Extra space for insulation in joists
I’m insulating a floor that’s above an uninsulated garage. I have 2×10 joists. I got R30 rockwool as that was the thickest one I could easily get. According to the packaging, it’s 7.25″ thick, meaning I have an extra 2″ in my joist bays. I was thinking of getting 2″ EPS sheets and cutting them in and putting them directly against the subfloor, sealing around the perimeter with spray foam before putting the batts over that. However, I’m concerned that this may lock in moisture if there’s a bulk water spill on the floor above — let’s say I spill a pot of water and it leaks through the subfloor and pools against the foam. Is this a valid concern? Are there other alternatives I should consider for that 2″ of extra space, or just leave it open?
Thanks
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Replies
reimhagen,
The place for any rigid foam board in that floor would be under the joists, where it can reduce thermal bridging. In the joist bays I'd either leave the 2" space open, or fill it with r-12 fg batts. If you put those batts up against the sub-floor, the R-20 rockwool will easily compress them so everything fits in the cavities.
Another option is to stack a combination of 2x4 batts (R15, 3.5” thick) and 2x6 batts (R23, 5.5” thick). This would give you a total thickness of 9”, so almost the entire depth of the joists, and R38 worth of insulating value. Running the rigid foam under the joists would perform better, but would need drywall over it in a garage for fire resistance.
Note that you could save some money here using fiberglass batts and just overfilling the joist bays. You could do that and use rigid foam under the joists the same way you could with mineral wool.
Bill