The window is in, and is evenly spaced according to the inside wall measurements. I put screws through the side holes at the top and bottom. The middle screw holes were too far out and wouldn't hit wood. I shimmed the sides at top and bottom, and also the top in several places.

I put flashing tape on both sides. Since the nailing fin was a mechanical one, I put the tape over the entire nailing fin to guard against any water seeping in where the fin attaches to the window.

I've fashioned some metal flashing for the top of the window. I still have work to do with that piece though. I need to cut it down smaller and do a better job of bending it.

My main question now is regarding the J-channel. I think I'm supposed to put J-channel all around the window for the siding, and given the location of the flashing tape - and its color - I think the J-channel should cover the inside edge of the flashing tape for aesthetic purposes. I think if I do that then the nail slots for the J-channel will be on top of the flashing tape, and possibly even the nail fin underneath. What's the correct way to install J-channel when dealing with a mechanical nail fin that's been covered with flashing tape?

 

I would probably install the j-channel as close to the window as possible, covering all the tape, as you mentioned, and then I'd probably tape the j-channel as well. After the siding is all put back up, "neatly" caulk the j-channel to the window on all 4 sides.

 

I did eventually finish all of the exterior work for this window installation. I thought I had a picture of it but I can't find it now. I'll take one tomorrow and post it. Anyway, I have another question about this installation now. The window installation instructions include the following text:
I somehow overlooked these instructions when installing the window, and I think that's the reason for the gap that is clearly visible in the picture below.



Trying to adjust the jamb jack now does absolutely nothing, presumably because the window is held fast by all of the yellow spray foam around it, as well as the nails through the fin. Does anyone have any experience with fixing this type of issue? The other side of the window does not have this issue. I'd rather not have to repeat this installation all over again if I can help it. Is it possible that I could cut/chip away the foam just on this side of the window and then adjust it?

 

First, measure the width to find out if there is even a problem. The sides of the window should be parallel. If you find it is bowed then yes, cut the foam with a drywall "keyhole" saw. The jamb jacks won't work if the space around the window inside rough opening is too wide, so you may have to physically shim the jamb so that the window is not spread in the middle. There are usually 2 things you measure when installing a window- diagonals (for square) and the width between the jambs (middle should be equal to width of top and bottom). On a vinyl double hung, you usually want to measure that width both inside and outside to make sure the jamb isn't "pinched" or "twisted" in the middle. Spray foam can sometimes pinch or bow a vinyl frame if you use the wrong kind of spray foam.

 

Aside from the width measurements, it's a problem because that gap leads directly to outside air, which affects the temperature of the room in winter or summer. It's very easy to feel the air coming in through that gap when holding my hand in front of it.

Anyway, I just measured the width at top, bottom, and middle, and the middle width is 1/8" more than top and bottom. This visible gap is on the left side of the window (when viewing it from inside). The right side does not have a gap. Am I safe to just cut the foam on the left side and then try to shim the jamb in away from the rough opening?

 

Yes. If you have a 4' level you might put it vertically on the jamb as a straight edge. Just to verify you are getting it straight.