Cover up entry ways. Bats commonly get into the house by way of the attic. They like attics because they are dark and the bats can sleep without being disturbed by humans. The best ways to keep your home bat free is to spend some time covering all of your attic vents and windows with sturdy screens.
Place something that smells strongly in your attic. The smell will discourage bats from using your attic for a nesting place but, because they can be toxic to children and pets, you should avoid using mothballs. Menthol and Eucalyptus are good choices.
Place screens over your attic vents and use another screen to cover your chimney.
Fix any ripped or torn window screens. While you are doing this you should have a tube of caulking glue on hand that you can use to seal any cracks or gaps. Not only will caulking the gaps help keep your house bat free, it will also help lower the cost of your heating bills during the winter months.
Go outside and watch. If you have gone through all of these steps and still have bats invading your home, you will have to spend some time sitting outside. When the sun starts to go down you need to sit outside, facing your house. You are watching to see where the bats exit. The bats are entering your home through the same hole that they are using to leave it.
Seal the hole once you have determined where the bats are entering.
Routinely check all of your screens and make sure that they are tightly fastened and haven't torn.