Due to a couple of plumbing leaks the fiberglass insulation and belly wrap under both bathrooms has fallen apart. Replacing with closed-cell foam is not an option at this time. I've been thinking of using the foam board insulation instead. There are two ways I can think of to install the foam board insulation:
1) Cut into 4' x 4' sections (for ease of installation). Attach the foam board to the bottom of the joists using screws and 1"by material (plywood strips etc.)
2) Cut the foam board to fit between the joists and secure by either screwing blocks to the inside of the joists or 1"by material across the joists. This would leave the board free-floating and better able to react to expansion/contraction.
Both methods would leave an air gap between the insulation and the bottom of the sub-floor. I believe the first method would better insulate the plumbing and HVAC ducting.
Which method would be the better choice? Any potential problems with either one?
Also, the roof insulation is practically non-existant. The roof construction doesn't allow for more than 3 1/2" of insulation. There's the asphalt shinges, the roof deck, 2"x4" rafters, then the 5/8" ceiling drywall. Lowering the ceilings isn't an option, the walls are only 7' with a 9' ceiling peak. Drop the ceiling any more and I'll be bumping my head on it. We've started painting the ceiling with a heat-reflective additive in the paint, but I doubt it's going to do much good. Any suggestions?