Faced fiberglass batts directly against rigid foam board – GreenBuildingAdvisor

1912CentHome,

The kraft paper facing and foam will act as a vapour-retarder stopping moist interior air and vapour from moving into the rafter bays. There is no issue with having the two stacked up together. The very small amount of moisture that diffuses through the EPS will continue to move outward and diffuse through the Kraft paper. Kraft paper is 1 perm, but when any moisture accumulates it becomes more vapor-open (up to 4 perms) so moisture is not trapped.

You don’t say if you are planning on using baffles to keep the vent space open. If you are not, then I would suggest using either rockwool, or high density fiberglass batts to reduce wind-washing which reduces the R-value of permeable insulation.

I would consider not furring down the rafters, but rather installing the foam as a continuous layer under them. As this would reduce thermal bridging though the rafters, if you wanted you could use 1″ EPS rather than 1 1/2″. The effective R-value of the assembly would end up similar, and it is fairly easy to install drywall over 1″ foam.