Moisture Sandwich

angus242

I occasionally help a fellow contractor do some tile work. He called me the other day to come look at shower he was redoing. It's a 12 year old house that the old shower was ceramic directly over greenboard (and of course mold). After remediation, he was going to durock, kerdi and then ceramic. He got confused and called me to take a look. Apparently the HO's decided to do a little upgrading themselves before he put the shower back. They used 3" of 2lbs closed cell foam to insulate the exterior frame cavities. So now if he kerdis the shower walls, will this create a moisture sandwich between the kerdi and foam? Since the shower is in a corner sharing 2 exterior walls, I'm not sure what advise to give. While the foam is not 100% to 3" everywhere, it looks to be mostly so with 2x4 framing, that leaves areas between the foam and durock with about a 1/2" gap. With the crazy Chicago climate, we can have 70° one day and 45° the next. That sounds like trouble to me but I'm not sure.
Any input about this situation?

send_it_all

You might want to skip the Kerdi on this one. My limited knowledge of spray foam is that it s it's own vapor barrier? If so, you probably would just dur rock over it without paper or anything. I would probably try to contact the spray foam manufacturer and ask them some questions.

nwtile guy

I would forget about the insulation guys and talk with a schluter rep. They would let you know for sure. I would think nothing is getting behind the kerdi including vapor but I am no expert on it.

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Chad

angus242

My concern wasn't any moisture getting past the Kerdi. It's the small void between the foam and the Kerdi. If someone's taking a hot shower and it's 20 outside, could there be a condensation build up between the 2? And since the foam and kerdi are both moisture barriers, what happens to the condensation? The Durock says it won't deteriorate in the presence of water but I'm not sold about this idea. Besides, the 2x4's are not totally foamed (side facing interior exposed) so wouldn't they just suck up any access moisture?

OR

Will the foam actually insulate well enough where my described scenario can't happen?

cleveman

Colonel,

You are getting pretty academic. I don't think we'll see an issue in the next 40 years at least.

Mike Finley

The Kerdi should act as a moisture barrier so no moisture will pass through the wall. That's the easy part, like you said.

The 2nd part is about condensation, and the question is how cold is it going to get behind that shower wall and the resulting dew point, like you said. That's the question, who's got the answer?

The easy thing to do is to tear that stuff out and put in a permeable insulator. The harder thing would be to create air flow behind that wall to allow moisture to escape, but of course that enters into the whole voodoo land of are you allowing colder air in to a space that wouldn't be a problem until you have let this cold air in due to your trying to let moisture out?

You might want to have your buddy start here:

http://www.buildingscience.com/build...oisturecontrol

orson

If you're worried about the studs you could redguard the exposed framing, at least then the condensation wouldn't have wood to soak into.

Cut 1/2" notches every foot or so to cross ventilate your stud bays behind the durock. Cut a hole through your top plate into the joist bay that has the exhaust fan in it. Consult your HVAC company to calculate the diameter hole to drill in the side of your exhaust fan housing to vent the wall cavity.

Sorry, I know that was a stretch.

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angus242

I turned down the "opportunity" to help with this job.

I am curious what should happen with this situation. Foam is a viable insulator. Kerdi is the only way I'd do a shower. Something's gotta give