I have a lot of experience with them. The tricky part is that where I live, the Indiana Brown Bat is an endangered species so you can't get an exterminator to do anything. You have to stay on top of it, or they will keep trying to come back, once they nest in the house they will continue to try to get back in.
Here are the things I know work from Experience.
1) Moth balls in atttic, open the vents and throw them in. They don't like them and will leave.
2) Put lights up the attic and leave them on, they don't like light.
3) The most important is, get some Great Stuff spray foam, they kind that cures hard and fill every opening. Look for openings where they are coming in and out. A sure sign of a an opening being used, is this black grease like substance around the opening. Once you get the foam in it, use some wood or metal to close the opening permently.
3) Use silicon caulk to seal up any small opening on siding and and vents, they can get in very small cracks. Think mice, as they can get into the same types of spaces.
4) Get some of the electronic rodent/pest deterent devices that plug into your electrical system. The best ones are the ones that work by going through the electrical system. Put one on each floor.
If you pay attention around dusk you should be able to see where they are coming in and out. Where I live they are really common, and can be a real pain. Good luck, if you put in some effort you should be able to get rid of them.