This design from Bat Conservation International is based on the latest research into what attracts bats. It is appropriate for bat species found in the 48 contiguous United States and Alaska. (Hawaii's sole bat species does not roost in bat houses.)
Young people will need adult supervision to construct this house.
You will need: 1/4 sheet (2 ft. by 4 ft.) 1/2-inch CDX (exterior grade) plywood
One piece 1 in. by 2 in. (finished size: 3/4 in. by 1-3/4 in.) by 8 ft. pine (furring strip)
1/8-in. HDPE (plastic) mesh netting, 20 in. by 22-1/2 in. (such as Internet product No. XV-1670; call 1-800-328-8456). Craft stores sell a similar product for needlepoint.
Twenty to 30 1-1/4 in. exterior drywall screws
Pint of dark exterior paint
Tube of paintable caulk
5/16 in. staples (for exterior use)
table saw or handsaw
variable-speed reversing drill
Phillips bit for drill
Measure and cut plywood into three pieces: 26-1/2 in. by 24 in.; 16-1/2 in. by 24 in; 5 in. by 24 in.
Measure and cut furring into one 24-in. piece and two 20-1/4 in. pieces. Apply a bead of caulk where the furring strips will go. Screw the back to the furring strips. Start with the 24-in. strip at the top.
Staple the netting to the inside surface of the house's back. Start at the bottom. Be sure the netting lies flat (curved side down) and does not pucker. (Internet netting comes in 7-foot rolls for $21; try a craft store for plastic needlepoint mesh if you're only making one house.)
Screw front pieces to furring, starting with the top piece. Don't forget to caulk first! Leave 1/2 in. space between top and bottom front pieces for ventilation.
Caulk outside joints, if needed, to seal roosting chamber.
Paint the outside of the house a dark color. Apply at least two coats.
Mount the house at least 12 feet above the ground in a sunny location on a building or pole. (Bats shun tree-mounted houses.)
The most likely reasons that bats may ignore a bat house are lack of warmth or food. Bats require summer temperatures of 80 to 100 degrees F. inside the bat house in order to raise a brood successfully. Put your house in a sunny location. The dark paint will absorb the sun's heat. (Only if summer temperatures reach 100 degrees should you paint it white.) In colder climates, eliminate the vent to keep the house warmer. Bats need to be near a supply of insects, so an area of low pesticide use within 1/4 mile of a body of water is good.
For more information on bats and bat houses, see Bat Conservation International's Web site: www.batcon.org
Or call them at: 1-800-538-BATS
Source: 'The Bat House Builder's Handbook,' by Merlin Tuttle and Donna Hemsley (BCI, Austin, Texas).