You may want to try removing the inside trim on just one side of one window and take a look-see. If there's any gap at all between the window frame and the studding, you've got a few choices... caulk the gap (if it's narrow and not too deep), lightly chink it with pieces of scrap fiberglass batt (if it's alittle wider and deeper), or my favorite, low-expansion spray foam (if you can get the little tube into the crack). Be careful with any of these techniques, so you don't cause the frame to push in, binding on the sashes when you raise/lower them.