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House Bat Management


Batproofing Materials

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 heavyduty staple gun.

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


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

To be eligible for caulking income tax credit, the IRS considers caulking as pliable material used to fill small gaps at fixed joints on buildings to reduce the passage of air and moisture. Caulking includes, but is not limited to, materials commonly known as "sealant", "putty," and "glazing compounds" (IRS 1980).

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 and also serves as a bat repellent. 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, according to Harrje (1978), is clear, long lasting (10-year guarantee), and almost invisible, thus matching any decor." Recently self-expanding urethane foams for caulking have appeared in pressurized containers and are dispensed similarly to shaving cream. 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. Princeton University has used such self-expanding urethane foams as "Polycel One," "Great Stuff," and "Touch-n-Foam" (D. T. Harrje, personal communication).

Houses may need to be caulked in the following places where bats may enter:


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 (Fig. 12). A recently developed nylon strip brush barrier called "Therm-L-Brush" is set in a galvanized steel channel and housed in either aluminum or vinyl (Fig, 13). 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.

GIF -- Types of weatherstripping.
Fig. 12.  Types of weatherstripping. (Courtesy of Exxon Company, USA)

JPG -- Nylon Strip Weatherstripping.
Fig. 13.  Nylon strip brush weatherstripping—"Therm-L-Brush." (Photo courtesy of Sealeze Corporation)

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 (Anonymous 1980b).

To help determine one's eligibility for weatherstripping income tax credit, IRS defines weatherstripping as narrow strips of material placed over or in movable joints of windows and doors to reduce the passage of air and moisture (IRS 1980).


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 the past 10 years (R. E. Stebbings, personal communication). 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.


Where screening is necessary the openings 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 × 14 mesh.

Bats can enter ventilators that are not properly screened. Hardware cloth for ventilators should be 8 × 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 (Fig. 14; Anderson 1970: Fig. 102; Anderson and Sherwood 1974).

GIF -- Ventilators.
Fig. 14.  Ventilators should be properly screened: A. Triangular; B. Cross section; C. Half-circle; D. Square; E. Vertical; F. Soffit. (From Anderson 1970).

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 × 2.5 cm (1/4 × 1 in.).

Bats may use an unused or old chimney because the rough surfaces of chimney walls offer suitable places for bats to hang. 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.


The use of insulation as an effective bat repellent was first reported by Scott (1963) when fiberglass insulation was blown into roof and wall spaces occupied by bats.

To help determine one's eligibility for insulation tax credit, the IRS defines insulation as meaning any item that is specifically and primarily designed to reduce, when installed in or on a dwelling, the heat loss or gain of such dwelling (IRS 1980). To qualify as insulation, the item must be installed between a conditioned area and a non-conditioned area. The term "conditioned area" means an area that has been heated or cooled by conventional or renewable energy source. Insulation includes materials made of fiberglass, rock wool, cellulose, urea-based foam, urethane, vermiculite, perlite, polystyrene, and extruded polystyrene foam.

Insulation materials are manufactured in a number of forms and types. Each has advantages for specific uses. Materials and methods of application are rapidly changing and improving and no one type seems best for all applications. Some basic information was received from insulation experts concerning the possible use of insulation to solve bat management problems (U. S. Department of Energy 1978, 1979).

Materials that are inorganic are fire and moisture resistant. The safest appear to be glass fiber (fiberglass) and rock wool. Inorganic insulation is made in batts, blankets, and loose fill and used on attic floors and in frame walls. Organic insulation such as cellulosic fiber is chemically treated to make it resistant to fire, decay, and vermin. Cellulosic fiber, although treated to be fire resistant, may break down in a hot attic (U. S. Department of Housing and Urban Development 1975). Caution should be taken with urea formaldehyde foam insulation, also known as urea-based foam insulation, which may release formaldehyde gas when exposed to heat and humidity (U. S. Consumer Product Safety Commission [CPSC] 1980). CPSC warns that infants, the elderly, and those with allergies and respiratory problems may be particularly sensitive to formaldehyde gas. Once the problem occurs, it can be very difficult and expensive to remedy.

No insulating materials blown into frame walls serve as a barrier to moisture so condensation may become a problem in winter. Wet insulation will not insulate and moisture may collect to cause structural rot. Ventilation paths should not be blocked by insulation (Fig. 15).

GIF -- Insulated roof for batproofing.
Fig. 15.  A properly insulated roof will keep bats out (see Fig. 5). (By J. Newel Lewis. Dip. Arch., F.R.I.B.A.—Trinidad and Tobago)

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