The problem: While most homeowners weatherstrip around the jamb, they often overlook the area beneath an exterior door.
How to spot it: Close the door on a piece of paper placed on the threshold and give it a tug. If it pulls out easily, air is passing through.
How to stop it: Install a sweep seal. This metal strip with a piece of vinyl attached uses spring action to close the space between the threshold and door. There are also foam, vinyl, and felt seals that fit under the door or on the threshold to prevent air transfer. Whichever type you choose, it's an easy DIY installation that'll cost just $10 to $20.
The payoff: Coupled with weatherstripping, a sweep seal can prevent 11 percent of the outside air that typically seeps in around exterior doors from getting into interior spaces.