A bat house is a specialized structure designed to mimic the tight, warm crevices bats naturally use for shelter, particularly during the summer maternity season. Installing a bat house provides an alternative roosting site, which is an important conservation effort, especially for colony-forming species like the little brown bat and big brown bat. The primary goal for most homeowners is to encourage a colony to move in, which naturally results in effective, local pest control through the bats’ nightly consumption of insects. Success in providing this suitable habitat depends entirely on meticulous attention to placement and construction details.
Choosing the Optimal Location
The placement of the bat house is the single most important factor determining whether bats will choose to occupy it. Bats are highly sensitive to temperature and need a roost that can maintain a warm, stable internal climate, especially for raising young. Therefore, maximizing solar gain is a priority, which means the house should face a direction that receives the most direct sunlight throughout the day.
Orientation toward the south or southeast is recommended to capture the sun’s warmth for at least six to eight hours daily. In cooler climates, ten hours of sun exposure is often required to achieve the necessary internal temperatures, which can range from 80°F to 100°F for maternity colonies. The house should be mounted on a pole or the side of a building, which acts as a thermal mass to help buffer temperature fluctuations. Mounting on trees is far less successful due to excessive shade and increased vulnerability to predators.
The house must be mounted at a significant height, with a minimum of 10 to 12 feet off the ground; 15 to 20 feet is even better. This height protects the bats from ground predators and provides ample vertical clearance for their pre-flight drop. A clear flight path is essential, requiring a radius of at least 20 feet around the house to be free of obstructions like tree branches or wires. Locating the bat house within a quarter mile of a permanent water source, such as a pond or river, can increase the likelihood of occupancy, as bats often forage near water.
Structural Requirements for Bat Houses
The physical design and construction must cater specifically to the bat’s need for warmth and grip. Bat houses should be constructed from untreated lumber, such as exterior-grade plywood or cedar, and must be tightly sealed with caulk to prevent drafts and retain heat. The exterior color is a factor in thermal regulation, with darker colors absorbing more solar radiation. Black is ideal in cooler regions where the average July high is below 85°F, while lighter shades of gray or brown may be necessary in hotter climates to prevent overheating.
The interior must provide secure footing, achieved by roughening the wooden surfaces. This can be done by scoring or grooving the wood horizontally at intervals of a quarter to a half-inch, or by securely stapling a durable plastic mesh. The roosting chambers must be narrow, ideally spaced three-quarters of an inch apart, as bats prefer these tight crevices. Taller houses, at least 24 inches high, are more successful because they create a greater temperature gradient, allowing bats to move vertically to find their preferred temperature zone.
Timing the Installation
The time of year for installation influences how quickly a colony discovers and occupies the new roost. The optimal period for installation is in the late winter or early spring, before the bats return from their hibernation sites or migratory routes. Having the house in place before the start of the summer maternity season gives newly returning female bats a chance to find it while searching for a suitable place to raise their young.
If a bat house is being installed as an alternative home because a colony is being excluded from a building, it should be mounted at least two to six weeks prior to the exclusion. This overlap gives the displaced bats an opportunity to locate and investigate the new structure before their original roost is sealed. Even with perfect placement and construction, the process requires patience; approximately 90 percent of successful houses are occupied within two years.
Why Bats Might Not Be Moving In
If a bat house remains empty beyond the expected waiting period, the issue is often a subtle environmental factor rather than a major design flaw. One common deterrent is the presence of artificial lighting near the house, as bats prefer dark, secluded entrances and will avoid structures illuminated by nearby streetlights or security floods. The use of strong chemical preservatives or pressure-treated wood is a major obstacle, as the toxic chemicals can be ingested by the bats during grooming and are lethal, causing them to abandon the house.
Another frequent problem is the presence of other tenants, such as wasps or mud daubers, which build nests inside the narrow chambers. These insects make the roosting space unsuitable for bats, and any nests should be safely removed during the late fall or winter when bats are absent. An incorrectly calibrated internal temperature—either too hot or too cool—will cause bats to avoid the house, meaning the exterior color or sun exposure may need adjustment.