The concern about a bat crawling under a door threshold and into a home is valid because bats possess a surprising ability to exploit small openings. Their unique anatomy allows them to compress their bodies into spaces that appear far too small. While a poorly sealed door gap can certainly provide an access point, bats more frequently utilize existing structural breaches higher up on a building. This ability to locate and utilize tiny entryways is why bats successfully cohabitate with humans, often without immediate detection.
How Small a Space Can a Bat Squeeze Through
A bat’s physical structure allows it to pass through gaps that seem impossible for a mammal its size because their bone structure is more flexible than most other animals. Many common microbat species can squeeze through an opening as narrow as 1/4 inch (6 millimeters). This dimension is comparable to the width of a dime or the thickness of a pencil pressed into a seam. Bats do not “crawl” but leverage their pliable ribs and chest cavity to flatten their bodies and wriggle through narrow crevices. Their wings fold tightly against their frame, and their body tissues are highly compressible, enabling them to navigate tight voids. Therefore, a gap beneath a door threshold can be large enough for a small bat to exploit, though other structural vulnerabilities are usually preferred.
Common Structural Entry Points
While a door gap is possible, the most frequent entry points for bats are located along the roofline and upper structural junctions of a home. Bats are attracted to small, dark, elevated breaches that mimic natural roosting sites like tree hollows or rock crevices. These elevated locations offer undisturbed shelter and are often concealed from ground-level view. Common access points provide direct entry into attics and wall voids:
- Gaps in the eaves where fascia boards meet the roof deck.
- Ridge vents without proper end caps.
- Unsecured chimney crowns.
- Loose or damaged flashing around roof penetrations, such as chimneys or plumbing stacks.
- Improperly screened attic louvers or foundation vents.
Why Bats Enter Human Structures
The motivation behind a bat entering a human structure is almost always related to finding a stable, protected environment to meet their physiological needs. The primary drivers are thermal regulation and the establishment of safe maternity colonies. Bats are heterotherms, meaning they can modulate their body temperature, and they seek warm, dark spaces to reduce the energy cost of maintaining warmth, especially during periods of rest. Attics or wall voids provide an ideal microclimate, benefiting from solar gain during the day, which allows bats to passively rewarm. This is particularly important for pregnant or nursing female bats who form maternity colonies during late spring and summer. The warmth accelerates the growth and development of their single pup, increasing its chances of survival.
Safe Bat Exclusion and Sealing
Removing bats from a structure must be done safely and legally to protect the animals and prevent them from being trapped inside. The process involves installing a one-way exclusion device, often referred to as a bat valve, over the primary entry point. This specialized fitting allows bats to exit the structure at dusk for their nightly foraging but prevents them from re-entering when they return before dawn. It is mandatory to time exclusion efforts carefully to avoid what is known as the “maternity season,” which generally runs from late spring through late summer, though exact dates vary by region. Sealing a colony’s entrance during this time will trap flightless young inside, leading to their death and creating odor and sanitation problems within the structure. Once the one-way device has been in place for a sufficient period and all bats have exited, the opening can be permanently sealed with materials like caulking, weather-stripping, or hardware cloth.