How to Remove Bats from Your House
Bats in your belfry? The humane (and legal) way to remove them is through a process called "exclusion." Here's how
Much as bats may spook us, we need them around. According to Bat Conservation International, bats consume about 1,200 mosquito-sized insects an hour; some species of bats disperse seeds, pollinate plants, and feed on beetles that destroy crops.
Still, that doesn't mean I wanted bats camped out in my attic, even if it is an unfinished, unused space, and even if the barge boards of our Carpenter Gothic house do feature cutouts of what can only be described as bats in flight. In anticipation of beefing up the insulation in the attic, I first had to remove the bats along with the old insulation that they had polluted. Because bats are valuable (and protected by law in some states) and at risk from a new plague, White Nose Syndrome, extermination and the use of pesticides against bats are illegal.
Rather, bats are removed through "exclusion", meaning a building is sealed except for primary exits which are outfitted with one-way doors, permitting bat exit and preventing re-entry. For professional bat exclusion and a great bat education I turned to Jim Dreisacker of Westchester Wildlife, a wildlife control expert with 27 years of experience and the inventor of the batcone, a bat exclusion device.
Over time, a colony of little brown bats such as I discovered roosting in our "belfry" can grow to a population of a few hundred, leaving behind an accumulation of guano that is benign if undisturbed but that, over time, generates an undesirable odor.
What we had, high up on the beams where they meet the chimney, was a maternity roost. With bat exclusion, timing is everything. Females give birth, one pup a season, any time from May through August. (The one upside: In a single day, nursing mothers can eat up to half their weight in insects.) So that no young bats are trapped in the building, exclusion must be conducted either in the spring when insects have appeared but before the pups are born or after the young bats are capable of flying, ideally early fall. By the time frost appears, little brown bats will have migrated to caves where they hibernate for the winter.
Having stood on the lawn at dusk and lost count at 100 as the bats swooped out into the night, I didn't know exactly how many bats were roosting in the attic but I knew which exit they favored: a corner of a defunct brick chimney capped by a slab of stone. On closer inspection, Jim identifies a gap between the bricks where the mortar had fallen away as the primary egress and the perfect location for a batcone. Molded of either acrylic or plastic, the tube-shaped device permits easy exit but its downward-angled pitch and slippery surface prevent reentry by bats. The batcones will remain in place for a few days—longer if the weather is inclement and the bats stay inside—before being removed prior to the hole being plugged.
Because the attic colony is a dry roost as opposed to one that's taken up residence in a damp cave, there is no risk of histoplasmosis, a lung infection (associated with bird and bat droppings) caused by a fungus that needs moisture to thrive. Still, the crew needs protection from dust and insulation fibers. In preparation for vacuuming the old cellulose insulation and guano, Josh Dreisacker and Craig Conway suit up in Tyvek coveralls with attached booties, googles, gloves, and HEPA face masks. Gas fuels a 13-horsepower TAP (Thermal Acoustical Pest control) vacuum, the most efficient method for quickly and thoroughly disposing of loose-fill insulation. One 6-inch-diameter hose snakes through the house from the attic to the vacuum; another deposits the waste in large reclaim bags.
While Josh and Craig vacuum the floor of the attic, sucking up not only insulation and guano but a defunct wasp's nest and the remains of a nest of starlings, Neal Trigger and Russ Howard comb every inch of the exterior for gaps and holes, paying particular attention to the roof's ridgeline and areas that may have opened up between clapboards or shingles. Bats can squeeze through an opening as small as 1 inch by 3/8 inches, so every possible entry must be caulked, except for the bats' favored exits which have been fitted with batcones. Jim has found that silicone caulk works best. "It's stable and has body so it's easier to apply, it dries clear so it disappears, and it stays flexible so no gaps open up due to shrinkage," he explains. The best time to apply caulk is early in the day so that it can harden before dark.
To zero in on exits that might be overlooked, Neal scans the white clapboard for small brown streaks and tiny black pellets, evidence of bat urine and guano. Gaps in oddly configured spots are fitted with batcones designed for corners and angles. Just to be sure no means of reentry remain, Neal reinforces the batcone installation with hardware cloth as well as caulk. Though bats do not gnaw through wood, squirrels and other rodents do, so wire cloth is a preventive measure to ensure the longevity of the batproofing.
The crew found bats living up inside the louvers of an attic vent. Russ cuts a piece of hardware cloth roughly the size of the circular vent, allowing for a hole at the bottom to accommodate an excluder. Though this installation is temporary, a more finished piece of screening will be screwed over the vent, after the bottom louver damaged by birds has been replaced, to permanently prevent bats from taking up residence again. The crew uses only galvanized hardware to avoid future deterioration and rust spots.
With the old insulation completely removed, Neal uses a hand-pumped sprayer to apply D-Molish Now, an organic stain cleaner and odor remover, where the bat guano was most concentrated. Enzymes in the formula digest the source of the odor.
"With any wildlife control, it's not enough to remove the animal; you have to eliminate the odor, too," says Jim. "I've tried a lot of different products. This one worked on skunk spray, so I suspected it would be very effective on bat odor." Neutralizing the odor is not only desirable, it's essential because scent draws bats back to the roost. Bats have excellent homing instincts and can live to the age of 30, so every measure must be taken to dissuade and prevent them from returning to a roost they may have occupied for years.
With the bat odor gone and even the orange scent of D-Molish Now fully dissipated, Neal begins to lay new insulation between the joists. For maximum effect, he will put down one layer of R30 Fiberglas insulation, then top that with another layer laid in the opposite direction. Had any odor remained, the crew would have sprayed the wood joists and beams with a water-based sealant to encase the smell before installing the insulation.
Bats will continue to return to a roost, especially one that is long-established, so providing them with a new place to roost nearby, such as a bathouse, increases the chances that they will take up residence there instead of inside your house. Since their homing skills direct them back to their favorite points of entry—in our case the gap between the chimney bricks—Jim positions a bat house as close to the old "bat door" as possible. The higher you can install a bat house, the more likely it will be effective; minimum height is 15 feet above the ground. Though small, the cedar bat house can accommodate a colony of a few hundred. By late spring, we should know if the bats have fully adopted the bat house as their new home.