[crb07]

crbo7 - Is it common practice in your area to put plywood between two moisture barriers ? Does that moisture barrier breathe? If humidity can't escape, the wood will rot. Seen it quite often around here in walls, before tyvek, people were putting a plastic vapor barrier behind the sheetrock, combined with tar paper under siding, walls would retain moisture.

I don't think many companies are doing this. I looked at a lot of info online and liked the idea of having the foam sandwiched between plywood.

There may be some issues with spray foam on the underside of plywood in the attic. This was something else I looked at but there are complaints of moisture getting in between the spray foam and the plywood in the attic. I believe most metal roofs in the north west have an air space under the metal to keep condensation out of the house. I know a house built in the north Georgia mountains with timber frame construction covered in foam panels with plywood on either side of the foam panels. The builder put an air space under the metal and there has been no issues with moisture in the house. The drywall is screwed directly to the underside of the plywood on the foam panels. That is the one thing I wanted to do but could not find a roofer to do it is put an air gap under the metal. Someone out west is probably familiar with this.