Weather sealing your window air conditioner in winter keeps the unit safe during the colder seasons and prevents the loss of heat due to the open spaces around the window unit. To weather seal your window AC for the winter, you need to buy a flexible window air conditioner foam seal, which you use to fill up and seal any gaps you may find.

Be sure to caulk around the outside of the unit to seal the space around it and reduce the chance of air flowing in and out. For best results, caulk during dry, cool weather that's at least 40 degrees Fahrenheit. This increases the likelihood of the caulking sealing correctly.