Hi–
I got a weird remodel and looking for advice.  I have 1976 ranch in the puget sound area, IECC 4C and it has fiberboard sheathing with ply in the corners for shear strength.  When we vaulted the living room, we reframed with ply sheathing and since all the drywall have been removed I’m trying to determine what I should do with the 2×4 bays that are sheathed with the fiberboard.  

The exterior is 48 year old clear cedar in good shape and want to keep it rather than pull everything off and re-side the entire house.  Looking at the fiberboard it’s 1/2″ and from some sections removed during demo, there is no way it’s close to airtight material like ply or OSB.  Plus seeing some the different in dust/debris between the fiberboard bays (none with outlets)  and the plywood bays, leads me to believe it’s air leaky as heck to the outside.

So one idea is cut 1/2″ polyiso and mount that on top of the fiberboard insulation and seal the edges with spray foam, creating the vapor barrier there and filling the rest of the with unfaced R15 insulation. The insulation will lose some R-value due the compression to 3″, but my wall just need to meet R-15 and the Rmax polyiso is rated R3.2 for 1/2″ 

I’m concerned that the wall may need airflow between the polyiso and the fiberboard, but seeing how much air filtration I got in the bays with fiberboard vs plywood… it may be OK, especially with the hard vapor barrier and unfaced insulation and drywall. 

Any other ideas?  Or am I overestimating the permeability of the fiberboard? 

Thanks,
Ron