Follow-up issue with 6 inches of open-cell foam
Following up on the earlier question someone had about 6 inches of open cell foam on the underside of the roof. Icynene is saying 5 1/2 inches of open cell spray foam (R20) is equal in energy efficiency as R38 cellulose or fiberglass because the foam blocks all air infiltration as opposed to the cellulose or fiberglass. I may be over simplifying the answer so you can read it for yourself here. I hope this link works.
I have a question about this. If I do the spray foam and create a conditioned attic space can I add unfaced fiberglass batts or cellulose at a later date to the floor of my attic or will 2 layers of insulation cause an issue? I could raise the total R value to R38 or R50, or whatever is required. I can’t imagine what kinds of problems it might cause but I thought I’d ask.