How do I seal a drafty window without replacing it?

The easiest fix is to apply weatherstripping foam tape around the window frame where the sash meets the frame. Clean the surface first, then press the adhesive-backed foam into the gaps. For gaps around the exterior edges where the frame meets the wall, use a paintable latex caulk and smooth it with a wet finger. For a temporary winter solution, you can apply shrink film kits, which use double-sided tape and a plastic film that you shrink tight with a hair dryer to create an insulating barrier. Rope caulk is another removable option you can press into gaps by hand and peel off in spring. Checking and replacing the window's glazing compound if it's cracked also helps significantly.