Getting ready to build a 24″ x 32″ glass block window in the bath using the Pittsburgh Corning Provantage system (blocks, channels, spacers, silicone caulk — no mortar). I plan to install directly into the rough opening (2×4 framing), but I’m hung up on what to do about flashing, drip cap, sill, etc…? Want to avoid moisture infiltration, of course. Also, how should I position the window — flush with the exterior sheathing and allow the siding to butt up against the trim; or align it with the drywall inside? Doing a complete reno, so the siding is off.

Other things being equal, the former is preferable…aligned to the exterior. If it’s done the other way, you end up with a place for water to gather and sit on the outside.

You confirmed what I was thinking. I’ll install the blocks flush with the sheathing. The old aluminum siding and wood clapboards have been removed. At the moment we’re down to 3/4″ diagonal plank sheathing covered with tar paper, so things are still somewhat flexible.

Am I correct in assuming the exterior trim should be attached directly to the sheathing, and the siding (probably vinyl shake panels) butted up to it? Also, I plan to line the window opening with Vycor self-adhesive flashing, but don’t know if a sill pan or drip cap is also needed. Any thoughts? The finished interior wall will be tile over 1/2″ Durock, leaving a 3/4″ or so recess around the window. How should this be trimmed out? (I could ask my drywall guy, but I can never get him on the phone…) We’re at the tail end of a 5 year gut remodel of a severely neglected 1940s Cape. I’m running out of steam and money, and more than anxious to wrap things up.