The easiest way to make your home more energy efficient is to seal any air leaks, and one that is often overlooked is the bathroom ventilation fan and exhaust vent. The back-draft flap these units come with do a very poor job of stopping leaks. To address this issue, I use a replacement insert fan from the Larson Fan Company (online). Their fans has a true damper built in, that does a great job in keeping warm air in during the winter and hot, humid air out in the summer. This product has reduced my annual energy bills by over ten percent. It saves the most when air conditioning is being used.