New window installation question


I am about to add a new vinyl window to an existing exterior garage wall. The inside of the garage wall is now just open studs, but I will be adding insulation to the wall and then sheetrock.
The outside of the wall is aluminum siding on top of some type of 1/2 inch siding (I don't know what it is called) that is nailed to the studs. Cutting the opening for the window and framing it out with a header etc. will not be a problem -- I know how to do that.
My question is about what to do on the outside. If I buy a new window, it will have a flap/flange(?) around the outside that is for nailing the new window to the studs. But, if I cut back the aluminum siding to allow for the flaps to be nailed to the studs, how do I finish off the space between the edge of the aluminum and the window itself?
Is there some other trick for doing this such as to cut off the flaps and nail the window in place through the sides so the aluminum will meet the edge of the window?
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And then caulk with high-quality caulking
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BetaB4 wrote:

Either way will work. A real window supplier will be able to get 'old work' windows without the nailer flange. You surround the opening with J-channel to catch the cut ends of the siding before you pop the window in the opening. Or if you cut back the siding, you trim around the window with 1x4 or whatever, again with J-channel to catch the siding. I'd key off whatever the other windows in the house look like. Usual cautions about sealing membranes and shoving flashing up under the siding above the window apply. Leaks around the outside are the main failure point for a cut-in window. Absolute safest way is to strip enough of the siding to install it just like a new construction window, and then side back up to it. If you can pull the old siding off without destroying it, it should go right back up with no problems. People scorn at aluminum, but unless it was nailed too tight, it actually reinstalls better than vinyl, which takes a set from the UV. If you number the pieces and keep track, you can even keep the fade patterns the same.
-- aem sends...
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Thanks. I did the job today and it is working out okay. I did it by cutting the flanges off. I didn't have any J-channel, but instead I used the window frame itself to overlap and cover the aluminum siding. I still have caulking left to do, but it started raining so I'll go back tomorrow to finish the job.
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wrote:

flange usually makes the neatest job. I think you did it right.
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