ItÕs tempting to try any Òquick fixesÓ. To be blunt, these are a total waste of time....mothballs for example. They are called MOTHballs....not BATballs. Bright lights, noise, chemicals, trapping and supposedly relocating...most of those methods do nothing to deter bats, some may be harmful to humans or domestic animals, and none do anything to fix the problem permanently.

If your problem is just a few bats temporailly night roosting on an open porch, mylar balloons, strips of foil, or pet repellent may help (when applied to the roost area). Never spray repellent or any chemical directly on bats as this is illegal in all states and all of Europe.

So going back to our list of why bats like buildings in the first place, about the only thing anyone can realistically do anything about is #1...the crevices.

Again going back to a problem of temporary bats in an open area, or perhaps a historic structure that you do not want to permenantly alter, consider an airbag to slip into a specific problem bat crevice. Most often however, there will be far too many crevices to use airbags in, but perhaps they might be used in the right circumstance.

The problem is that the building is accessible to bats. Little can be done with temperatures and attic crevices, but the access holes can be systematically sealed...this is the only way to ever keep bats out.

This is a representation of what kinds of entrances bats might use. As with a lot of info found on the internet, this isnÕt as complete. Other classic areas are around the ridge vent if so equipt, and everywhere the trim meets the siding, and finally on metal or slate roofs, where the roof bends over into the gutter.

Stucco and brick finished buildings with aluminum soffit are usually a bit less bat friendly. But this construction will have uneven sections as well. Bats would have no issues with landing on the rough surfaces and entering these crevices. The rule of thumb is if you can stick a pencil into the crevice, it will be fair game for bats. All sides of the building must be inspected with this in mind.

The first thing to do is a site survey, essentially scope out what kinds of crevices bats are getting into, decide how to get to the work areas, and decide if there are any unusual repair materials (such as chimney caps, lumber for rotted soffit, etc.) Areas that will need need checked up close on this building are everywhere the white trim meets the siding. Once under the trimm, bats can generally go anywhere they want to in the house. Other typical problem spots are anywhere angles come together. On older buildings, often the angles donÕt come together when seen up close. Crevices that don't seem like much from the ground will almost always be bat friendly when seen up close.

This is an example of a typical stone building where there are numerous gaps under the straight trim. This must be sealed on all sides of the building or bats will simply start using other crevices.

Materials I use on every job include construction adhesive that sets up in about 15 minutes. The fast set is important so that the material sticks into overhead crevices without falling out wet. Roof sealant is handy to have around for anything that is directly exposed to the weather, say around ridge vents or chimney flashing. We use small chunks of 6ÕÕ wide fiberglass screen for filling deep or wide holes before caulking. Large overhead crevices must also be filled with screen first, otherwise adhesive will drop out wet. Plan on any vertical or overhead crevice 1/4ÕÕ or wider will need at least some screen tucked in to hold the glue. The large 36ÕÕ wide screen is for making one-way doors, which is discussed later.

This is an example of twisting fiberglass screen and stuffing into wide crevices so that the caulking has something to stick to while drying.

The usual construction adhesive dries the color of tan. This color might really stick out on some buildings. You can spray paint it black after 15 minutes or so... from far away this will look more like a shadow rather than a big hole full of glue.

An example of someone who tried to solve a bat problem with steel wool. This material rusts and deteriorates and should not be left exposed to the elemments without being caulked over.

The screen is covered with caulking material. Be fairly liberal with the caulking, it will shrink slightly when it dries. Be sure to inject it into crevices as much as possible rather than simply covering over crevices.

Most cans are hard to control and clog. No matter how careful you are, youÕll inevitably make a mess that is conspicuous and hard to clean up. This is some homeowner who went wild with a commercial foam machine. Despite all this, he still has a bat problem. When drying, this material kills any bat it comes in contact with.

Some other material to consider is clear silicone caulking. When dry it is the least conspicuous material, but itÕs also more expensive. The cheap clear caulking is mixed with latex, and can be very runny, which takes hours to set, and always seems falls out of vertical and overhead cracks. In cold weather it freezes in the tubes and cannot be applied.

