A common single chamber bat house is capable of housing 50 bats, while larger multi-chamber design can attract nursery colonies of 200 bats or more.
All bat houses should be at least 2 feet tall, 14 inches or more wide, and have a 3-6 inch landing area extending below the entrance. Most houses have 1 to 4 roosting chambers. Roost partitions should be carefully spaced 3/4 inch apart. All partitions and landing areas should be roughened. Wood surfaces can be scratched or covered with durable plastic screening (or 1/4-inch mesh). Include vents 6 inches from the bottoms of all houses to be used where average July high temperatures are 85 F, or above. Front vents are as long as a house is wide, side vents 6 inches tall by 1/2 inch wide. In Wisconsin, depending on your summer temperatures, you may choose not to use these vents.
A combination of exterior plywood and cedar is best. Do not use pressure-treated wood. Any staples used must be exterior grade or galvanized. Caulk all seams, especially around the roof.
Paint the exterior with three coats of outdoor water-based paint. The color should be a very dark shade of black, brown, green, or blue.
Most bats choose to roost within close proximally of water, preferably a stream, river, pond, or lake. Greatest bat house success has been achieved in areas of diverse habitat, especially where there is a mixture of differing agricultural use and natural vegetation. Bat houses are most likely to succeed in regions where bats are already attempting to live in buildings.
Bats houses should be mounted on poles or buildings. Trees are not an appropriate structure. Wood or stone buildings with proper solar exposure are ideal, and locations under eaves often have been successful. All bat houses should be mounted at least 10 feet above ground; 15-20 feet is better. Bat houses should not be lit by bright lights. In the northern United States, houses should receive a minimum 6-8 hours of direct sunlight.
Houses mounted on sides of buildings or high up on poles provide the best protection from predators. This may be a key factor in determining a bat’s choice. Locations at least 20-25 feet from the nearest tree are best. However, houses may be found more quickly if located along forest or water edges where bats tend to fly.