Bats in Houses

Bat encounters in the home usually happen at the most inopportune times. One minute you're watching the news, and the next minute, without any warning, a bat zooms through the living room. Soon the dog, cat and family are in an uproar either trying to swat or to run away from the intruder. While this does not happen very often, if it happens to you, you'll want to keep reading to find out more about what it means and what you should do.

A Single Bat in the House

(living quarters: dining room, kitchen, bedroom, den, etc.)

Anytime you encounter a single bat in your residence you are urged to call your county health agency. Your local health agency will discuss the encounter with you to determine if potential contact has occurred with any of the residents or pets withing the house. As all bats have the potential to carry the rabies virus, it is important to rule out any potential contact to ensure the safety of all the residents involved.

Summer Visitor--The occasional lone bat that enters a house during warmer months in Maryland is usually a young bat that simply made a mistake. Their instinct leads them to investigate new areas, including the inside of houses, usually with fatal results.If you are certain that no contact has occurred between the bat and other people or pets inside the house, then you can follow one or several of these suggestions:

  1. Trap the bat in a room and open a window. The bat will find the opening and exit on its own.
  2. Call a bat removal expert (wildlife control cooperator).
  3. Put on thick gloves and wait until the bat lands on a wall to rest. Place a bowl over the bat and slide a piece of cardboard under the bowl. Take the bowl outside and release the bat.
  4. Inspect the outside of your house for signs bats are roosting elsewhere in your house.

Winter Visitor --The occasional lone bat that is found inside the house during colder winter months is likely to be a Big Brown Bat. Big Brown Bats are the only species that can over winter in homes. The warmth of the house and/or outside temperatures can rouse these bats from hibernation, sending them in search of water and insect food.

If no contact has occurred, release the bats outside using the suggestions above. Big Brown Bats are hardy and are able to survive sub-freezing temperatures. They also are aware of and will use alternate roosts in the area.

Colonies in the House

Discovering a colony of bats living in your house can be disturbing and unsettling. The following answers to Frequently Asked Questions will help you understand the problem and guide you towards diagnosing your own situation and deciding the best course of action. These guidelines will help you effectively remedy the problem and guard against future problems.

What bats are in my building?

It's helpful to know what species has taken up residence in your house. Maryland has ten species of bats.

Bats that typically form colonies in and on buildings are Little Brown Bats (Myotis lucifugus), Big Brown Bats (Eptesicus fuscus), Northern Long-eared Bats (Myotis septentrionalis), and Evening Bats (Nycticeius humeralis).

Indiana Bats (Myotis sodalis), a federally endangered species, has recently been documented as inhabiting buildings in Pennsylvania. Eastern Small-footed Bats (Myotis leibii), a species in need of conservation, and Eastern Pipistrelle Bats (Pipistrellus subflavus) may occasionally use buildings.

All of these species except Big Brown Bats will leave your building on their own in the fall to go to their hibernation or wintering sites.

Why are the bats in my building?

Historically, bats lived in old hollow trees, which unfortunately have been effectively removed from the landscape by humans. Bats have responded to habitat loss by adapting to other available habitats, including houses, barns, country churches, condominiums, townhouses and apartment buildings.

Bat colonies living in buildings during the summer are called nursery or maternity colonies because they consist of mothers and their young, known as pups. Bat pups need the high temperatures found in attics and buildings in order to grow fast and put on enough fat to survive hibernation. Female bats are exceptional parents and usually return with their female offspring to the same roost each year. Little Brown Bats are known to live thirty years or more.

How did bats get into my building?

Illustration of home showing areas where bats could possible enter home 

Bats enter buildings from gaps or cracks that are 1/4" to 1/2" in diameter or larger. Some of the ways bats gain entry include:

  1. an unscreened attic vent
  2. a hole or crack under a rotted eave
  3. a crack or separation where the chimney meets the house
  4. loose or warped siding
  5. an open cellar hatch
  6. chimney
  7. openings where pipes or wiring met the house
  8. rotted window sills or a loose fitting screen

How can I get them out of my building?

The only effective way to get the bats out of your house is through exclusion. The exclusion process involves setting up one-way doors for bats to exit your house and then sealing the holes so they can't get back inside.

Care must be taken that bats are not sealed inside your house. Sealing bats inside will create an odor problem when they die and make it more likely that bats will find their way into your living quarters in their efforts to get out.

You can do the exclusion yourself with inexpensive materials, however DNR recommends waiting until September 1 to exclude if at all possible. If you are unsure when to do the exclusion, you can call the Nuisance Wildlife Hotline at 1-877-463-6497 to help you diagnose the situation.

The Wildlife and Heritage Service also has a List of Wildlife Control Cooperators by County who will exclude bats for a fee. Only cooperators with a state permit that explicitly mentions bats are authorized to implement the exclusion process in Maryland. National exterminating companies must also have a valid Maryland state permit. DNR recommends that homeowners get several estimates just as you would for any other contracting job.

During the active colony period (March 1 to August 31), if there are 10 or more bats, a rare/threatened/endangered species, and/or if pups are present, then both the homeowner exclusion form and wildlife damage control operator exclusion form (if applicable) must be completed to generate a Letter of Exemption.

Letter of Exemption

The letter of exemption is required to ensure DNR is aware that bat exclusion work is being performed.

Homeowner Application for a Letter of Exemption

Wildlife Damage Control Operator Application for a Letter of Exemption

A letter is not needed for inspection work or sealing off ways that bats gain entrance into the living areas of the house, but needed only when the actual exclusion phase begins with the installation of one-way doors.

For detailed information about this permit process, see: Wildlife Control Cooperators and Nuisance Bats

There is no known repellent or device that will drive bats out and keep them out. No chemicals can be legally used while bats are in the house. No aerosol fogger is USDA or MDA-approved for use in bat situations.

