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Community and Q&A

Insulating behind tower/tub

fall50 | Posted in General Questions on

I upgraded my membership to GBA pro and poked around some on the strategies & detail pages. For drawing 5-01008 is that essentially the “cut and cobble” method leaving a 3/8 gap around the inner most layer of XPS.

Would this be enough R -value for Zone 6b assuming I were to use 2” XPS for the first and second layer or would I be better served using 2” and say 1” for the inner most layer that requires the 3/8 channel?
Is this detail to be used around the entire perimeter of the shower/tub?
Any Permeability concerns of having double layers 2” XPS for Zone 6b when being utilized on the interior

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Would this be enough R-value for Zone 6b assuming I were to use 2 inches of' XPS for the first and second layer? Or would I be better served using 2 inches and say 1 inch for the innermost layer?"

    A. This is a complicated question, believe it or not. The 2012 IRC -- which may or may not apply in your jurisdiction -- requires either 20+5 or 13+10 wall insulation. In other words, the code requires that walls have a layer of continuous insulation, which is almost always installed on the exterior side of the wall sheathing.

    This requirement is controversial, as I explained in the following article: The 2012 Code Encourages Risky Wall Strategies.

    In your climate zone, the best approach is a 2x6 wall with a continuous layer of insulation with a minimum R-value of R-11.25. For more information on why this is so, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Before giving you advice on how to insulate the stud bays behind your shower, it would be good to know how the walls in the rest of your house are insulated.

    Q. "Any permeability concerns of having double layers 2'' XPS for Zone 6b when being utilized on the interior?"

    A. If you end up installing two layers of 2-inch-thick XPS, don't expect the wall sheathing to dry to the interior. Four inches of XPS isn't very permeable. That said, having two layers doesn't add any additional problems compared to one layer; the two layers just lower the vapor permeance even more than one layer does.

  2. fall50 | | #2

    This is a remodeling situation, so no chance of adding exterior insulation. After reading the article you linked to my head is spinning. Thankfully, that detail only seems applicable in the event I was dealing with new construction or a complete tearoff of the exterior siding, neither of which is applicable in my situation.

    My current project is full bathroom remodel going down to the studs and replacing with a walk-in tile shower using the Ditra/Kerdi system. To answer your existing "how the walls in the rest of your house are insulated" I am fairly certain it either consists of rockwool or balsam wool insulation 2x4 wall studs and aluminum siding house was built in 1938.

    Considering these variables , what would be a recommended insulation detail behind the tile. It would be nice to have 2 options from a product standpoint you could recommend. My location is Zone 6A Minneapolis.

  3. GBA Editor
    Martin Holladay | | #3

    In most cases, there is no need to bring up an old house to modern code standards when remodeling a bathroom. (Of course, it never hurts to contact your local code authority if you have any code compliance questions.)

    The "cut and cobble" approach shown in the GBA detail you refer to was developed by architect Steve Baczek. While it is one possible approach, it is far from your only option. You might also consider other insulation materials -- for example, mineral wool batts, dense-packed cellulose, or open-cell spray foam.

    If you want to limit thermal bridging through your studs, you might consider installing a continuous layer of rigid foam on the interior side of the studs. For more information on this approach, see Walls With Interior Rigid Foam.

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