Cedar is thicker for support and already dark, which eliminates any need to paint. Typically, a bat house is about 14 to 20 inches wide and 24 to 32 inches tall with a 3-inch landing pad, or floor, extended below the entrance. Bats need rough textured wood for the inside walls, partitions and landing area so they can get a grip and easily climb about. If the wood is not rough hewn, you can create the right texture by sanding the wood with coarse sandpaper. Cover the outside of the house with two to three coats of latex paint to protect it from moisture, air leaks and wood deterioration.
Paint also helps retain heat in the cool months.
Do not paint or varnish the inside of the house. The paint color can range from deep brown to white, depending on your climate and the amount of sun exposure hitting the house. If July temperatures typically exceed 85 degrees F where you live, cut out vents 6 inches from the bottom of the house. If you live in a hot climate, the house needs less sunlight than if you live in a cooler climate. In moderate climes, face the house in southerly direction to provide a combination of morning sun and afternoon shade.