Why Bats Become a Problem and What is a Bat Exclusion?

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Why Bats Become a Problem and What is a Bat Exclusion?

The long-term solution for bats that enter buildings and cause a nuisance problem or present a public health hazard is systematically repairing the structure. Chemical toxicants never solve house bat problems and often create different, more dangerous problems. This section describes batproofing techniques that when used in conjunction with an effective bat box will effectively deal with house bat problems. Recent declines in bat populations and greater appreciation of the ecological importance of bats have led to this solution which encourages bat conservation, protects human health, and permanently solves nuisance problems.

Why bats become a nuisance

Bats are usually forced to roost in buildings when natural roosts, such as caves and trees with exfoliating bark are destroyed. Some caves are ruined by flooding (natural progression but often as a result of surface development), dam construction, burning of debris, and ground water pollutants. Cave roosts also are destroyed by explosives used in mining and quarrying, vandalism, and tourism. Deforestation, particularly removal of diseased or old trees with hollows, have also reduced the number of available natural bat roosts.

Bats that have adapted from their natural roost type to human structures are now imperiled not only by some peoples intolerance, but also changes in building construction. Old barns and homesteads fall down, are torn down, or are remodeled, leaving the remaining modern structures tightly constructed with no room for bats.

The general requirements for buildings to be used as bats roosts are known. Colonial bats that live in structures usually occur in areas near water and at the edges of woods where insects are found in adequate numbers and variety. Less understood is the importance of other factors that govern specific site selection such as temperature, humidity, disturbance, and the physical characteristics of roost sites.

Why not just exterminate?

Hiring an exterminator or "doing it yourself" may seem like the simple, very direct solution. Consider these thoughts before attempting any exterminating:

  • It is inhumane when better options exist.
  • Will ultimately fail because the roost is still available for bats. You just get them again.
  • In many states, including Pennsylvania, it is illegal to kill bats in buildings. Exclusion is the only recommended method.
  • Chemicals that can kill bats are also just as hazardous to humans.
  • There are no chemicals licensed for use on bats in the United States. Any company proposing to do so should have their practices questioned. In the United Kingdom, any exclusion is illegal without first consulting local officials.
  • Some services will offer to catch and release bats far away, but bats have been proven to return from up to 400 miles away; a few weeks later they will be back.
  • Sealing the structure while providing an alternate roost (a bat house) is the permanent solution.

Bats use human structures because the traditional roosting platforms (tree bark with exfoliating bark) are more scarce today compared with what our landscape appeared like nearly 300 years ago. A few species have adapted and even flourished where they have found proper temperatures, humidity, and crevices in certain man-made structures such as attics and steeples. As new superior construction replace the old, bats are having increasing difficulty finding even a marginal home.

This makes it important to seal the structure's entrances, which may be cracks less than 1/4'' wide. Otherwise, a new colony will surely take up residence after the old one is gone. By observing the summer evening exit, these entrances can be located. There may also be staining visible just below these entrances. Sealing should take place November-February when bats are hibernating in local caves and mines.

In some warm regions bats occupy structures throughout the year. In this case a more aggressive approach is taken to seal all entrances except the main. At that point a simple device is installed to allow bats to escape but not reenter. Structures should not be sealed in summer or early fall because flightless young may be trapped inside. Patience is required to win, as may take three seasons or more to completely bat-proof a structure. Persistent bats will use less desirable entrances once the main ones are blocked.

Installing a bat house during an eviction project will improve the chances of success significantly, as bats will opt for the better available roost in face of awkward, undesirable entrances. However, bats will never completely abandon a structure for a bat house until some degree of repair work is complete.

