Originally Posted by eladsgarr
I watched Bussy McBusface just apply 2X4 boards over top of the buses flooring in a pattern, and use styrofoam insulation between the boards and finally capped it with plywood. Is this an option that wonít cause later issues? Any advice is greatly appreciated.
I don't know why people do this. It's not a huge deal, but XPS foam board has an R-value of 5 per inch, while wood has an R-value of just 1 per inch. So to maximize the overall insulation value of your subfloor, you want it to be as much insulation and as little wood as possible.
XPS has a typical compression rating of 25 PSI, so with plywood on top of it, it doesn't really need furring strips at all. If you want to use furring strips anyway, a 1" wide strip of 2X (1.5" thick) or even a piece of 2X2 would be just as strong (in terms of resisting the bending of the plywood) while using less than half as much wood.
I even saw one build where the person used 1X6 on its side in a grid with tiny little squares of XPS in between - it was more than half wood, so what should have been an R-value of 3.75 was instead less than 2.25.