The worst material we have used is the expanding spray foam insulation.

The foam also degrades with sunlight, limiting itÕs use to maybe a big complex hole that is hard to reach from the exterior, but maybe can be approached on the inside. Whatever happens, youÕre left with big globs of foam that you either live with or have to make a second trip up to cut off and paint. Let it get even more out of hand and the foam can even pop your roof off.

At some point when doing this work, hopefully the batÕs favorite entrance will be found. This is recognized by droppings stuck to the siding below the entrance. If this an old problem, the entrance will be marked by a Òdirty spotÓ, which is stained from the oils on the batÕs fur and urine over many years. This is a particularly obvious entrance, there have been hundreds of bats here for as long as anyone can remember.

This entrance staining is a little more suttle. There were 50 bats or so in this house for a number of years. The droppings (black specks) are usually realized before the staining. The crevices here have just been caulked over but the entrance staining is visible.

If bats enter thru the ridgevent for example, you may never find droppings or staining because of weathering. ItÕs time to throw a bat party some summer evening and have people watch for an emergence. Usually from a corner of the building you can watch 2 sides at a time. ItÕs important to not be distracted during this, if you are dealing with a small number of bats you will miss them.

You already know about the local time when to expect the emergence, itÕs the same time at dusk when you see the fist bat flying overhead. DonÕt shine a light looking for the hole-- the bats will not emerge. Instead, watch for them to be silliouetted against the sky.

Once the main entrances are determined, seal absolutely everything else, on all sides of the building. Here you see out caulking extending almost to the entrance crevice at the peak of this roof. The black specks are guano splattered on the siding.

Place a large piece of fiberglass screening over the entrance, allowing it to extend about 2 feet below the crevice. Seal the screen on all sides except the bottom-- use a staple gun or duct tape. And leave the screen loose...bats must drop out of the crevice and fly out the bottom as unobstructed as possible.

This is a good example of how inconspicuous clear caulking can be. The joints leading up to the duct tape are sealed but are almost invisible. Keep in mind the bigger escape hatch you leave, the more likely bats will use it. the screen should be about 2Õ or so below the hole. Somehow seal it tight on 3 sides, if a staple gun wonÕt work, duct tape will. Leave the middle poofy so bats will drop as fast as possible. This is kind of a bad example...the screen should be much wider to allow it to be not so tight against the wall.

Bats will emerge and drop out under the screen but not remember how to properly return. Instead they will land near the top at the old entrance area, look confused, and eventually go elsewhere. This works because bats are following the airflow of the roost to reenter. If we used a solid plastic sheet instead of the screen, the air flows around the sheet, the bats will follow the air and crawl around the sheet to reenter.

This is an example of a one way door made from some extra wood because the duct tape would not stick to the stone to seal the sides of the screen to the wall.

Bats could be heard inside and all across the roofline on this building, so this situation called for a very long one way door.

Most barns were never intended to be bat proof. First the owners installed wire screening on the entire inside of the barn to seal the crevices between boards. Still a few problem areas existed marked by the one way door screens. Sometimes it takes several seasons to fully batproof a building.

This is an engineering feat. Look at the custom fit screen, the antigravity tubes, the neat glue job. We would have used a lot more duct tape and a bigger screen and would have looked pretty trashy in comparison. Remember we said the bats are following the airflow from the roost? This one way door is probably too airy...bats might have been confused by the big screen and had trouble finding the way thru the tube. This could be helped simply by taping some solid plastic over the screen.

Example of some sort of "chute" and probably a later attempt at a one way door on the lower half. The chute is worthless but the screen might work if it extended further over the hole.

Bats behind shutters. There are at least 3 options. One iss simply remove the shutters. Another is to prop the shutters away from the house a few inches. Bats will not like the large crevices and many will go elsewhere. A third option is to place screen behind the louvers and caulk the shutters to the wall, completely sealing the shutter against the house.

Here was a problem where bats were exiting a gap in the ridgevent of a house. Tape will generally stick to asphalt shingles long enough to do a bat exclusion (about a week).

