Waterproofing a shower ceiling with closed cell spray foam insulation
Question about waterproofing a shower. I’m worried about creating a moisture sandwich. This shower is unique because it has a low sloped ceiling, so one of the walls only 5 ft tall. The ceiling also has closed cell spray foam insulation. (Just an FYI: above the 7 inches of spray foam is a 1″ channel, then the roofing. The channel releases condensation out of the ridge vent.) The shower is not a steam shower.
I’m using Kerdi over greenboard for the shower walls and sloped ceiling. But to be warrantied by Schluter, Kerdi must run up to the height of the shower head, which is 6’6″; 18 inches higher than my lowest shower wall (5′ tall). This means I need to install Kerdi on the sloped ceiling too. So the shower ceiling layers from inside to out will be: tile, Kerdi membrane, thinset, greenboard, closed cell spray foam insulation, 1 inch channel, roof. Will I create a moisture sandwich between the 2 waterproof & vapor proof layers (Kerdi, greenboard, closed cell foam)? I’ve read that you should never use 2 vapor barriers, because the area between them cannot dry. If, 5 years from now, moisture gets behind the Kerdi or spray foam, it needs to be able to move out, right? In my case maybe a Schluter system isn’t the way to go? I’ve heard that Schluter will grant a special warranty in certain cases. Help appreciated! Thanks.
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