2. Seal the bottom of external doors first
The gap under an external door is usually the single largest opening in the house. A 3 mm gap across a standard 36-inch door is about the same open area as a drinking-glass-sized hole punched straight through the wall, and cold air pours through it all night because that is where the pressure difference is strongest.
You have three options, and they are not equal. A fabric sausage draft excluder is the cheapest at around $8, CA$12 or £6, but someone has to put it back every time the door opens, so in practice it sits against the wall by November. A brush strip that screws to the face of the door costs roughly $12 to $20 and works whether or not anyone remembers it. A full threshold seal — a channel screwed to the floor with a rubber blade — seals best but needs a level threshold and a door that clears it.
For most homes the brush strip is the right answer. Measure the door width, buy the next size up, and cut the aluminium carrier with a hacksaw. Close the door, hold the strip so the bristles just kiss the floor with a slight bend, mark the screw holes in pencil, then drill pilot holes. Set it so the brush sweeps rather than jams — if the door drags, you have gone 2 mm too low.
Then do the hinge side and the latch side with self-adhesive E-profile foam. Clean the frame with rubbing alcohol first; foam tape fails on dusty gloss paint more often than it fails on its own.