An easy and cheap way to keep the house warmer and more efficient is to stop air leaks around windows; most importantly the frame seat and between double-hung panes. For this, you can use a variety of cheap materials, including ready-made products. Caulk around the frame, around the storm window seat, and anywhere else that can take caulk and flickers a flame held close-by. If you can get everything air-tight you likely won't need film.