Tom Meehan, owner of Cape Cod Tile Works, replies: A window in a tiled tub or shower enclosure is common, but you are smart to be concerned about the potential for moisture damage. There are a few different ways to approach this project. I like to continue the tile into the window jamb as if it’s part of the wall and add a pitched sill.
In these types of situations, I encourage homeowners to use vinyl-clad windows housed in a wooden jamb. The vinyl provides maintenance-free protection from moisture for the window itself. The all-wood jamb provides a flat and sound base. Make sure that you locate the window outside the range of the spray from the showerhead.
As with all tiled shower installations, run cementboard throughout the shower area and on the same plane as the window jamb. Caulk the joint between the jamb and the cementboard with a good-quality silicone caulk. Although the caulk provides an initial seal, it should not be relied on as the only defense against water infiltration through this joint. To seal the joint completely and to cover the exposed wood of the window jamb, wrap a waterproof membrane such as Schluter’s Kerdi (www.schluter.com) over the joint and into the window jamb. Bond the membrane to the wood with a latex-modified thinset.
Sealed joints. When installing a window in a tiled shower enclosure, make sure the joint between the jamb and the cementboard is sufficiently sealed with silicone caulk and a waterproofing membrane. The membrane bridging the two materials creates a stable substrate for the tile.
I like to install a pitched sill in these windows to allow water to drain back to the shower area. In lieu of wood, I use a piece of Corian, which I install with latex-modifed thinset. The simplest way to pitch the sill is to build up the thinset under the back part of the Corian; mixing it a little stiff will make it easier to work here. Once the sill is in place, I install tile over the entire area in the same manner as I do the rest of the shower.
When the tile is complete and the grout is sealed, use a good silicone caulk where any two planes come together, like where the window meets the newly tiled jamb.
Remember that the window is on an exterior wall, so temperature changes will cause expansion and contraction in many of these materials. Check the area every year for cracked joints; caulk or grout as needed.