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Polyiso between sheetrock and studs

FarmBoss17 | Posted in Energy Efficiency and Durability on

I am in the process of remodeling the inside of my house. I’m doing one room at a time since I have to live in it with my family. The existing walls are 2X6, 24″ OC with unfaced fiberglass insulation in the wall cavities. Plastic sheeting between the studs and the inside of the drywall acts as a vapor barrier. Can I replace the plastic sheet vapor barrier with foil faced polyisocyanurate, situating it between the studs and drywall on both the exterior walls and the ceiling? I have been told no, but all the reasons given do not make sense to me, given I am replacing one vapor barrier with another. Will the increased insulation of the polyiso promote condensation in the wall cavities? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    If you decide that you want to insert rectangles of rigid foam between the studs, the method is called the "cut-and-cobble" method. This method has pluses and minuses. If you decide to use the cut-and-cobble method, you want to locate the rigid foam toward the exterior of each stud bay, not the interior of each stud bay. (After all, you want to minimize the chance of condensation against the cold wall sheathing. That means that it's a good idea to have a layer of air-impermeable insulation -- rigid foam -- on the cold side of the assembly, to prevent the warm, humid interior air from contacting a cold surface.)

    For more information on the cut-and-cobble method, see Cut-and-Cobble Insulation.

    If you decide that you want to install rigid foam on the interior side of your wall assembly, it's important to install the rigid foam as a continuous layer rather than cutting the rigid foam into small rectangles and inserting the rectangles between the studs. That way, the rigid foam will interrupt thermal bridging through the studs. If you are working from the interior, a continuous layer of rigid foam is possible. (Although you'll be installing the rigid foam on the "wrong" side of the assembly, it will still work).

    For more information on interior rigid foam, see Walls With Interior Rigid Foam.

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