I recently ordered 2x Andersen 400 series tilt/wash DH replacement/insert windows without flanges to replace leaking 17-yr-old NVR Homes / Ryan Homes builder-grade vinyl windows with installation flanges. Vinyl j-channels were not installed around the builder-grade windows given they had some sort of integral channel as part of the actual window. I’ve only addressed one of two windows, and was able to cut out the window from the interior using a Dremel multi-max (enabled a more controlled and safe demo than a reciprocating saw...only saw I’ve come across that I’m literally afraid of) by essentially separating section of the frame that sat inside the rough opening from the entire integral channel; cut the roofing nails and removed tape which held the flanges in-place behind the siding; and slide j-channel in-place without having to remove all siding. I followed Andersen’s instructions by flashing the 2x4 rough opening that Ryan Homes didn’t bother to do 17 yrs ago using Vycor Plus 6” Self-Adhering flashing tape (the bottom wood had large water stains covering it which synced with the water that began pooling and pouring from the window stool), which calls for flashing tape to overlap installed j-channel for manufactured siding. Logically speaking, this step seems appropriate to to stop the possibility of water infiltration into the rough opening. Although, I keep wondering if the j-channel’s nailing/installation flange should have also been flashed with tape after nailing it in-place and prior to inserting siding. I cannot locate anything online that suggests the j-channel’s nailing/installation flange should be flashed or caulked.

I would appreciate insight and/or recommendations from pros before I proceed to the next window which explicitly discusses whether the flange should be flashed or caulked, or if opting to overlap flashing onto the j-channel as Andersen advised is/was sufficient.

Ref: Andersen Installation instructions, manufactured siding, bottom of page 9. https://awwebcdnprdcd.azureedge.net/-/media/aw/files/technical-docs/installation-guide/installationguide-9177035.pdf

Note: I have already caulked the boundary between the new Andersen window and the new j-channels although I can certainly remove the caulk and temporarily remove siding to add flashing tape if indeed appropriate to prevent water infiltration.