The draft usually shows up in the kitchen first. You're standing at the sink, the coffee has gone lukewarm, and there's a thin cold line along the floor from the front hall. Mom would have rolled up a bath towel and laid it against the door by October without ever mentioning it. Your dad has probably been stepping over that cold spot in his slippers for weeks and telling you he's fine. He may well be. A door that lets the cold in is still worth fixing, and this is one of the few winter jobs that can be settled in an evening, with a flashlight and a clear head.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Test the gap tonight
Before you buy anything or call anyone, find out where the air is getting in. Close the door as you normally would, then go outside after dark, turn on a flashlight, and shine it at the edges of the door from the outside, slowly, from the top corner down to the threshold. If you see light, you have a gap. Do it again from the inside with the lights off and a flashlight held out in the hall, and look for the same thing.
Then try the paper test. Slide a sheet of ordinary paper into the gap where the door meets the frame, close the door on it, and pull. If the paper slides out with no resistance, the seal is gone along that stretch. Do this at the top, the latch side, the hinge side and the bottom. Write down which spots fail. Most people find the problem is worst at the bottom and on the latch side, which is good news, because those are the easiest places to fix.
Last, check the latch. Close the door and see whether it catches on its own or whether you have to lift it or push it. A door that needs lifting has dropped on its hinges, and no strip will seal it well. That changes the whole decision.
When you can do it yourself
If the door closes square, the latch catches without help, and the gap is only at the bottom or along one side, you can usually handle it with self-adhesive foam or vinyl weatherstrip and a door sweep. The materials are simple, and you can work through the door in an afternoon. Clean the frame first with a damp cloth and let it dry, since the tape won't stick to dust. Cut the strip a little long, press it on in one even run, and close the door a few times to check that it compresses without jamming.
A door sweep is the part people skip, and it's the part that matters most along the floor. Measure the gap under the door at its lowest point, not at the middle, because the bottom edge is rarely level. Buy the sweep that fits that measurement, and if the door is hard to measure, bring the door itself to the store, or take a photo of the bottom edge next to a ruler. Screw it on so it brushes the floor without dragging.
When to call someone
Call a carpenter or a handyman if the latch has dropped, if the door rubs at the top when it closes, if the threshold is cracked or pulling away from the floor, or if the door has no weatherstrip at all and the frame is soft or rotting. Those are structure problems. A new strip laid over them will look like a fix for about a month and then the cold will come back around it.
Also call someone if the job means taking the door off its hinges or planing the edge. That work needs a steady hand and the right tools, and it's easy to make a small problem worse with the wrong cut.
What to ask before anyone comes
Ask whether the quote includes replacing the threshold and the sweep, or only the strip around the edges. Ask whether they will check the latch and the hinge screws, and tighten them, not just look at them. Ask whether they will take away the old strip and the packaging, because old foam tape left stuck to the frame is the kind of thing that ends up on your dad's list of things to deal with. Ask them to close the door with him standing there so he can see it work. He will trust a door he watched close properly.
Make it his job, if he wants it
Your dad may want to be the one who fixes it. Let him. He can hold the flashlight while you test the gap, or read the instructions out loud while you cut the strip, or run the paper test himself and tell you where it fails. A man who has spent decades fixing the house does not want to be handed a project, but he does want to be asked what he thinks.
Tonight
Do the flashlight test from outside and the paper test at the four edges. Write down the spots that fail and whether the latch catches by itself. You now know which of the two paths you're on, and you can decide in the morning.
If he is often cold, sleeps in his chair under a blanket, or seems slow to warm up even in a warm room, that is a question for his doctor, not a door job. Bring it up at his next visit and let the weatherstrip be a separate task.
The good-week card
One line for the card: the front door draft got tested with a flashlight and a sheet of paper, and the two of you decided together who fixes it.