My front door had a gap at the bottom big enough to slide a magazine through. In winter, cold air pooled on the floor. In summer, hot air crept in and my AC worked overtime. I lived with it for two years because I thought fixing it required replacing the whole door.

Fifteen dollars and thirty minutes later, the gap was sealed. The hallway stopped being five degrees colder than the rest of the house.

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The Bottom Gap: Door Sweep

Most drafty door problems start at the bottom. A door sweep is a strip of rubber, vinyl, or brush bristles that screws or slides onto the bottom of the door. Measure the width of your door, buy the right size at any hardware store, attach with the included screws. You need a drill or screwdriver and five minutes.

There are two types: under-door sweeps that slide onto the bottom of the door with adhesive, and surface-mounted sweeps that screw into the face of the door. The surface-mounted kind lasts longer. The slide-on kind works for renters who cannot drill.

Sides and Top: Foam Weatherstrip

For gaps around the sides and top of the door frame, self-adhesive foam weatherstrip tape does the job. Measure each side, cut to length, peel and stick. Clean the surface with rubbing alcohol first — dust and grease kill adhesion.

I made the mistake of not cleaning first. The foam peeled off in two weeks and I had to redo the whole thing. Now I keep rubbing alcohol under the sink just for this kind of prep.

The Keyhole and Letter Slot

People forget about the small openings. A keyhole cover — a small metal flap that swings over the keyhole — costs two dollars and installs with one screw. For mail slots, add a brush seal or an exterior flap. These tiny holes leak more air than you would think.

I held my hand in front of my keyhole on a cold day and felt a breeze. A literal breeze. Through a hole the size of a dime. The keyhole cover paid for itself in one winter month.

Test Your Work

After sealing everything, do the incense test again. If smoke still wavers near any edge, add more weatherstrip or try a thicker foam. The goal is zero air movement when the door is closed.