first of all, the house never had insulation there in the first place. in the second place, the homeowners probably didn't want to repaint if the trim was removed to put the insulation in correctly. if you don't want to either, then just caulk the trim to the siding and the trim to the windows. this will create a dead air space, which is the best insulator. most good replacement windows come with foam wrap to insulate the inside of the window frame anyways. personally i think your energy auditor is just trying to blow smoke up yours, so you'd think that he was worth it. if you really want to do it right; cut the paint/caulk around the trim on the inside and then pull the trim off. then, remove the sash weights, fill the void with fiberglass cut to size, then re-install the trim. set the nails, putty the holes, caulk the trim, and then repaint it. that's why the previous owner didn't go the extra mile, especially if they were thinking of moving. good luck.