Types of bat problems around your home

Bats Outside Buildings. Some bats temporarily roost behind shutters, under wood shingle siding and roofing, roof gutters, awnings, trim with overhang, under flashing around chimneys which has separated or loosened from the solid structure, open garages, patios, porches, breezeways, open livestock shelters, and under sheets of tar paper. Shutters on brick houses are especially attractive as day roosts for transient bats in migration and for bachelor males. In exceptionally hot weather, females may abandon an attic and reside behind shutters. Big brown bats are partial to roosting behind the trim below roofs of houses. Unusual roosting areas include sewers, wells, and graveyard crypts. Generally speaking, this activity is short term, involves just a few male individuals, and largely goes unnoticed.

A Few Bats Inside Living Spaces. The discovery of one or two bats in a house is probably the most frequent problem. The big brown bat accounts for many of these sudden appearances. Common in towns and cities, it often enters homes through open windows and doors, but may use any crevice it can find. This usually occurs in the early Fall when bats are checking for potential roost or hibernation sites. These bats may occur singly, in pairs, or in small groups. The big brown bat can hibernate in below freezing temperatures, so it is common to find them asleep in cold garages, houses or public buildings during early winter. These bats may suddenly appear in midwinter during a warm weather spell and even attempt to feed. Migratory bats occasionally enter buildings overnight during their spring and fall migrations.

Repeated occurrences of bats in your living spaces in mid to late summer suggest that a maternity colony is close by, most likely in the attic. As juvenile bats begin fending for themselves and exploring, one may explore it's way into the your living room. The presence of any bat in your living spaces is purely accidental on it's behalf, and keeping this in mind it is often a simple matter to allow it to escape.

Any bat will usually find it's own way out. The simplest solution to rid the building of the bat is to open all windows and doors leading to the outside. Bats usually will not attack a person even if chased. Never swat or throw linens at the bat, or run around waving. All this tends to do is confuse the bat and leave you exhausted. Above all else, calmly WATCH the bat to make sure it leaves. If the bat refuses to leave, it will calm down and land on something. Drapes and hanging clothes seem to be the preferred rest areas. Place a small box or can over the bat, then gently slide thin cardboard under the "trap" to collect your bat.A more direct approach is to simply take it in a gloved hand then release it outside. All bats can cling to surprisingly small surfaces. For the bat's sake, do not use an overly thick glove when handling, remember this is not molten steel you are carrying. At last resort, local health authorities can be called to collect the bat, though this may result in it's demise. If the bat, or any wild animal, has come in contact with pets, children, invalids, etc. contact your local health department. Health department recommendations vary from state to state.

Occasionally big brown bats may overwinter in a building and arouse during warm weather in mid-winter or early spring. Bats found at this time are usually underweight and need special care to survive. If you find a bat in a building in the winter which must be immediately removed, capture it using a method described above. Keep the bat in a warm, dark, escape-proof container with water, and call a local wildlife rehabilitator.

Batproofing materials and general ideas

Unlike rodents, bats will not gnaw their way through wood or building materials. Soft materials such as insulation batting can be easily attached to a building with a heavy duty staple gun.

Effective materials to exclude bats are expansion foam caulking, flashing, screening, and insulation. Weatherstripping, stainless steel wool, or stainless steel rustproof scouring pads are excellent materials to block long, narrow cracks.

Caulking. Cracks and crevices develop in a structure as it ages and bats will take advantage of these openings. Caulking will seal the openings.

Since wood expands and contracts with the weather, it is best to apply the caulking during dry periods when the cracks will be their widest. Occasionally cracks enlarge and a filler is necessary before a caulking compound is applied. Oakum is a tarred-hemp fiber commonly used to caulk ships. It packs easily and firmly by hand into small cracks. The tar or creosote binds the fiber so that it is not easily dislodged. In addition to oakum, other fillers are caulking, cotton, sponge, rubber, glass fiber, and quick-setting putty.

There are various caulks which may be applied with a caulking gun. Latex, butyl, and acrylic have a durability of about 5 years and can be painted. Elastomeric types, such as silicone rubber and polysulphide rubber, will last indefinitely, expand and contract with the weather, do not dry or crack, tolerate temperature extremes, and come in colors. However, some cannot be painted. Silicone rubber is clear, long lasting (10-year guarantee), and almost invisible, thus matching any decor.

