No Stupid Questions

Drains!!! Make sure you have water in all your ubends

I used to have lots of flies and noticed one summer when I put tin foil in the windows to cool my house down, I stopped finding flies inside completely. Maybe because it made my house dark, they didn't want to come in anymore? Idk, but I've been gloriously fly-free ever since. You've mentioned you have a basement—do you find fewer flies there? I suspect they might avoid dark places.

They come in through the door when you do. That's the only thing I can figure out, I have the same problem. I've caught them doing it a couple of times, but of course I don't see them every time.

I think you're doing everything you can to keep them out. Unless, you want to build some sort of airlock contraption. Make it with smooth, white walls, a bright light, and a fly swatter. But, the thing is, even then, a few would sneak through, and that would absolutely drive you mad after going through all that effort.

For me it is in the hottest months and its in the recycling. The recycling bins are not as contained as the general bins, residual food is in the recycling from improper washing. Lone fly in through an open door and lays eggs in recycling, in normal times process from egg hatching to maggots to flies takes longer than the recycling collection period so it is all thrown out. Under very hot conditions it is accelerated and you wake up to 10 newly hatched flies on the inside of your garden door every morning. Solution is when it's very hot put the recycling out more often even if it attracts foxes. This is very specific to me but something similar may be happening for you, I don't think flies squeeze through small gaps.

Supplement your offensive salt gun strategy with spiders.

You could recreate the experiments of Jan Baptist van Helmont to determine if your house is the one place on Earth where life spontaneously generates from inanimate matter.

I was having the same problem and we couldn't figure it out. I tried hanging up some old school fly paper strips and withing a week or so all the flies were dead on the fly paper and they haven't come back.

Start by identifying the fly. Habitats very by species.

Could be something died in your chimney, could be drain flies, could be vents. Could be doors or crawlspaces. Doesn't take much.

If you've got screens, check that they're in the correct position. I had a sliding screen patio door get knocked off the bottom track, and the gap was wide enough for wasps to get through

They were laying eggs in the window sill at my apartment. They wouldn’t stop and we didn’t realize.

There’s numerous other places, holes in screens of windows, of bathroom exhaust, dryer exhaust. Most annoying might be central heating if you have.

The mentioning of a salt gun just sent me down a rabbit hole...

And this below must be the most US thing I have seen in quite a while! 😆

Also found a fantastic direct translation with hilarious WTF-factor on the page (for anyone speaking German):

Get those sticky paper strips. You might have to wait a few generations, but after they "hatch" they don't immediately reproduce, but instead look for food first. That's the small window where you have to get them.
Importantly, you have to keep the strips up at least 1 "cycle" after you've caught the last fly!

What kind of flies? House flies or fruit flies?

dead animal in your walls or similar.

they will go away eventually. once the food source does.

just kill them.

They probably aren't getting in as flies. But also they probably get in when you enter or leave.

Bathroom and kitchen exhaust fans might be an entry point, especially if some critter crawled into the duct and died.

The windows I didn't replace are all jacked up - including the screens. That's where I assumed it they were getting in which is why I caulked them closed. I guess at least that'll save me on heating and cooling...

Need more info.

Do you live somewhere that has lots of bugs (Like a high humidity place, lots of plant life outside) or in a dry climate?

In an area with lots of bug life it's hard to completely keep them out - your house is like a sieve and you have a high "bug load". So investigate gaps in window frames, screens, etc.

In a place with few bugs (say US southwest), you're just playing odds and aren't likely to get many anyway.

Someone else mentioned they're probably coming in with you through the open door - in my experience that's the "big gap" (haha) that's hard to reduce.