Air sealing windows w/ jamb extensions – GreenBuildingAdvisor

GBA won’t let me see previously asked questions for some reason, so please excuse the double post, as I had another thread about air sealing windows.

For our fiberglass Pella windows, we planned to air seal on the interior by inserting a backer rod before adding low expansion spray foam (to ensure a drainage gap was maintained). After foam, we planned to use backer rod and caulk to get a good seal.

They are windows with factory applied jamb extensions. We didn’t know this before they were installed, so we didn’t seal the seam between the window frame and jam extensions. We were hoping the spray foam would help with that. 

But now I’m not so sure I’ve been  thinking about all this correctly. Pella’s instructions specifically say NOT to use any foam between the jamb extensions and opening. It’s only to be used between the window frame and the opening. 

Does the air sealing need to happen back at the window frame itself?

If so, how can I effectively get back there to ensure it’s air sealed? And what do I do with the rest of that empty space in the opening (from jamb extensions inward)?

It’s a pretty deep cavity, and some of the openings are very narrow.

Would love some guidance here!