We live in framing country too- plenty of manure around, and muck spreading from time to time. You makes your choices ...
We do get flies in the Summer - so we've put screens on one window per room- but the problem is the kitchen door, as I + dog go in and out every few minutes, so closing the screen is useless. We will invest in one of those beaded curtains this year, and see if that does the trick.
Just too many of us want our cake and eat it though. One of the joys of Summer here is watching the swallows and housemartins dash in and out of our large roof barn, where they live - no flies, no swallows- and on balance, I'd rather have the swallows ... so will have to accept the flies.
People want pristine gardens, and wonder why wildlife is no longer abundant...