I am in the middle of a deep energy retrofit of a three story 1880s farm house. The bottom 2 stories our wall assembly will be double studs with 8 inches of cellulose, 140 year old sheathing, Blueskin, 3 inches of GPS foam, and then siding.

For a couple of reasons we are considering making insulation strategy modifications to the 3rd story which itself was an addition by a previous owner. We will be using closed cell foam on the underside of the roof and have started to consider using closed cell in the stud bays of the 3rd floor walls as well.  We would still like to use the 3 inches of GPS on the outside in which would give those walls a similar R value to the double studded ones downstairs but would no longer have the ability to dry to the interior. The new assembly would be 4 inches of closed cell, 80 year old T&G sheathing, Blueskin, 3 inches of GPS foam, and then siding. 

Neopor GPS is rated to 4 perms for one inch material, but we are using 3 inches. Am I asking for trouble with a wall design like this or would this be acceptable?