If a crack in the window is causing the draft, then you will need to replace the window to fix the draft. Using expanding foam not only prevents air infiltration, but it also makes the treated area watertight. It looks a whole lot better than covering windows with plastic film, and you can easily pull it WebHow to Seal Leaks Between the Lower Window Rail and the Windowsill. than you can with just caulk. You may have to remove the operable portion of the window to find the weatherstripping:

\n