A number of methods have been tried to evict colonies of bats from buildings. Whether you do the work yourself, or call a pest control company, the ONLY CONSISTENTLY SUCCESSFUL METHOD IS PERMANENT PHYSICAL EXCLUSION.
Timing is critical for excluding bats. In the spring and summer, if a maternity colony has taken up residence, you will need to delay excluding the bats until August when the young are able to fly. If you exclude the adult bats while the young are flightless, the young bats will needlessly starve to death and may create an odor problem. Frantic mother bats, excluded from their young may mistakenly get into your living area when trying to find a way back to the roost to care for the pups.
First step: Bat-proofing
Whether or not you decide to delay excluding the bats for the above reasons, concentrate your efforts first on bat-proofing the living areas of your home. Make sure doors to attics and basements are sealed with draft guards and holes in any interior walls and ceilings are repaired so that a bat can't mistakenly enter the main parts of your house.
Second step: Locating entry points
The next step in excluding bats is to locate the exterior entry points. During the spring or summer, station several people around the outside of the house 1/2 hour before sunset to watch for bats leaving each evening to feed. The watch should continue for about 1 hour after the first bats emerge. Bats can enter through holes as small as 3/8" (the diameter of a dime) or spaces 3/8" by 7/8". Typical entry points include chimneys, louver fans, air intakes, exhaust vents, openings around plumbing, power or cable lines, spaces around doors and windows and where exterior siding has shrunk, warped or loosened. Close inspection during the day will help determine the exact location of the entry points.
If there are many entry points you can close the less frequently used holes. Hardware cloth (1/4"), can be used to cover chimneys and vents. Caulk, weatherstripping, insulation materials, screening, steelwool or even duct tape can be used to close these and other entry points. Efforts to batproof your home will also often improve energy efficiency. Make sure to leave the most frequently used entry sites open until mid-August.
Third step: Deter entry
In August, the next step is to drape 1/2" X 1/2" structural grade (weight 1.3 oz. / yd), bird or bat netting over the remaining outside entry points (see figure at right). This netting should be available at most garden or hardware stores. Drape the netting over each entry point extending several inches above, 1 ft. to the sides and 2 ft. below the opening. Attach the top and sides of the netting to the structure with tape, staples or velcro strips. The bottom should left open so the bats can crawl out when they exit. Don't stretch the netting too taut or the bats will not be able to get out. When the bats return from feeding they will land on the netting close to the hole but will not be able to enter.
Experiments indicate that bats quickly become frustrated and fly away. Unlike rodents, bats probably will not attempt to chew through the netting and they can't squeeze through because the netting is not stable enough. Not all bats leave the roost each night, therefore, the netting should be left in place for several days or up to a week if the weather is cool or wet. Removal of the nets and permanent screening or repair of these last entry points should be done simultaneously, once all bats have left.
Or just wait for them to leave
Provided you have secured the interior of your home, an even easier solution is to wait until late fall when the bats will have left on migration to their winter hibernating sites. Buildings are usually unsuitable as winter hibernacula as bats prefer mines, caves, and tunnels underground where humidity is high and temperatures consistently cool. Once the bats have left on their own accord, the hole closure is most easily accomplished using chalking, screening or other repairs discussed above.
Occasionally, an individual bat will attempt to hibernate in a building. See the section "removal of a single bat" for advice on how to evict such a creature.
What NOT to do!
Attempts to poison bats, or exclude them using inappropriate methods can actually increase human contact, as sick or homeless bats may disperse through the neighborhood thereby increasing chance encounters with people or pets.