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Questions sealing cut & cobble in a remodel tile shower area etc.

TomTom12 | Posted in Energy Efficiency and Durability on

I’m looking for any suggestions on best method installing 3.5″ thick surplus closed cell rigid commercial roofing foam insulation to cut and cobbling in the exterior 2 x 4 walls in my bath remodel and new shower area.

The bath room has a bump out in the bath area for the shower that has always resulted in cold area here in PA due to exterior wall bring on the one long side plus 2 partial side walls. The existing 2 by 4 wall in this area is drywall +R13+ 1″ Celotex foil + 5/8 sheathing plus 5/8 T1-11 wood paneling. After installing the 3.5 inch foam in the stud cavities, I’m also planning on installing a 1/2″ rigid foam foil covered polyisocyanurate panels in this area (from Home Depot) to pad out the existing studs and to provide some thermal isolation from studs. Should the reflective side face to the exterior. On top of this will be a Wonderboard? cement wall board product that will be covered using the Schlueter shower kit and membrane and then tiled. The tiled shower will be about 72 by 39 tiled when complete. The total exterior wall length is about 12 ft.

Questions: Should the 3.5 foam board be foamed with great stuff door window etc on the edges before inserting or after?. I plan on a snug fit. Any other sealing or treatment after installed. I plan on taping the overlay 1″ foam panel seams with foil tape. I’ve just read Mr Holladay’s excellent article on cut and cobble from 2013. Thanks for any comments.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should the reflective side [of the foil-faced polyiso] face to the exterior?"

    A. In this case, it makes no difference. The aluminum facing on rigid foam insulation provides no thermal benefit unless it faces an air space. If you change your plans and build in an air space, then the aluminum foil should face the air space.

    Q. "Should the 3.5-inch foam board be foamed with Great Stuff door window etc on the edges before inserting or after?"

    A. After. For more information on this insulation method, see Cut-and-Cobble Insulation.

    Q. "I plan on a snug fit."

    A. That's a bad idea. You want a sloppy fit, to make it easier to foam the edges. Here's what I wrote in my article on the topic: "Most cut-and-cobble veterans suggest that it makes more sense to cut the rigid foam for a loose and sloppy fit rather than for a tight, perfect fit."

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