Can i stack foil faced polyiso?
Well my insulation plans have changed as I recently have access to a good amount of “recycled” polyiso 4x8x2 foil faced foam boards (both sides) that read r13. Also a good amount of 1.5x4x8 r9 foil both sides. Pretty much perfect condition from a warehouse project for next to nothing.
I need to insulate a vented vaulted ceiling in my home. Bays are 2×8 and about 16 foot in length. The 2x8s measure 7″. My plan was to do 1.5 air gap with blocking and install 5.5″ of these foam boards to fill the bay flush with the jousts where ill install drywall. So drywall will be touching back the 1.5 foil board. And foiled faced boards will be stacked as well obviously.
Will i have any issues with this set up in regards to moisture etc? I plan on cutting the boards to fit snuggly for a pressure fit so i wont need to foam the sides. Id worry about foam expanding and not allowing me to stack the boards perfectly as well. I will stagger the butt seams.
Do i NEED to fur out the bottom of the rafters for a thermal bridge? If I do that, how thick do I need and would it be okay to have a gap between the back of drywall and the foil insulation now because the furring.
My plan wasnt to fur out joists an just screw drywall directly to it and drywall would also be touching the insulation.
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You'll have no problems using the stackup you described with multiple, stacked, sheets of foil-faced polyiso. The vent gap under the sheathing will take care of any moisture that might sneak its way up through the assembly too.
You don't NEED to furr things out. You could put a sheet of polyiso directly under the rafters and tape the seams. This would take care of any thermal bridging, and will also provide you with a good air barrier. This is what I'd do.
Drywalling may be easier with a layer of 1x4 strapping over the polyiso, which will eliminate the need to align the screws through the polyiso into the rafters when hanging drywall. The strapping will also help keep the seams in the drywall from opening up over time, which is an added plus IF you align the stripping so that gaps between drywall sheets are aligned with the strapping. This is what I'd do. The ~3/4" air gap between the drywall and the polyiso isn't a problem.
Bill
If i dont furr out the rafters the drywallers will just screw directly to the rafters since the insulation wont be protuding past the rafter depth itll be even with them.
You mean if i add 1 more layer of poly across everything including the rafters and tape the seams then id add additional furring strips (nailers) over that for the drywall. That would then leave me a gap between the drywall and the bottom of the poly in whatever size the nailer is
Yes, one layer over the bottoms of the rafters with no gaps -- you'd see just a big ceiling of foil after that, with seams taped (I like Nashua 324A foil tape for this). Strapping would then go under that, screwed to the rafters through the foam, and the drywall would be screwed to the strapping. This is a pretty common way to do this.
Bill
Thank you sir! Can i use 1.5," for that
Yep, 1.5" would be fine.
Bill
I agree with the thermal break, taped seams & strapping approach ...
Mostly because just relying on "friction fit" between rafters, even with a vent space, would make me nervous.