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Reinsulate an exterior bath wall in climate zone 6

keithrichard | Posted in General Questions on

I’m working on a shower wall remodel in climate zone 6 of a home built in 1950. When I pulled down the old cracked tile, mortar and plaster walls they were about an inch thick. Underneath those walls was R-5 kraft faced batts on 2×4 studs. The exterior of the house has Tyvek and wood sheathing. 

The bathtub is staying put so I’m not able to get behind it other than to slide new batts in place. My plan is to replace the batts with R-15 mineral wool. Then, to bring the walls back to the same thickness, attach 2 layers of 1/2″ drywall, encapsulate it in mud and a couple of coats of primer. Then put an 1/8″ laminate type shower surround over it and seal around the tub and window.

Is this assembly enough of an interior vapor retarder for this climate zone? Is there another approach I should be taking before fully encapsulating everything?

Edit: Images attached. I removed all black kraft paper shown before putting in mineral wool.

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  1. Expert Member
    Akos | | #1

    Air sealing behind the tub is also what you need to do. This is a very big air leak in any older house.

    Looking at the pictures there is a lot of dark spots on the sheathing, this generally means a lot of air leakage and some mold.

    Any type of batt insulation, even mineral wool, won't stop air leaks.

    You need to air seal with a sheet good for some kind. Around me this is done with 6 mil poly that is caulked around the perimeter with acoustic caulk. You want th poly to completey cover the wall from bottom plate to top plate along the whole exterior wall.

    A higher R value, a bit more expensive option is instead of the poly install 1/2" Kerdi board all round the tub. This would let you skip the interior drywall as well as the usual waterproofing and bump up your assembly R value. Just make sure to tape the kerdi where you need to cut it to mount the tub lip to the studs.

    You already have 95% of the bath opened, moving that tub out of the way and doing a proper air seal, means not only a more comfortable bathroom down the road and also one that will be mold free.

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