Chimney tops should be screened from February to September to prevent birds and animals from nesting inside. To prevent fire and safety hazards, check with a knowledgeable source before attempting this. Seal all cracks and holes larger than a one-quarter inch in diameter to keep out rats, mice, bats and snakes. Bats Exclusion is the best policy in preventing bats from gaining entrance into buildings. It isn't always easy, though, because of their size.
Bats can squeeze through cracks as narrow as an inch wide.
Some of their preferred entrances are in older frame structures where boards are loose or have shrunk. They may also enter houses through loose vents, eaves, spaces around water pipes, electrical outlets, corrugated roofing, doors or windows. Caulking cracks is most effective if applied during dry weather when cracks are the widest. If you're certain the noises in your attic are being made by bats, wait for them to leave, then seal the entrances before they return. Unfortunately, you'll need to do the work at night when bats depart for their nightly feeding forays.