This was a louvered vent that had no screen. The louvers were not large enough for birds but bats managed to find it. The quick solution was a custom built frame with metal screen which was then attached into the brick using masonary screws.

The most awkward exclusions are when forced to one-way a horizontal crevice. Bats are not too amused by having to crawl around and figure things out, but they will. Usually these one way doors are left up for about a week, more or less depending on circumstances. Each project is different.

So here is Òthe planÓ, keeping in mind that there are no guarantees with wildlife. One track is to simply wait until winter when you are least likely to have bat present, then seal everything. If anything is missed though, the project tends to start spanning years if you just work when bats are absent, because with observation bats will show you hole that you missed. Also, this approach risks trapping some bats inside, which at the very least makes bats unhappy.

Early spring, late summer are best. Seal everything except the main entrances, install doors, watch what happens during the next few nights. Keep sealing anything else the bats reveal to you during the emergence.

If a building is sealed in the birthing season, thatÕs an open invitation for all those bats to come for a visit down into your living space.

However, it is fair game to site survey, emergence count, and seal all crevices EXCEPT the main entrances. This way come fall you are ready to slap on the one way doors and face a smaller job.

A few bats may refuse to exit the one way door, no matter how inviting you make it for them. Any exclusion attempt may lead to a temporary increase in bat encounters. Also, bats will be desperate to re-enter the building using any means, including more obscure crevices, open windows, or garage bays. However, they will not chew their way back into building.

Having a few bats in the living spaces does not necessarily constitute a health hazard. DonÕt swing things at it, or it may just end up crashing into someone. Try to close doors to adjacent rooms and confine the bat into one room as much as possible. Open doors and windows to the outside. After 30 minutes or so she will either fly out on her own, or land on something. It may be possible to pick the bat up, gently but firmly wearing a thin leather glove. You might also be able to trap it in a box.

If no one was bitten, the animal can just be released outside. If there is any question that someone was bitten, the bat should be taken to your local public health department who will kill the animal for rabies testing.

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Some people do tolerate bats enough to try and work around them, or under them. But as this plastic stretches, somebody someday will have quite a mess on their hands. Short term fixes like this just lead to more work later.

Gone unchecked for many years, the problem is bound to worsen. Everything about bats becomes a bit more...shall we say, noticable, and some people understandablly decide that maybe sharing the house with a pack of wild animals is not necessarily a good thing.

A few bats in an attic usually doesnÕt pose much of an issue. They are quiet, elusive, donÕt have much smell, and are not destructive. There are usually droppings found though...they look like black grains of rice.

Problems start as old structures collapse and are replaced with bat unfriendly steel buildings. As a result, small colonies that were previously unnoticed merge in the fewer remaining buildings.

Why have certain species moved into buildings? Frankly, we managed to build bat roosts faster and better than Mother Nature can replace our forests. First, buildings have crevices. Usually lots of crevices. In crevices bats are protected from predators. Bats are looking for temperatures around 100¡ so that fetus develops as rapidly as possible. Roofs are giant solar heaters. But roofs can get too hot. Bats love buildings because they can switch to cooler sides, or lower elevations where they can find the right temperatures without having to abandon the roost. Finally, a high launching pad reduces tangles with ground based predators, like cats.

ItÕs small colonies that homeowners really run into. Bats would like nothing more than to spend these summers in a nice big dead tree. Unfortunately, if these ÒhazardÓ trees are removed or logging practices donÕt allow forests to fully mature, certain kinds of bats look elsewhere for a home. Sometimes they find no better place than a convenient building in disrepair just enough to allow easy access.


Since many buildings are much more attractive than trees, gone unchecked for a number of years this nuisance can get out of hand. In fact the largest summer concentration in Pennsylvania pre-WNS was a barn that contained about 40,000 bats. Usually homeowners encounter just a handful of bats rather than a truckload though. Any building can be batproofed, some more easily than others. Expect to spend time observing where they come from, and systematic with repairs.

Bats and Buildings

Bat exclusions: What to look for and how to do them

BCM specializes in bat-related fieldwork,