I have a unheated garage with the living room above (cold floor). I was given 10 sheets of 4X8 4″ foam panels. Can I remove the drywall from the ceiling, remove the R-13 FG insulation, then rip the foam to 13.5″ and place them between the joist and against the bottom of the floor above and then use some spray foam on the sides to seal them against the joists. I know it would be allot of work but I want warmer floors. Any ideas?
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I'd just screw the sheets to the ceiling then drywall over it. You'd get better insulation with less work. They make long drywall screws just for that purpose.
I agree with the suggestion above as this will mitigate thermal bridging through the floor joists. Extruded polystyrene is a good choice as it will act as a vapour barrier and has a good R-value (depending on the thickness). The only draw back on this approach however is that you will lose some headroom in the garage and will have to make sure that if you want to install anything on the ceiling that you provide blocking in the floor. The question I have is what do you do at the rim board of the floor? This is where the floor meets the outdoors and this is where you would certainly want EPS (expanded polystyrene) blocks or some other insulation. Did the builder install insulation along the rim as well as between the joists?