Bats in Buildings


Bats found in interior (living or working) spaces are usually there by accident and a single bat flying inside does not necessarily indicate that a colony is in residence. Occasionally a bat may fly through an open door or window, or be carried in by a pet. These accidental visitations can be prevented by keeping doors and windows screened, chimneys capped, and exterior basement or attic doors closed and in good repair.

Bats that roost in buildings are usually in structural voids, the spaces between the exterior and interior envelopes of a building.

Bats enter voids through openings on the exterior of buildings. A colony may remain unnoticed unless someone sees, hears or smells them. When a resident colony is present in the structure, lost bats may find their way into the living area once or twice a year. These events are usually rare, but may occur during the summer maternity season (lost juveniles), or when they awaken briefly from winter hibernation.

Myotis Yumanensis
CREDIT: Bruce D. Taubert
Pallid bats roost inside bridge crevice
CREDIT: Bruce D. Taubert


Potential access areas for bats include structural penetrations as small as 5/16” (8mm) x 1½” (38 mm), or holes 5/8” (16mm) x 7/8” (22 mm), which can include expansion joints, holes, cracks or crevices on the exterior of a structure. As a general rule, if you can get your pinky finger in, a bat can enter. Most bat species choose roost entries in high, out-of-the-way areas (above 10 feet), but in some cases, bats will enter basements, cellars and other areas beneath a house.

Small bat colonies can usually be tolerated and simply left alone, but bats should always be prevented from entering human living quarters.

The first step in exclusions is to inspect the building’s interior for small openings through which bats could enter. All openings connecting the attic or other potential roosting areas to living quarters should be sealed, while entry points on the outside of the building are left open. Caulking, flashing, screening or insulation can be used to seal most openings on the inside. Draft guards should be placed beneath doors to attics; electrical and plumbing holes should be filled with steel wool, caulking or weather stripping.

Caulking, flashing, screening or heavy-duty mesh can be used to bat-proof most openings on the outside. Expanding foam or similar products should never be used to seal cracks in a building where bats are active because they can become caught in it. Caulking should be water-based and applied early enough in the day so it has time to dry before bats emerge in the evening.


Catching bat with box


Interior access can be prevented without disturbing the colony by closing interior openings such as those around plumbing or gas pipes, electrical wiring, or heating and air conditioning units found in utility closets, cabinets, behind appliances, and under sinks. Ensuring an interior “bat-free zone” will prevent the stress (for bat and human) of the sudden appearance of a flying bat in the home or workplace, and allows any further exclusion actions to be carefully considered and planned.

Although interior openings can be closed to prevent bat access to the living space, closing exterior penetrations can be fatal to roosting bats and should never be done without a solid plan to prevent entrapment.

Because entrapment occurs inside walls and other areas that are away from human view, exclusion attempts (or any event that prevents bats from leaving a building) can cause significant mortality that is rarely documentedand often unnoticed, making it nearly impossible to accurately quantify. Evidence of entrapment may include numerous bats suddenly appearing in the living space of a building. If recognized and acted upon quickly, bat mortality may be avoided. Signs that trapped bats have died of dehydration and starvation include strong putrid odors, and dark stains on interior walls or ceilings caused by seepage from decomposing bats.

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