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Lift the door by the doorknob. If you can move it upward, then the hinges are likely loose. Use a screwdriver to tighten the screws holding the hinges in place.  Some drafts are caused by the door shifting due to loose hinges. Because of this, before you seal the edges of your door, take a few minutes to tighten all of the hardware. If the hinge screws spin but won't tighten, it means that the wood in the hole is stripped out. You will need to replace the screws with wider or longer screws so that they can dig into wood that is not damaged. You may also need to fill the holes with wood plugs and re-drive the new screws into those plugs if there is a lot of wood damage. If your door had weather stripping previously installed, it might be damaged or deformed. Check it by running your hand around the perimeter of the door while the door is closed. Feel for air coming in between the door and the frame. Mark the areas where you feel air coming in with a light pencil mark or a piece of painter’s tape. If you can feel a draft while checking areas that already have weather stripping, you will need to remove the old weather stripping and replace it with a new seal. Remove old weather stripping that is broken or ineffective by peeling or sliding it away. Then use a damp rag to wipe away any visible dirt and debris from the door frame and the edges of the door itself. This will help you attach your new weather stripping securely.  Drag the edge of a paint scraper along the top, bottom, and sides of the door and door frame to remove any stuck-on debris from these surfaces. Also clean the threshold, which is the bottom of the door frame. If there are grooves along the threshold, run a nail through the grooves to scrape away any gunk that has gotten lodged in there. Then run a lightly damp rag along the threshold and door frame to ensure that the surfaces are clean. There are several different types of weather stripping available, so you'll need to decide which one you wish to use. For the top and sides of your door, wrapped foam weather stripping is durable and works well with gaps of various sizes. Weather stripping with a wooden shell is durable yet easier to work with than metal weather stripping, so it makes a good choice for someone who has no prior experience sealing doors. For the door sweep, consider using a metal door sweep with a flexible vinyl flap for greater durability.  A metal door sweep with a flexible vinyl flap is also fairly easy to install, as the metal piece is screwed down and then the vinyl is inserted into it. More advanced options include storm-proof seals and roller door seals, such as brush or auto-lift vinyl seals that attach to the bottom of roller doors.  If purchasing a weather stripping kit, note that most kits only contain weatherstripping for the top and sides of your door. You will need to purchase a separate door sweep. Rigid door sweeps will not work if the carpet is higher or even with the threshold. When rigid door sweeps won't work, use flexible bulb weather stripping made of vinyl. This attaches to the threshold right under the door.
Tighten the hinges on your door. Check the state of the old weather stripping. Clean the areas that need to be sealed. Buy new weather stripping at a home improvement or hardware store.