Actually, depending what the flies actually do inside that might indeed be relevant.

I live in the Midwest and I get these flies called cluster flies. They usually only come in in the fall when looking for somewhere warm to overwinter, but I’ve had them other times as well.

They aren’t like houseflies in that they wont really reproduce inside the house, they mostly hang out near windows. I wait for a lot of them to build up on the screen, close the window most of the way, crack the screen open a bit (storm window) and go outside to open the rest of the way.

But once everything is closed up for the year, any left behind just die.

If those are what you’ve got, you probably wont be able to stop them getting in without completely ruining the insulation and humidity profile of your place, especially if its an older place which aren’t neatly as airtight as newer ones. They will get in through any ducts going outside, cracks under your siding that aren’t sealed, you name it.

Best you can do is get rid of them when you get them.