In the past, chemicals that were used illegally to kill colonies posed health threats to humans and their pets. Any company that proposes to use chemicals while bats are present in the attic is in violation of their pesticide applicator's license and should be reported to the Maryland Department of Agriculture - Phone: (410) 841-5870

When can I exclude bats?

​Below is a general bat activity guideline to help you time the exclusion activities:

Bat Activity Guide

Bats leave hibernation site and return to summer areas; colony formation begins; big brown bats may remain in structure year-round; exclusion is recommended to guard against sealing up hibernating big brown bats
Bat pups are born; pupping can occur over a 2 week period in the same colony; flightless young are likely present; exclusion will result in bats dying and creating an odor problem and/or backing up into the living areas to seek another way to get to their mothers; mothers will be flying around the outside of the house looking for a way to get to their young; EXCLUSION NOT RECOMMENDED
Bat pups begin to fly with mothers; bat deaths from exclusion less likely depending on when the pups were born; August is better month to reduce possible problems resulting from improper exclusion.
September-October:Bats leave summer areas and move to winter sites; best time for exclusion to begin
November-February:Hibernation period; periodic bat wakening and flying in search of water and/or insects may occur during warm weather in winter; exclusion activities should be completed before hibernation begins

The Wildlife and Heritage Service recommends that all exclusions and sealing of entrance holes take place from September 1 to March 1 provided bats are not hibernating in the building. For bat colonies consisting of greater than 10 bats, a rare/threatened/endangered species, and/or if pups are present, the Wildlife and Heritage Service should be contacted (410-827-8612 x. 108) prior to the eviction of a colony from a building.

For general information on excluding bats, please contact the Nuisance Wildlife Hotline at 1-877-463-6497. During the active colony period (March 1 to August 31), if there are 10 or more bats, a rare/threatened/endangered species, and/or if pups are present, then both the homeowner exclusion form and wildlife damage control operator exclusion form (if applicable) must be completed to generate a Letter of Exemption.

Very young pups cannot fly and females leave their flightless pups behind in the roost when they go out to feed. The young are flying by late summer depending on when the females gave birth. Installing the doors prior to September would allow the females to exit the roost, but not allow them to get back in, thereby trapping the young and ultimately resulting in their death as they will not be cared for by the female. Females will also seek other holes to get back to their pups, making contact with humans more likely.

During winter when the bats are hibernating, DNR recommends that the exclusion and sealing of entrances occur after spring emergence to prevent bats from being trapped and killed inside the building.

How do I exclude bats?

  1. Go out at dusk (between 8:30 PM and 9:00 PM during the summer) and watch where bats exit your house. You may notice staining on the outside of the exit hole and guano (waste) beneath the hole. Bats exit to feed on insects every night except windy or rainy evenings. If possible, count the number of bats exiting. They usually exit one at a time. You may have to station people at different points around the house.
  2. Identify all possible entrances even if the bats are not currently using them. Seal entrances that aren't currently being used by bats leaving the ones in use open. Building materials such as caulking compounds, cement, oakum, lath, sheet metal, hardware cloth, and window screens can be used depending on the gap.
  3. Once bat entrances are identified, install one-way bat doors so that the bats can exit but not get back in.

    A successful design for a one-way door was developed and field-tested by Dr. Stephen C. Frantz, the senior research scientist for the New York State Department of Health (Click here to view illustrations of one-way doors). Frantz's design uses polypropylene bird netting. Other materials that can used include hardware cloth and garden netting. Mesh size should be 1/2". Openings of more than 5/8" will allow bats to squeeze back through the door.

    Attach the netting to the building above the exit hole(s) using duct tape. It should project clear of the hole so that it does not block the bats' exit. The sides of the netting are attached to the building, but the bottom is left open and should hang 2-3 feet below the exit hole. This will make an open-bottom sleeve or skirt. No gaps should exist between the sides of the netting and the walls of the building, or bats will be able to fly around the sides.

  4. Leave the netting in place for 5-7 days to be sure that all bats have exited. Bats will not exit on rainy evenings. Bats find their roost entrance by smell and bats may try to re-enter or hang on the netting, but will eventually leave after repeated attempts have failed.

  5. Seal all entrances when the colony has left the house. Unlike rodents, bats can't gnaw through wood or other materials. Cheap and effective materials for sealing holes include caulking, flashing, insulation, window screening, and hardware cloth. These can be nailed or stapled in place to block long, narrow cracks. Caulking cotton, sponge rubber, fiberglass, quick-setting putty, oakum, and self-expanding foam are other possibilities. Spark arresters or bird screens should be installed at the tops of chimneys to prevent bats from entering.

  6. If necessary, clean up guano once the colony has left. When cleaning up bat guano, use a respirator capable of filtering particles as small as 2 microns in diameter. Spraying water on guano will reduce air-born dust during the cleaning.

  7. Bats will return to the same roost each year. If you find that bats have re-entered your house, repeat steps 1-5.

What about bat roosting boxes?

The Wildlife and Heritage Service encourages homeowners to install bat roosting boxes to house the displaced colony either before or directly following eviction from a building. Females return year after year to the same roost and the displaced colony will likely move into your neighbor's house or building. Bats will readily use a bat roosting box rather than leave the area. Installing a bat roosting box will help prevent future problems in the neighborhood. You may have heard that bat roosting boxes don't work, but failures are usually due to poor design or improper installation.

What's in my chimney?

That chittering noise you hear in your chimney may actually be birds and not bats. Click on the link below to learn how to determine whether birds or bats are the culprits.

Chimney Swifts

Check on the following for a link to:

Listings of Local Health Departments

List of Wildlife Control Cooperators by County​​​