How to bat-proof your house

By Bearerofthecup (Own work) [Public domain (], via Wikimedia Commons

Bats make great neighbors, but maybe not the best roommates. On one hand, they play an important role in controlling flying insect populations as their only major predator at night. On the other, they can put you at risk for rabies and histoplasmosis — a lung infection caused by inhaling a fungus that comes from bat poop.

Having a colony roost in your attic for a whole summer isn’t ideal, so the best course of action is prevention. Sealing off entrances before they return in April will keep them out of your attic without impacting the local bat population.

House bats

Most bats prefer to roost in secluded locations, but two varieties commonly inhabit buildings during the summer months. Little brown bats are the most abundant species in the Ohio-Pennsylvania area, while big brown bats are common in agricultural areas.

Before you decide the bigs and littles are both disgusting and you want nothing to do with either variety, consider their diets. According to PennState Extension, a colony of just 100 little brown bats can consume more than a quarter of a million mosquitoes and other small insects a night. Over the course of a summer, a colony of 150 big brown bats can eat 38,000 cucumber beetles, 16,000 June bugs, 19,000 stinkbugs and 50,000 leafhoppers. They can also prevent the hatching of 18 million corn rootworms by devouring the adult beetles.

The bottom line is both play a big part in the local ecosystem by controlling insect populations.

Maternity colonies

Little brown bats and big brown bats once roosted in hollow trees. They adapted to life in man-made structures as forests were eliminated by early settlers. Returning to the same sites ever since, they tend to seek out hot attics to provide good incubation for their pups.

Once a maternity colony has settled for the summer, their pups won’t be ready to take flight until mid-July. This is why it’s so important to bat-proof your home before they come to roost. The protection of maternity colonies is important to the survival of both the little brown bats and big brown bats as they only have one or two pups a year. Wiping out just one maternity colony can have a long-term impact on the local bat and insect populations.


Bat-proofing must be done before April to prevent trapping any inside. It consists of sealing any potential entrances and providing an alternate roost.

Bats usually enter at points where joined materials have warped or shrunk, according to PennState Extension.
  1. Identifying the entrances. Bats can sneak into holes as small as 0.5 by 1.25 inches. You need to located all of the holes they could potentially use to enter or exit your attic. Common places to check include any place where joined materials are damaged, louvered vents with loose screening, the roof peak and areas where flashing is separated from the roof or siding.
  1. Figuring out whether any entrances have been used. In order to figure out whether or not the access points you’ve identified have been used, look for bat droppings near the opening. You can also identify entrances that have been used over a long period of time by a brown discoloration around the edges.
  1. Sealing entrances. After you’ve located the entrances, you need to seal them. You can use window screening or hardware cloth to cover louvered vents or larger openings and expanding foam insulation or caulking compound to seal the smaller ones. Bats will not gnaw new holes, so sealing the existing openings will be enough to keep them out.
  1. Building a bat box. When bats are prevented from using their traditional roost they must find a new home. This can be stressful as they risk being kicked out again or even exterminated. Generally, they will try to stay in the area by inhabiting a new building. However, they will not relocate to buildings with preexisting maternity colonies. If they can’t find a suitable roost, they may leave the area all together. Bat boxes solve these problems by providing an alternative site for females to raise their pups. You get all the benefits of having bats around without having them in your home.

Again, bat-proofing should never be done from May through mid-July as pups are confined in the roost until they are old enough to fly. Bat-proofing during this time can increase the potential health risks you face as bats may enter your living space in search of a way out or attempt to reenter your home in search of their young.

The best time for bat-proofing is in the spring before bats enter their roost or in the fall after they’ve vacated it.

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  1. Thanks for the article! I live in western PA and have a colony living in my siding/soffit and fascia where it meets the chimney. Would like to seal them out but how will I know if they hibernate there all winter or migrate elsewhere? Is sealing it in the middle of the night when they’re out and about an option? Or would that be detrimental to them?

    • Patrick, I actually had a bat colony living in the soffits above my garage for a couple of years and failed to exclude them properly because I didn’t seal all of the cracks the first time. Definitely don’t seal anything off until they move out in November. Bat colonies are made up of mothers and their pups. The mothers will leave at night to eat, but the pups will remain and rely on their mothers to nurse until they are old enough to fly. At the end of August, you could try installing one-way doors as the pups and their mothers should be leaving for the night at that point. One-way doors should be installed from Aug.-Oct. before you seal off their entrances and exclude them completely to make sure no bats end up trapped inside your soffits — this will also help you ensure they’re not hibernating in there when you seal everything off. Then from Nov.-Dec. you should completely seal the entrances so they don’t return next year. All entrances should be completely sealed off before the beginning of April as that’s when they return. If you attempt to seal everything off after March, you’ll run the risk of trapping bats inside. After you’ve excluded the colony completely, it’s also a good idea to install a bat box so they have somewhere to move to when they return in the spring. For more on excluding bats and sealing entrances, check out this how to I wrote — — and for more on installing bat boxes, read this one —


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