Originally Posted by The Nebuchadnezzar
I've seen some different methods of floor insulation, some folks simply install plywood over insulation board and others do some framing to support the plywood above the plywood. I would appreciate your thoughts
I put plywood directly over 2" XPS foam board and it is doing well. No soft spots. To make this method work you need to pay attention to two things:
One, use XPS foam board, not the white beaded or foil faced poly-iso stuff. Both of those crush to easily, in my opinion. If you're concerned about compression you can use the higher PSI foam board. It comes in 15 psi, 30 psi and up. I used 15psi for the floor (tongue and groove Foamular 150) and it is holding strong. For my shower pan I used 30psi (Foamular c-300) since my body weight won't be as widely spread - a tile or two vs a 4'x8' piece of plywood spreading the weight. For reference, the foam shower pan kits that Schluter
sells are 42psi.
Second, make sure you use a sufficiently thick plywood. I used 5/8" and that's the minimum I'd consider. 3/4" would probably be best. Again, you want to spread the weight out as much as possible so as not to crush the foam over time.
I used Tapcon style concrete screws and PL Premium adhesive to hold everything together.
These screws required pre-drilling which made the task arduous, but they're freaky strong and affordable. Very pleased with how it worked out.
I tried using PL 300 foamboard adhesive, but hated it. I was working in cold conditions and it wouldn't initially tack to the foamboard. After struggling with that glue for a while I did a test with PL Premium. It worked MUCH better, despite what the aisle jockeys at Home Depot may tell you. I installed the sub-floor 1.5 years ago. Recently I cut a good sized chunk out to deal with the shower pan and found the glue to be holding exceptionally well... I had to rip the foam out in little chunks and wire-wheel the rest off of the steel floor.