What kind of sweep works when the bottom of my door is flat?

The U-shaped slide-on ones are made for doors that have a kerf or channel cut into the bottom edge, or doors thin enough for the two legs to pinch it. On a flat-bottomed door they either sit loose and rattle, or you squeeze them on and they drag and scuff the paint every time the door swings. Skip those entirely for your situation. What you want is a screw-on sweep. The two styles that actually hold up are the aluminum track with a flexible vinyl flap, and the one with a brush strip. The vinyl flap style seals better against cold air, which is your problem. The brush style is more forgiving if your floor or threshold is uneven, but it lets a bit more draft through. Either way, you screw the housing to the face of the door near the bottom and let the flexible part hang down to the threshold.

Before you order anything, close the door and measure the gap between the bottom edge and the threshold. You want the sweep to just barely touch or drag the tiniest bit, not hover above it or squash down hard. If the gap is bigger than an inch, no standard sweep will reach, and you might have an adjustable threshold you can raise instead. Look at the top of the threshold for screw heads, turning them usally lifts the whole thing up.

One more thing worth checking while the old sweep is off. If the vinyl crumbled away like that, moisture might have been getting into the bottom of the door. Poke around the bottom edge and see if it's soft or swollen anywhere. If it is, seal the raw edge with a couple coats of exterior paint or primer before you mount the new sweep, otherwise the new one goes on over rot and you'll be back here in two winters. Cut the sweep to length with a hacksaw, pre-drill your screw holes so you don't split anything, and it should last you several years no problem.

New here so hope this is ok to add. I put a screw-on sweep on my back door last year after the old one fell apart too, and the vinyl flap style worked fine at first but I swear it went stiff in the deep cold and stopped sealing right for a couple months. coud have been a cheap one though. Is that something to worry about, or was mine just junk? Thinking of doing the front door next and not sure which style to pick this time.

That stiff-in-cold thing the last poster mentioned is real, I had the same problem on my side door. Swapped to one with a neoprene flap instead of plain vinyl and it stayed flexible all winter, no more draft. Cost a couple bucks more but I haven't touched it in three years now. Also when I mounted mine I ran a thin bead of clear silicone behind the housing before screwing it down. Water was sneaking behind the old one and that's probably what ate the vinyl in the first place. Ten extra minutes with a caulk gun and the screw holes aren't wicking water into the door either.

Been through this same fix a couple years back, and one thing that tripped me up was the threshold itself. Mine had a little lip on it and the flap woud snag every time the door opened. Took me way too long to figure that out, kept blaming the sweep. If yours has any bump or ridge, sand it down or the new one's gonna catch from day one.

Also, once the bottom's sealed up don't be surprised if you notice air sneaking in from the latch side next. Weatherstripping on the jambs goes first alot of the time, worth a quick feel with your hand while you're at it.

Gonna push back on the 'drag the tiniest bit' advice from the first reply. On a door that gets opened twenty times a day, any actual drag chews the flap up fast. Mine wore down to nothing in under a year because it scraped every single swing. Set it so it just kisses the threshold, no sound, no resistance when the door moves. You lose maybe a sliver of sealing power but the sweep lasts years instead of months. Cold air moves through a hairline gap slow, it's not the river of draft people imagine. Also worth checking that first reply's threshold tip against your own door, lots of thresholds out there are solid aluminum with no adjuster screws under those plugs, so don't count on being able to raise yours.

Update: so I checked the threshold after reading that last reply and sure enugh, mine has a raised ridge right where the sweep would drag. Guess I found the reason the old one shredded in the first place. Planning to sand it down, but does the neoprene flap need to just barely touch the threshold when the door is closed, or is a little contact okay when it swings?