Adding Insulation to attic floor
I have an unconditioned attic space that is 12ft x 12ft with a walk-in door from a conditioned-finished room. The space is floored with 3/4 OSB sub flooring. The attic floor has R30 fiberglass with 1/2 Polyiso and drywall at the ceiling below.
I’d like to beef up the R value of this attic floor and decrease thermal bridging.
If I lay down 2″ XPS I can still easily walk in here to access electric, minimal storage, house antenna, etc.
1) If I use rigid foam am I creating a foam sandwich to be concerned with? Stack effect is the only real concern with moisture.
2)If I lay down foam, should it be thick enough to follow the guidelines of 61% foam Thickness to prevent condensation, the same as a vaulted ceiling/roof assemble? This is zone 7.
3)Am I just safer rolling out Fiberglass over the sub floor?
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Replies
Foam sandwich is going to depend on the permeability of both foams I would imagine. Also remember that the SIP wall assemblies often have a foam sandwich (I believe I read that on here recently) so not all foam sandwiches are "bad" inherently.
The 1/2 foam is foil faced...so Zero perm
Just use more fiberglass. Use batts if you have batts already, or loose fill if you have loose fill. You can put loose fill in over batts, but I typically like to match what’s already there. If you add batts, use unfaced batts and install them perpendicularly with respect to what is already there.
If you need access, build a catwalk. You shouldn’t need to access wiring often enough to worry about it. Note that you can’t put foam in in a storage area without a thermal barrier (typically drywall) over the foam, so laying the foam and storing stuff on top of it is a code violation. You would also have the moisture trap issue you’re worried about, which is another reason not to se rigid foam here. Lastly, rigid foam is expensive compared to either batts or loose fill.
Bill
The current floor batts are already enclosed on all 6 sides, so new batts would lay on top of the current floor. Since the floor space is only 12 x12 the cost of foam is not that extreme. I may eventually turn this into a conditioned space. This is an old picture before insulation was installed in the floor and outside of the knee walls. You are correct, loose fill would be more adorable.