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Community and Q&A

Wall Cavity Insulation

GBA Editor | Posted in Energy Efficiency and Durability on

I am adding a bathroom in a Cold zone (Rochester, NY) and will have a tiled shower that has 2 exterior 2″x4″ walls. Planning to do 1.5 inch foil faced rigid polyisocyanurate sheets on the exterior of the studs and vinyl siding over that. There appears to be many opinions on how to insulate the cavities. It is a small area so some say just add more sheets of the foil faced polyiso in the cavities and foam seal them. Others say just use unfaced fiberglass batts in the cavities. The shower area will then have Hardiebacker cement board over the studs. The rest of the bathroom ( 2 sides also on outside walls) will have Gypsum Gold Bond XP (purple mold resistant drywall) over the studs. House has gas forced air heat for winter. In the summer occassional air conditioning will be used. Which of these cavity insulation approaches is best or what other option should be done?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what's "best." Either will work.

    If you install piles of polyisocyanurate sheets in the stud cavities, you will end up with a higher R-value. But that's fussy and time-consuming work, especially if you do it right and foam around the edges. But, in a small area, the technique works, and results in a higher R-value and better performance than fiberglass batts.

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