8″ of exterior mineral wool (Comfortboard 80)
I am in zone 7 (Northern Minnesota). Looking at building timber frame structure without SIPs. I love the idea of using Roxul Comfortboard 80 for exterior insulation. I want to avoid foam as much as reasonably possible even though I will most likely need to use it for the roof. Wall assembly would be 2x4s sistered onto outside of timber frame, exterior plywood, WRB (Delta Vent SA), 8″ of Roxul Comfortboard 80, vertical furring strips and then siding.
I have not seen example of anyone using 8″ of mineral wool on exterior wall. This would be 2 layers of 3″ and 1 of 2″. 8″ would give me approximately R34. Adding mineral wool between 2x4s would give at least R40. The city of Vancouver in British Columbia has an excellent document http://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf that details how to attach 8″ of exterior mineral wool.
Besides worrying about hitting a stud through the furring strips, is there any reason why this wall would not be feasible or performant? Anyone used 8″ of Comfortboard 80 on exterior?