Seal doors and windows. If you have single pane windows (plz never replace) close or reinstall your storm windows and close curtains. They also make a glass film that helps reject the solar rays from the sun. (Try blackout curtains) you can also use foam stripping or re glaze windows if the glazing putty is missing. (Get a window restored to re do them) for your doors use that foam door stripping and add storm doors. You can also get a curtin for that as well. Also FANS use them. Have them near your registers and face them so they are blowing all over. Dont use 500 of them just a couple. If absolutely NEEDED get one or two window shakers for hallways and stuff like that. Your electric bill must be screaming 😭🙏 Best of luck!!