I'm preparing to replace the old aluminum framed windows in our house with with either vinyl or fiberglass (haven't decided mfg yet). Our house has vinyl siding with J channel around the windows. The interior jambs are drywall with J channel butted against the interior face of the windows. Walls are 2x4 construction. After a quick peek behind the siding, the house doesn't appear to be wrapped but has foam sheathing.

Originally I had thought of tearing out the old frames and installing block/box frame windows that don't have a nailing fin so that vinyl siding would have minimal disturbance. After reading comments from this and/or other forums, I'm having doubts about how long the windows would be sealed relying on caulk/silicon sealant rather than flashing tape over a nailing fin to keep it water tight. I then decided that a regular new construction window would be best because it's not that big of a deal to remove some siding.

This brings me to my question. Is it possible to insulate around the window without disturbing the interior drywall? With a finless window I would have sprayed great stuff from the outside followed by a bead of sealant. If I go with a finned window I now can't access the gap from the outside and don't see anyway of accessing it from the inside without ripping out the drywall from the jambs. I was hoping that at most I would only have to trim the drywall if the windows were thicker, not replace drywall and remud and repaint. In addition to the difficulty insulating the window, I would think shimming would be more difficult as well. Any tips would be appreciated otherwise I may be adding drywall work to my to do list.

Chris