Exterior wall insulation advice desperately wanted!


Old 01-13-20, 08:52 AM
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Exterior wall insulation advice desperately wanted!

Sorry for my book and I very much appreciate any responses!

Scenario: I'm remodeling small bathroom to add walk-in shower, approx 50" x 44". The back/long wall of the shower is on an exterior wall (no window). When I demo'd the bathroom, I took everything down to the studs and removed the insulation. My dilemma is how to properly insulate the exterior wall behind the shower without creating a moisture envelope, resulting in condensation issues.

The person who built the house was cheap, I think, and only used foil-faced rigid foam board as the exterior sheathing. That means I have this nice shiny foil surface facing the inside of the room. What was previously in the walls was bat insulation (non paper faced) and a poly barrier over that. All of that has been removed.

I have installed an exhaust fan in the shower area to help with moisture. I plan on using Kerdi board as the tile backer board for the shower.

Here's what I've attempted so far.

Attempt #1 - I put Thermafiber mineral wool insulation in between the studs. No plastic vapor barrier. I thought all was well, until it got pretty cold here in Illinois. Then I noticed quite a bit of condensation along the bottom plate of the exterior wall. When I checked the Thermafiber, it was wet where it met the foil backer board and the bottom of the insulation next to the bottom plate was acting like a sponge and had a fair amount of water in it. I pulled this insulation out and proceeded with attempt #2.

Attempt #2 - I used 1/2" pink foam board in between the studs, sealed with spray foam along the edges and then put Thermafiber on top of that. I was hoping to reduce the condensation point in the wall cavity. However, the insulation was still getting wet next to that foam board. Not as much as before through.

Attempt #3 - I decided to pull the Thermafiber out and use closed cell spray foam. I bought one of those DIY kits and it turned into an epic fail. The foam did not bond and disintegrated and I thought I followed all instructions regarding temps and surface prep. So, I pulled all of that out.

Now I am back to 3 wall cavities between studs with only the foil faced foam board nailed to the studs on the outside of the house as the insulation/sheathing with vinyl siding over that.

And, yes, I have called around to various contractors, insulators and not one return phone call. I think my job is too small for them and nobody wants to take it.

So, how can I properly insulate this exterior wall to prevent condensation within the wall cavities, knowing that Kerdi board will be used over the studs as the tile backer board?


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Old 01-13-20, 09:15 AM
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So there is nothing really special about the wall behind your shower than any other wall in the house.

What is special is that you will need a vapor barrier integrated into your shower which you have stated will be Kerdi board, excellent choice!

As far as the foam board, that is an item I am not familiar with and will yield to others!
Old 01-13-20, 09:20 AM
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Thanks Marq. I guess what is bothering me is the amount of condensation that was present with the mineral wool next to the foil faced sheathing. Maybe not having a poly vapor barrier up and the cold weather is the issue, but I was relying on the Kerdi being the vapor barrier next to the insulation.

Maybe I'm just attempting to do this during the wrong time of the year?!
Old 01-13-20, 02:39 PM
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Something I was thinking of doing is taping the seam between the foil board sheathing and the studs, then place 3/8" plywood inside the wall cavity next to the foil foam board to mimic what should have been done to the house (i.e. Plywood sheathing, then foam board on top, then house wrap). I'm hoping that provides enough of a break so that condensation doesn't occur in the wall cavity.

Good idea or bad?
Old 01-13-20, 08:05 PM
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When you are working on a house in the winter, you will get condensation and frost on the exterior side of the insulation / interior side of the exterior sheathing because there is no air barrier to isolate the warm humid air inside from the cold wall outside. The cold surfaces act like the coil on a dehumidifier... and you get a lot of condensation.
keys2heaven voted this post useful.
Old 01-14-20, 04:29 AM
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All the previous posts have good information. There is no need / benefit to treating the shower wall differently than any other exterior wall beyond using KERDI-BOARD per the Schluter installation instructions. Keep in mind that KERDI provides sufficient protection by itself so don't add vapor or moisture protection which can create a sandwiching problem.
Old 01-14-20, 07:02 AM
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Thanks. I was thinking this might be a reason.

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