Self-expanding urethane foams for caulking have appeared in pressurized containers and are dispensed similarly to shaving cream. Though quite messy and difficult to clean, when the material is placed in a hole it will expand several times to fill the space. After it cures and hardens, it may be trimmed, sanded, and painted with any type of paint or stain. Spray foam will weather, limiting it's use to very deep crevices and interiors.

Houses may need to be caulked in the following places:

  • Between window drip caps (tops of windows) and siding
  • Between door drip caps and siding
  • At joints between window frames and siding
  • At joints between door frames and siding
  • Between window sills and siding
  • At corners formed by siding
  • At sills where wood structure meets the foundation
  • Outside water faucets, or other special breaks in the outside house surface
  • Where pipes and wires penetrate the ceiling below an unheated attic
  • Between porches and the main body of the house
  • Where chimney or masonry meets siding
  • Where storm windows meet the window frame
  • Where the wall meets the eve at the gable ends the attic
  • Where wall meet the eves anywhere on the structure.

Weatherstripping. When bats crawl under doors, the space between the floor and the door bottom may be sealed with weatherstripping, a draft shield, or a gap stopper to close off the space between the bottom of the door and the door sill or threshold. Weatherstripping is made of a variety of materials including natural fibers, aluminum, fine wire, felt, hard rubber, vinyl, and nylon. A nylon strip brush barrier is set in a galvanized steel channel and housed in either aluminum or vinyl. It has several advantages over ordinary weatherstripping. The flexible nylon filaments, which comprise a substantial brush, move easily in any direction permitting the bristles to conform to uneven floor surfaces, including carpet. This seals any gaps, stops drafts, and reduces heat loss. It is said to resist rodents and insects.

A simple draft excluder for the bottom of seldom-used doors is a long, flexible, sausage-shaped cloth tube filled with sand, which is simply pushed against the crack at the bottom of the door.

Flashing. Wherever joints occur in a building, e.g., walls meeting the roof or a chimney, flashing may be installed to keep the building watertight. Flashing consists of strips of metal or other material to cover cracks, crevices, and holes. The materials most commonly used are galvanized metal, copper, aluminum, and stainless steel. A self-adhesive flashing, called "Flashband," was developed in 1965 and has been used to batproof buildings in England and western Europe for years. Flashband has advantages such as flexibility, self-adhesiveness, and a grip that reportedly improves with time despite extremes of weather. It is available in the United States and Canada.

Screening. Where screening is necessary the mesh must be small enough to prevent the access of bats. Steel hardware cloth should have 0.63 cm (1/4 in.) mesh with three meshes or more to the inch. Insect screening for windows should be 18 x 14 mesh.

Bats can enter ventilators that are not properly screened. Hardware cloth for ventilators should be 8 x 8 mesh. Inlet and outlet ventilators should be properly installed. The type of ventilator used, its location in the building, and the direction of prevailing air currents may be important factors because buildings of identical design, but different orientation, vary in their attractiveness to bats. Many ventilators are made with metal louvers and frames, others are custom made of wood to more closely fit the house design.

The soffit (the underside of an overhanging cornice) usually has ventilators which may be continuous, round, single-framed, or the soffit itself may be of perforated hardboard. Regardless of soffit type, the slots should not exceed 0.63 x 2.5 cm (1/4 x 1 inch).

Bats may use an unused or old chimney because the rough surfaces of chimney walls offer suitable places for bats to hang. Bats will almost never use an active chimney. To prevent bats from entering chimneys, spark arresters or bird screens should be installed. These should be of rust-resistant material and carefully attached. They should completely enclose the flue discharge area and be securely fastened to the top of the chimney. Except when in use, dampers should be closed.

Screening is also used as a filler for very large crevices. Strips of fiberglass screening is pushed into holes then finished off with a coating of sealant.

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  • John Chenger
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