulation, nothing in the stud walls. We’re in western NY State, zone 6. Our current plan for the wall layers is:

1) stud wall

2) zip system sheathing

3) 1″ Rockwool Comfortboard – for fire resistance plus insulation

4) 2-1/2″ polyiso

5) 3/4″ furring strips / rainscreen gap

6) exterior cladding

The roof will be essentially the same but with 4″ polyiso and 1-1/2″ rainscreen gap.

My question is about the permeable Rockwool between the WRB of the sheathing and the impermeable polyiso. Is there a risk that the Rockwool will get wet then fail to dry in either direction then cause some kind of problems?

A related question is about the necessity of the Rockwool for fire resistance. We’re concerned about the possibility of fire in the polyiso, especially given the vertical air gap of the rainscreen, but is this a valid concern in a 2 story house?