Replacement window minus the replacement

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 snipped-for-privacy@SPAMMYREMOVEweckesser.us
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.
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Gabrielli wrote:

That is because replacement windows aren't designed to be installed that way. If you have the time order the correct sized new installation window. Pulling off a little bit of siding will probably be much easier than what you are dealing with now.
If you can't wait then you need to weatherproof the window on the outside then worry about the inside. It sounds like you are mounting this window high so water from the shower shouldn't be a major concern. I would put some metal flashing on the outside so that water can't get in then try and make it look right. Replacement windows require moulding on both sides to cover up the gaps.
When you are ready to screw it into the studs make sure you use shims between the window and the studs so the frame doesn't warp when you tighten it up. Also spray in the expanding foam designed for windows & doors, again so the frame doesn't warp (gap sealing foam can easily compress a vinyl window frame.
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What you describe is normal. Generally a flat piece of 1x stock is added to the inside after the window is installed pushed to the outside to bring the opening flush to the inside edge of the drywall (actually 1/8" more is better). Then the window can be framed on all four sides with your choice of casing.
Your case is different because of location. A pricey option is having some faux marble slabs made and fitted. Lower cost options can be field engineered with a trip to the BORG. In the wood molding sections of most you will find plastic/composite trim pieces. It is just a matter of putting the right combination together. You might be able to use some of the cement or composite based boards that are sold for fascia in lieu of the 1x board. Then add pvc casing and caulk really well.
Did you actually cut out a portion of the surround? And is this going to be a problem?
--
Colbyt
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Well it looks like you have gotten yourself into a bit of a jam, unfortunately. The guy at the bigbox store was stupid, or at the very least very misleading. This is what you should probably have done, either: (1) Use a new construction window. You'd have to pull back the siding, and I suppose with aluminum you would have to be careful not to bend or crease it. With vinyl you could bend and stretch it to fit in the window's built-in J-channel. Or you could get a window without a J-channel and then add your own. Latter is probably the best option for you. or, (2), you could go with a replacement window. Start by building out your window in the old fashioned way: rough frame, then build a box of finish quality wood (sides, top, and window sill). and all the trim on both the inside and the outside, then paint the whole thing, then insert your replacement window inside this box. It would end up looking like you had a wood window there that you replaced.
At this point, I don't see any great options for you. You could rip out the studs and the top and bottom 2x4 plates, make the opening 1.5 inches bigger on all sides, then go with option (2) above. Or you could buy a slightly smaller replacement window. Or you could cobble something together to make it look not terrible. E.g. Finish the window frame and trim first. Then put the window in the middle of the width. Don't worry about resting it on the bottom 2x4 plate versus drywall or whatever... it is the four side screws that hold the window in place. In fact, it is fine if the whole thing kind of hangs a tiny bit suspended above the bottom plate. As for the trim, get somebody who can make (i.e. bend) aluminum trim, and have them make you an exterior frame. In this case, the aluminum would go under the siding, form a J-channel, then wrap back inside the window opening. Have the aluminum go all the way the full 5 inches deep right to the inside of the wall, so it covers the entire studs. Then you can put your window in place. Then you can layer some 1/4" thick trim on top of the interior aluminum showing, and make a frame around the window with trim pieces. And be sure to caulk the hell out of everything (i.e., under the aluminum, between aluminum and your window, under your trim, etc.), since it would be too easy for water to get inside your wall.
Good luck.
-Kevin
Gabrielli wrote:

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