Bat-Proofing Buildings - October 15, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Bats are beneficial animals. Most bats are insectivorous and may consume more than one-half their body weight in insects each night. A large colony can eat literally tons of insects annually. Other species are important pollinators of native plants. Many bats live in large colonies. If the colony is in your attic or other building, this can cause unwanted noise and pose potential health risks to people living in close proximity.

Before attempting to manage bats, it is essential to verify that bats are actually the cause of the nuisance. The only way to permanently rid a building of bats is to eliminate all possible entrances. Repellents and traps are not permanent control methods. Bat-proofing a building is the only efficient and permanent way to eliminate bat problems.

Bats should not be excluded from roosts when females are raising pups. This is generally May through August in Arizona. Mothers often leave young bats in the roost when foraging at night. If bats are excluded during this time, the pups will be inadvertently trapped inside the roost. Some bats also hibernate in buildings during the winter months. Winter exclusions should be performed only if it can be determined that no bats are hibernating in the building. If bats are present during the winter months, exclusion should be postponed until the spring.

Bats may enter buildings through openings such as louvers, vents, broken windows, worn-out siding or holes around eaves and cornices. Other problem areas are faulty ridge vents, crevices in the soffit and fascia, chimneys that have slumped away from buildings, loose flashing, and the interiors of abandoned chimneys. Smaller bat species can crawl through slits as narrow as 3/8 of an inch. Therefore, it is essential to seal all openings 1/4 inch or larger.

Examine the building during the daytime to determine where bats are entering and exiting the roost. Look for brown stains at the roost openings caused by oils on the bats’ fur rubbing off on the outside surface of the building. Guano (droppings) may also be visible beneath roost openings. Generally, most bats will leave the roost at dusk and within 30 minutes after the first bat exits. Observing the building within an hour after dusk is the best way to locate openings used by bats as they fly out.

The next step is to screen all potential openings with fiberglass window screen fastened in a way that allows bats to escape but not re-enter. Fiberglass screen material is available at home improvement or hardware stores. Do not use wire screening as it is not flexible. The top and sides of the screen should be affixed to the building at the roost exit and the bottom left open creating a one-way door. Bats will exit the roost and crawl down and out from under the screen, but when they return and attempt to land, their re-entrance is blocked by the screen. Leave the netting up for 5–7 days, and observe the bats exiting at night to see that progressively fewer bats are leaving each night and not using alternative openings.

Seal all holes after you are sure all bats are gone. Construction adhesive or caulking can be injected in cracks, crevices or any smaller entrances. Larger crevices may need to be stuffed with fiberglass screening as fill before being caulked over. Large openings should be covered with sheet metal or with 1/4-inch mesh hardware cloth if ventilation is necessary. Foam is very messy to work with and can kill animals that come into contact with it; it should only be used in very complex or hard-to-reach areas. Bats, unlike rodents, will not chew their way back into a roost.

In areas where bat populations are high and other suitable roosts are limited, homeowners should consider installing a bat house. These simple structures are commercially available. For on-line resources about bats and bat houses, visit the Bat Conservation International website at I would also like to acknowledge Dr. Jon Boren, New Mexico State University Extension Wildlife Specialist, for much of the information used in this column.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: July 16, 2009
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