I had a purple martin house for at least 10 years and nary a single purple martin. This year I decided to take it down and replace it with two bat houses. It isn't that bats eat more mosquitos than purple martins as some have asserted, since research of the stomach contents of both has found that neither consume many mosquitos. Rather, I regularly see bats flitting about around dusk so I know that they are already in the area. The habitat surrounding our home is quite diverse including forest, open fields, creek, wetlands, and a pond within 100 yards.
I used these plans from the U.S. Department of Fish and Wildlife as the basis for my houses. I placed them back-to-back on one post with one house oriented South and the other North. This will allow the bats to choose a warmer or cooler house depending ambient temperature fluctuations.
I made the bat house largely from Oriented Strand Board (OSB) for two reasons. First, I wanted the rough surface on the interior to provide gripping for the bats and second, it is inexpensive.
For the post, I joined together two 12 foot long 4"X4" treated timbers using a scarf joint and two 3/4 inch bolts. With 3 feet of the bottom post buried below grade, that places the bat houses around 20 feet off of the ground.
On the interior boards, I sawed shallow horizontal grooves equally spaced in 1 inch intervals. This is to provide additonal gripping points for the bats. I dyed the interior boards with a dark brown stain.
I made some small shingles from larger asphalt shingles to protect the roof of each house. I also placed a finial on top of the post for purely aesthetic purposes. I sealed all of the joints with caulk and I painted the exterior of the house with three coats of exterior latex stain. Bats like the interior of their summer roosting house to be between 80-100 degF. Your geography will determine whether you paint it a light, medium or dark color. Based on our location, I chose a medium color. In addition this should protect the wood and it is certainly more pleasing to the eye than the unpainted OSB.