XPS around windows? – GreenBuildingAdvisor

Hello!  We are using Marvin triple-pane Elevate windows in our home (mixture of fixed, awning, and casement).  For some reason, the gaps between the windows and the rough openings are really big (like upwards of 2″ in many places).  This creates some challenges for air sealing.  I’ve wrapped the Intello and Tescon tape into the RO, but now I need to extend the air barrier  to the windows.  I’ve stuffed a bunch of Rockwool back into the opening (based on guidance here, instead of spray foam), and originally thought about backer rod and Meltell, but it would be huge backer rod and a ton of Meltell.

I then bought some Siga Fentrim tape to try out.  It works okay, but it’s a real pain because the face of the window frame is pretty small, so I’m sticking the small flap to the side of the window facing the side of the RO, then stretching over the gap w/insulation in it and sticking it to the other side of the RO (while retaining a bit of a pocket for furring out).  Corners are especially challenging with the huge gap, and the tape wants to pick up little bits of insulation when I’m stretching it across.  It’s also not cheap and I have a TON of big windows.

I have a lot of long thin strips of XPS left over from our basement walls.  Is there some reason I can’t glue strips of XPS to the sides of the RO (to the 2x material), and then Meltell on each side of the XPS (not totally covering it like I would with backer rod)?  Seems like since XPS is an air barrier, as long as I seal it well up against the RO and up against the window it would be okay?

Just trying to save some money since I have the XPS, and I have a case of Meltell on hand, whereas I’d have to buy backer rod or more the Fentrim tape.  Thanks!!