Most bats give birth in May or June. The young are weaned about a month and a half later. So it should be fine to close the openings now (after first making sure there are not still bats inside the structure).
Watch the exterior of the building from dusk (half an hour before sunset) until it gets dark outside to determine the entrance(s) to the roost site. Bats typically enter buildings through openings in the roof, eaves, dormers or gables, via the chimney, attic or roof vents, and holes or gaps in siding. Seal all gaps of ¼ × 1 ½ inches or holes 5/8 inches diameter or greater. Seal minor points of entry first. After bats leave the roost in the evening, temporarily seal all major entrances with a one-way door made of wire or plastic mesh.
One-way doors can be made of ¼ inch wire screening. Use a 2- to 3-foot piece of wire to cover the hole (there should be 3 to 5 inches between the structure and the wire to give the bats room to crawl out). Secure the top and sides of the wire to the building using staples or duct tape. Leave the bottom section open. After all bats have left the building (this may take several nights) permanently seal all openings. One-way doors should be in place before the young are born in May and June to prevent the young from starving after the mother has been excluded from the site. If there are young bats inside the building, wait until they are weaned (late July-August) before excluding the bats and sealing all entrances. Bat Conservation International provides good information on exclusion techniques.
