how are flies getting in?

It's from the dead bodies in your crawlspace.

Every time you open the door they probably fly in so I guess that means you have to stay indoors at all times and never open your doors lol - just kidding but really they probably so sneak in that way. Or you had them from last summer and the eggs hatched and now you have flies. Get fly paper. The other couple of suggestions would be make sure you clean everything, leave no water sitting out and not even the smallest piece of food. Use pine sol and water to clean off everything. They look for food and water. The other advice which someone else gave me that I thought they were nuts, but it worked and they are still hanging up. Get yourself a ziploc bag fill it with water half way and place a penny in that water. Then hang it up near your doors outside or inside so that when BUGS - flies, gnats, bugs period see that they will back off and not bother. Why? & What the heck? Well that is because the watr in the bag makes the penny magnify and it makes it look like a HUGE spider. Bugs have magnifying eyes. They hate spiders! So this bag will ward them off.

When the weather cools off, flies will go into attics, like cluster flies, and stay for the winter. They can go down into the house through light fixtures.

Flies can get in around some windows.

And vents.

Some flies will get on the sunny side of the house and go inside through tiny openings.

If you have a cat, it might have brought a mouse in thats died and the maggots in it have hatched into huge flies, just a thought.

You need to kill the odors in the air, try making 50% water and 50% vengar. Wonderful for cleaning as well as killing bacteria, mold, viruses and air Odors.