How do I weatherproof a door to reduce heating costs?

Start by inspecting the door for gaps and drafts. Apply adhesive foam weatherstripping or V-strip tension seal along the sides and top of the door frame where the door meets the frame. For the bottom, install a door sweep on the interior bottom edge or a door threshold seal to block cold air from entering underneath. If the door itself is old or hollow, consider adding a storm door in front of it for an extra insulating layer. You can also apply rope caulk around the frame where it meets the wall for the season, and use a door draft stopper as a quick fix. Check and replace any cracked or missing exterior caulk around the frame annually.