Bats in Buildings
Excluding a Colony

Excluding a Colony


Excluding bats from buildings—How to do it

Removing, or excluding, bats from buildings requires establishing one-way exits through which the bats can leave but cannot return, while also sealing all other potential entry points. This process of eviction and exclusion is the only effective and permanent solution when bats in a building are unwanted.

Trapping and relocating is ineffective since bats have powerful homing instincts and will simply return, even when released at great distances. The use of pesticides against bats is illegal and counterproductive, and greatly increases the likelihood of bats coming into contact with people and pets. Naphthalene, the active ingredient in mothballs, and ultrasonic devices are often promoted as bat repellents. Ultrasonic devices have proven ineffective, however, and naphthalene, to be effective, would have to be used in such large quantities that it would pose a significant health hazard to humans.

Catching bat with box

How to Exclude Bats:

Excluding bats with tubes

In most cases, tubes make the best bat-exclusion devices. These include openings on buildings with rough exterior walls, such as brick or stone houses and log cabins. Tubes also work best for holes at corners where walls meet and on horizontal surfaces such as soffits.

Catching bat with box

Exclusion tubes should have a diameter of two inches (five centimeters) and be about 10 inches (25.4 centimeters) in length. Exclusion devices can be purchased commercially or made from PVC pipe or flexible plastic tubing. Bats are unable to cling to the smooth surface of these tubes, so the tube should project no more thanone-quarter inch (six millimeters) into the opening.

This will ensure exiting bats can easily enter the tube. Empty caulking tubes also work well after caps at both ends have been cut away. Caulking tubes must be thoroughly cleaned before they can be used for exclusions because dried caulk forms a rough surface that could allow bats to reenter. These flexible, plastic tubes let you squeeze one end so it fits into a crevice. Or you can cut one end into flaps that fit over an opening and can be caulked, stapled, nailed or screwed into place (see diagram).

Once the tube has been secured over the hole, a piece of lightweight, clear plastic can be taped around the tube’s outside end (see diagram) to further reduce the likelihood of bats reentering, though this is usually not necessary.

Catching bat with box

After the tube has been secured into or over an opening used by bats, any spaces between the outer rim of the tube and the building must be sealed shut. Also be sure to seal any other openings in the building that bats could use. Leave the tube in place for a minimum of seven days to ensure all bats have left. After the bats have been excluded, the tube should be removed and the opening permanently sealed with water-based silicone caulking, caulk-backing rod, hardware cloth or heavy-duty plastic mesh. In some cases, sealing may require repair or replacement of old, deteriorated wood. When bats are using multiple openings to enter and exit, exclusion devices should be placed on each opening. If the bats do not appear to be exiting or seem to be having trouble doing so, add new valves as needed.

Never simply wait for bats to fly out at night and then seal openings. Not all of the bats leave at the same time, and some may remain inside all night, especially during storms.


Keep in Mind:

Catching bat with box
CREDIT: MerlinTuttle.org

Seasonal Concerns

Bats often roost in buildings during maternity periods, when they give birth and raise their pups. Exclusions should not take place until young bats are able to fly; otherwise, they will be trapped inside, away from their mothers, and die of starvation. Separating pups from their mothers may also lead mother bats to search for other entrances to reach their young. In North America, the maternity season begins as early as mid-April in the southernmost United States and in the mid-June in the northern U.S. andCanada. Young bats are usually flying by late August.

Exclusions should not be conducted between April and late August.

Cool or Severe Weather

Catching bat with box
CREDIT: MerlinTuttle.org

Contrary to popular belief, not all house-dwellingbats migrate to warmer climates or enter caves or abandoned mines to hibernate in the late fall. However, a few species can hibernate in buildings. If hibernating bats are present in cold regions during the winter, exclusions should be postponed until spring when they emerge to feed. In mild climates, some bats may remain active year-round, but exclusions should be carefully monitored or avoided during cool weather when night-flying insects are not present, and at any time of year when rain, wind or severe weather is occurring or expected.




When bat exclusion should not be performed

Bat exclusion should NEVER be performed during any period when bats do not leave their roost on a regular nightly basis. This includes during maternity season in the summer, hibernation or torpor (a less lengthy period of inactivity) in winter and during periods of inclement weather. Maternity season dates vary by region and are species-specific; though not typical, some tropical and subtropical species in southern regions may give birth twice a year.

In North America, the maternity season begins as early as mid-April in the southernmost United States and in mid-Junein the northern U.S. and Canada. Young bats are usually flying by late August. Exclusions should not be conducted between April and late August. Some U.S. states and Canadian provinces have passed regulations governing bat exclusion dates. Please check with your state or provincial wildlife agency for the rules in your area.

Methods to Avoid:

Methods that pose a danger to bats or the public and are NOT recommended:

  • Any products or structural modifications that block natural ventilation, like hanging plastic sheeting over an active roost entrance, thereby altering roost microclimate
  • Silicone, polyurethane or similar non-water-based caulk products
  • The use of flexible netting as an exclusion device, or one-way door, is overly complicated and there are multiple draw-backs, including entangling bats, which can result in permanent injury or death. In the majority of situations, BCI does not recommend its use.
  • Any exclusion device attached with duct tape. Duct tape or similar adhesive tapes fail when surfaces are rough, coated with dust, mold or mildew, or when used in high humidity or during rain, and can result in re-entry or entrapment.
  • Expandable foam can block ventilation and break down in the heat, allowing bats to re-enter. It can kill bats that are exposed to the material before it dries--dead bats have been found entombed in foam.

Hiring a Professional:

Should I Hire a Professional or Do it Myself?

Safely and permanently excluding bats from buildings requires patience and attention to detail. It can involve working high on ladders, scaffolding or even a hydraulic lift. Though detailed exclusion instructions are included here, many prefer to contact a bat management professional. Bat Conservation International no longer maintains a list of BCI-approved bat exclusion professionals, but we do provide criteria for selecting a qualified professional.

BCI recommends that the company:

  • Have verifiable bat exclusion experience (both the company and the individual)?
  • Be licensed by the state and insured against any incidental damage that may occur?
  • Have demonstrated knowledge and familiarity with local bat species (e.g., specific roosting preferences, behavior and seasonal activity, including maternity season date range for the species identified)?
  • Provide at least three recent client references (with phone numbers) for similar projects?
  • Offer a workmanship and materials guarantee for at least two years?
  • Offer a written contract?
  • Use BCI-approved materials and methods

Be wary:

  • If they use scare tactics to convince you that your family is in danger from roosting bats and must act immediately?
  • If they advise or use ineffective methods such as ultrasonic repellents (one study found that ultrasonic devices may even attract bats), or materials that degrade quickly like expandable foam, paper, steel wool or rags to close holes?
  • If they advise or use illegal methods (in the U.S. and most European countries) such as chemical pesticides?
  • If they agree to exclude bats during maternity season.

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