Bats can enter extremely small holes. They typically enter buildings through openings in the roof, eaves, dormers or gables, or through the chimney. They can also get in through attic or roof vents, and holes or gaps in siding. You'd need to seal all gaps of ¼ × 1 ½ inches or holes 5/8 inches diameter or greater. It may be worth hiring a nuisance wildlife control operator with bat experience to help you locate the entry point(s) so that you can seal them up.
