Ok, here goes...

We're putting in a bathroom window where one did not exist before, so our
original intention was to order a new construction window. However, since
the (insert name of large home improvement store) guy said it would be hard
to get the nailing flange underneath the aluminum siding on the outside
(it's a small window in the middle of a wall so we don't want to remove the
siding) he suggested just ordering a replacement window. Ok, we ordered a
replacement window. So far so good.

We've rough-framed the opening with standard 2x4 studs but now things are
getting a little murky. The studs have their normal depth of 3 1/2" and add
to that about 1/2" for the outer wallboard and another 1/2" roughly for the
aluminum siding at its widest. Coming inside from the studs, there's 1/2"
for the drywall (and a negligible width for the tub surround that we'll just
cut to fit). So the wall depth totals 5 inches.

The window is 3 1/4" deep. The J Channels for the aluminum siding are
1/2" thick. So now we have 3 3/4" inches of stuff in a 5 inch deep hole.

Problem number one: the depth placement of the window. The window's
bottom plane is not precicely flat, but rather has two 3/8" wide 1/8" high
ridges almost at the front and back edges of the assembly. If we center the
window on the studs, it will fit stably there but the J channels won't sit
flush against the window and still be able to fit around the aluminum siding
because of the 1/2" wallboard in between. If however, we push the window
towards the outside so it sits flush against the J channels, the front 3/8"
ridge will be resting comfortably on the stud but the back ridge will be
sitting on the wallboard (which seems less stable).

Problem number two: the interior. Either way we place the window, there's
still the issue of what to do to cover over the width of the drywall.
There's not really room to add jambs around the top, bottom and sides of the
window because it already fits precisely into the rough stud framing
(roughly 1/4" on all sides). Add to that the issue of it being right above
the shower so we're trying to find a water-resistant solution. Does anyone
know of a plastic or vinyl L-shaped strip that can be caulked to the window
and the tub surround/wall?

Thanks for any brilliant insights,
~Gabriel


PS: We've nearly finished gutting and remodeling our bathroom, so we've been
able to figure out or research most things, but the web has precious little
info about replacement-less replacement window installing. Go figure.