Water can leak through gaps under exterior doors, damaging floors and carpets and causing dampness and mold growth. Install weatherstripping under the door to seal it from water leaks. The best form of weatherstripping for doors is made from reinforced, waterproof foam that will compress and stretch each time the door is opened or closed. Avoid felt or open-cell foam, as they may not provide adequate protection against weather, according to the United States National Renewable Energy Laboratory.
Calculate how much reinforced weatherstripping you will need to protect the bottom of the door. Open the door and measure the length of the threshold with a tape measure. Allow 5 percent to 10 percent extra as an error and waste margin. You will need to apply weatherstripping to both the bottom of the door and the frame threshold to form a watertight seal.
Clean the bottom of the door and the threshold by wiping them with a clean cloth dampened with water and a squeeze of dish detergent. Wait for the surfaces to dry.
Cut the correct lengths of reinforced weatherstripping from the roll with a sharp utility knife. Use a tape measure to get the right length. Cut two lengths – one for the bottom of the door and one for the threshold at the bottom of the doorframe.
Unpeel the backing tape from the weatherstripping. Push the weatherstripping along the center of the doorframe threshold and press it firmly into place. Apply the weatherstripping to the underside of the door in the same way. Cut away any excess weatherstripping with the utility knife.