Bats in the Chimney: What to Do and When

How to tell it is bats

Bats are quiet compared with birds or squirrels. Most people hear faint squeaking, scratching and a papery rustle around dusk, when the colony wakes up and leaves to feed, and again before dawn when it comes back. During the day the chimney is mostly silent.

The physical clues are just as telling. Small, dark droppings that crumble to a dry powder collect on the smoke shelf, on the hearth below a leaky damper, or at the base of a boiler flue. The New Jersey Department of Health points to droppings and rub marks at the entry opening as signs, and suggests watching the building from outside, from half an hour before to half an hour after sundown, to see where bats come out. A large colony can also bring a sharp, musty odor into the room around the fireplace on humid days, along with the noise.

Bats or chimney swifts?

In summer, a loud, rapid chattering from the flue in the middle of the day is more likely chimney swifts, a small bird that glues its nest to the inside of the flue wall. Swifts arrive in warm months and leave for the winter, and at dusk you may see a group circling and dropping into the chimney top.

The difference matters because the two are handled differently. Neither should be smoked out or sealed in. If you are not sure which you have, a wildlife professional can tell you from a dusk watch, and the answer decides who does the removal and when the cap can go on.

Why a chimney appeals to a colony

A masonry chimney is warm, dark and rarely disturbed, which is what a summer colony looks for. An uncapped flue is the obvious door, but not the only one. The New Jersey Department of Health notes that bats can squeeze through openings as small as three eighths of an inch and lists crevices around the chimney among common entry points.

On older brick chimneys in the region, those crevices are easy to find: a cracked mortar wash around the flue tiles, flashing that has pulled away from the brick at the roof line, open mortar joints on the weather side, and gaps where a chimney meets a gable wall. The same department notes that a colony left undisturbed will return to the same roost each spring for many years, which is why bats can show up in the same chimney summer after summer.

Seasonal timing for removal

Exclusion works on the bats' schedule, not the homeowner's. The Pennsylvania Game Commission advises against sealing bats out during June or July, because flightless young would be trapped inside, and suggests sealing entrances once the bats leave in the fall. It also notes that excluding bats correctly can take two years. The New Jersey Department of Health says young are born in June and July and that bats hibernate over winter in caves, mines and buildings.

Protections and who decides

The Pennsylvania Game Commission lists the little brown, northern long-eared, Indiana and tri-colored bats as endangered and protected, and the eastern small-footed bat as threatened in the state. The New Jersey Department of Health says bats are protected under the state's Endangered and Nongame Species Conservation Act, making it illegal to hunt, capture or kill them.

Rules and timing windows differ by state. In New York, Delaware or anywhere else, check with the state wildlife agency or a licensed wildlife professional before any exclusion work starts. None of this is legal advice; it is the reason the chimney crew waits for the wildlife side to finish.

What not to try

Several quick fixes make the problem worse or put people at risk.

• Smoke or a fire in the fireplace: it can kill young, drive adults into the house and harm protected species
• A cap fitted over an active roost: the bats are trapped and may push into the living space looking for a way out
• Poison: the Pennsylvania Game Commission notes no pesticides are approved for use on bats
• Glue boards or fly paper, which the New Jersey Department of Health says should not be used where bats may contact them
• Sweeping up droppings dry with a broom, which raises dust

If a bat gets into the house

When you are certain nobody, person or pet, has touched the bat, the New Jersey Department of Health suggests confining it to one room, turning on the lights and opening the windows; bats usually leave at dusk. Close interior doors and keep people out. Do not try to catch one barehanded.

Bats that reach the living space from a chimney usually come past a damper that does not seal: a warped plate, a broken handle, or a fireplace with no damper at all after an old insert was removed. Until the colony is excluded, the New Jersey Department of Health suggests interior bat-proofing such as weather stripping, caulking and draft guards under doors. For a fireplace, that means a closed damper, closed glass doors, and a technician checking that the damper plate actually meets its frame.

If anyone may have been bitten or scratched, or had bare skin contact with a bat, or a bat was found in a room where someone was sleeping, the same department advises contacting the local health department and seeking medical attention. Your doctor or health department will tell you what to do next.

Closing the chimney once they are gone

The chimney work keeps the bats out after the wildlife professional gets them out. Every flue gets a cap with fine mesh, fastened with no gaps at the corners or base. Cracks in the crown or mortar wash are sealed or the crown is rebuilt, open joints are repointed, and flashing that has pulled away from the brick is reset and counter-flashed. Gaps where the chimney meets siding or a gable wall are closed too.

Droppings on the smoke shelf and down at the flue's lowest point are removed by a crew wearing respirators, as the Pennsylvania Game Commission recommends for dust in bat roosts, and the flue is swept and checked before the next fire. A bat-proofed chimney is quiet, dry and no longer on the colony's list.