We’re putting a (used, but sound) vinyl window with a nailing flange in as a temporary replacement for a completely worn out wood double hung in our Victorian.  I’m planning to build a frame to set the window into the wall somewhat, so it looks slightly less tract-house-y, and then nail or screw through the nailing flange.  The window’s on the south side of the house, which is also the prevailing weather side here.

Temporary for us means, probably, a year or three (three if I’m counting, one if LOML’s ambitions for my to-do list prove out).

What’s the best way to seal the window at the flange?