I've got two types of walls in my house--stud walls and block walls. With the block walls, I am going to place foam board up against the masonry, then build 2x4 walls that contain fiberglass bats, and then follow up with drywall. With my already existing stud walls, I am going to fill the cavities with fiberglass insulation, and I was wondering if it would then be OK to place a layer of foam board over the entire insulated wall? That is, insulation between wood studs, then foam board over the studs. I would then cover the foam board with drywall, attaching the drywall with screws through the foam and into the studs. I'd like to include this layer of foam board to increase the R-value since my stud walls are only 2x4. Since I'd like to keep as much floor space as possible, I thought foam board would be the way to go instead of extra fiberglass bats and framing. I also figure the foam board would provide an extra air tight seal. Does this sound good?