I have a question for someone knowledgeable about exterior vapor barriers and Schluter Kerdi. I have a garden window on an exterior wall of a 5' tub. The problem is that I hired someone else to slant the window sill and they liquid nailed in a piece of wonder board. Unfortunately, they cut the edge of the durock without taking into consideration the vertical backer board underneath. As a result, I'm left with a horizontal joint and I'm worried about moisture slipping down it. I would like to use Schluter kerdi to overlap it, but since I already put 6mil plastic on the walls, can I durock over that and then put schluter kerdi overlapping the edge of the window down the wall or do I need to pull out the plastic and do schluter kerdi everywhere? Is it a problem having the top of the windowsill with schluter kerdi draped behind the tile vertically (just at the top) and plastic behind the backerboard? I am in Colorado and it is a s facing exterior wall inside a tub enclosure, about 5' by 2.5' by 8' tall. Thank you so much to anyone who can help. We need a bathroom!

The normal way to handle a window would be to install your moisture barrier, overlapping the flange of the tub and then cut out for the window. Place a bead of polyurethane caulk along the edge of the window and push your cement board jams into the caulk. Next, push the cement board sill into the caulk, using some shims under it gives a good slope, tack in place. If you wanted to, you could use Kerdi if you had some old stuff left over from a previous use, or you could just get yourself a gallon bucket of any of the liquid waterproofing membranes that are on the market. Paint them over the jams and the sill, extending them about 6" past the window on the surrounding wall surface, two coats. You need to embed fiberglass mesh tape into the first coat. If you can find 6" fiberglass tape in the drywall section of your local hardware store, great, otherwise, you can get 6" fiberglass tape where rolled roofing products are sold. The roofing stuff costs more because it's a tighter weave and always a much longer roll. 2" tape likes to "unbend" from outside corners. It does not need to be alkali resistant cement board tape because the liquid membrane will isolate it from the thinset.

You would use the same basic process for a niche, too. Blocking between the studs, top and bottom of the niche, line with cement board, using one piece in the back, then tack pieces to the sides to hold the back piece in place. You don't need to use the caulk between sheets, that's only because of the seal to the window. Paint with the waterproofing membrane and install your tile.

You need not worry about the 6" overlap of 6 mil plastic on one side and a short amount of liquid or sheet membrane on the other.

Thank you so much Bljack! A tile company sells kerdi by the foot so I used that. I appreciate all of your tips. They were extremely helpful!