Why would you want bats? They eat insects, are generally useful to human settlements, and bats are wonderful to watch streaming out of an evening to get on with their night's work.
If you look carefully at the front wall of the house, between Merinda's
bedroom window and ours at the right of the upstairs, you will see
a dark gray object that is about 3 feet by 1.5 feet, hanging between
the cream window shutters.
This is our new bat house.
Jonathan had the ladder end of the installation operation. We considered mounting the house on the East chimney (visible in the previous photograph), where we could watch it from our bedroom window, but the The Bat House Builder's Handbook reports that hours of direct sun exposure is such a strong factor in successfully attracting bats that we decided to go for the front, South-facing wall.
We have virtually all the other factors that correlate with success: less than 0.25 miles to water, bats known to inhabit the area, house more than 24 inches tall, more than 17 inches wide, chambers between 0.75 and 1.0 inches thick, no overhanging branches to conceal avian predators, more than 15 feet above ground level, dark paint coating, bat landing area, rough and untreated wood surfaces for good grip, average of more than 10 hours of direct sun per day, shade from Noon Summer sun, ventilated chamber available. Statistically, we have better than 75% chance of having bats move in within the next two Summers.
The landing area is the brown, stepped area at the bottom.
Note the ventilation slits visible like gills on the front
of the house, just near my head in the photograph.
The web will provide a wealth of information on bathouse design,
but in summary
the most popular bat houses offer chambers that are rectangular
and open at the bottom (so that guano falls out and warm air
stays in), with the above dimensions (to best allow groups to huddle and
hang on at the same time), and which best stay warm. Bats like it warm.
Ours has meticulously-grooved inner walls, and one innovation of
our own: scalloped side walls, so that the chamber is not
exactly rectangular inside, but has wavy or sawtooth sides
to give more crannies in which to huddle (as bats do) and
more secure-feeling hidey-holes.
In this photo you are looking into the two chambers of our house
and you can see the irregular sides.