The following information is provided courtesy of the Washington Department of Fish and Wildlife.
Spring and Fall Encounters
In spring and fall, migrating bats may temporarily roost outside on window screens, fence posts, piles of lumber, and other unlikely places. If a bat is seen roosting outside during daylight hours, leave it alone. It will probably be gone the following morning.
Bats in Your Home: Encouraging a Bat to Leave
If a bat flies into your home it's probably a juvenile learning to fly, a solitary male following prey, or an adult that has been excluded from its roost. Bats often enter through an open door or window, or by coming down a chimney into an unused fireplace.
If a bat is found inside during the day, confine it to one room. Place a towel under doors to prevent the bat from moving into other parts of the house. Leave the area alone until nightfall.
At nightfall (if you are sure the bat has not been in contact with humans or pets), turn off any lights in the room where the bat is confined, open all doors and windows that lead outside, and stand in the corner. This allows you to watch the bat while staying out of its way. (If you must move around the room, stay as near to the wall as possible.) Be prepared to watch the bat for up to 20 minutes. Normally, the bat will fly around the room to orient itself, and then leave.
If the bat seems to have disappeared but you didn't see it leave, it may be perched somewhere, such as behind a curtain, in hanging clothes, or in a houseplant. The bat will generally choose a high place to roost. Moving these things around with a broomstick may arouse the bat.
Bats in Your Home: Catching the Bat
If the bat doesn't leave, it can be caught and released outdoors away from people and pets. Approach the bat slowly and place a container (small box, large glass, Tupperware container, coffee can) over it. Next, gently slide a piece of cereal box paper or cardboard underneath the bat (be gentle—bats are fragile animals). Using the paper as a cover, take the bat outside. The ideal release procedure is to place the container against a tree, slowly slide the paper away, and then remove the container. Releasing the bat against a tree allows the bat to rest safe from potential predators—like the neighbor's cat.
You may also catch the bat using a pair of leather gloves and a pillowcase. (Never handle a bat with your bare hands.) Put your gloved hand inside the pillowcase and gently place it over the bat.Then fold the pillowcase over the bat so it is inside. Take the bat outdoors and safely release it on a rough tree trunk or lightly shake the pillowcase until the bat flies off. In the absence of a container or pillowcase and gloves, a thick towel can be used. Roll the bat up gently and release it outside.
Bats in Your Home: Referrals to Removal Services
Note: State wildlife offices do not provide bat removal services, but they can provide names of individuals or companies that do. To find such help yourself, look up "Animal Control," "Wildlife Control," or "Pest Control" in your phone directory.