Why you should build one
Bats are incredibly beneficial animals. Eat bat can consume thousands of insects a night, including the insects that cause the most problems for humans, such as disease-carrying mosquitoes, and the insects that cause the most damage to agricultural crops and gardens. The myth that bats are the primary carriers of rabies have been disproved. Very few people contract rabies from bats. The majority of cases in which humans are bitten by rabid animals are from unvaccinated dogs, not bats. That said, never attempt to pick up any wild animal, especially without gloves. If a bat is on the ground, it is most likely sick or injured. If you absolutely must re-locate it, wear leather gloves, and minimize handling the animal.
Many organic farmers and gardeners add bat houses to their property, knowing that welcoming a bat colony to your area will reduce the number of insect pests. They are also popular additions at beach houses and lakeside cottages, where mosquitoes are a particular problem.
Bats are also facing new challenges as an invasive fungal infection is spreading across the nation. Literally millions of bats are dying from White Nose Syndrome, and there is little we can do to halt the spread or help the bats who have already been exposed. You can read more about it in the article "Bats and White Nose Syndrome: Why We Should All Be Concerned." and "Saving the Bats." One small thing we can do to help is provide safe habitat for bats to roost.
I've noted some discussion about whether mounting bat houses on your property will result in a population of dead bats to deal with, should they become infected with White Nose Syndrome. To allay those concerns, you need to understand the hibernation and migration habits of bats. The bat houses are used in warmer weather, primarily from late spring to early autumn. In areas with cold winters, the bats will relocate to a cave, mine shaft, or other location with stable temperatures and ample space for large colonies of bats to huddle together during their hibernation periods in the winter. It is during hibernation that White Nose Syndrome is a deadly menace to the bats. It has primarily affected bats in parts of the country that experience below-freezing temperatures in the winter, rather than warmer locales where bats could potentially live in a bat house year-round.
Where they should be located
Bats like heat, particularly if they are establishing a nursery colony. Locate your bat house where it will receive at least 6 hours of sunlight a day. South or south-east facing locations will maximize the midday sun. You can also locate two bat houses back to back, in either a north-south or east-west orientation. You may find that bats prefer one house in some seasons, and the other in other seasons, as their temperatures fluctuate.
Surprisingly, many years of observation have revealed that bats prefer bat houses attached to buildings or free-standing poles, and rarely inhabit houses attached to living trees. This may in part be because predators can also use the tree to reach the bat house, while houses mounted on buildings and poles are only accessible by air. In colder parts of the country, houses mounted on buildings have a distinct advantage, as the house radiates heat and blocks wind.
One other consideration is the height of the bat house. Experience shows that bat houses are best mounted a minimum of eight feet from the ground, and up to twenty feet above the ground. This gives many options for mounting to houses, garages, sheds, barns, or poles. Avoid attaching bat houses to light poles, as bats will not live in a house with all-night lighting, despite the abundance of flying insects the light may attract.
In northern climates, bats seem to prefer bat houses attached to buildings, where they can take advantage of the wind-blocking properties of the structure, as well as the solar heat that is stored and radiated by the building. Just think carefully about where you will locate it. Since bat droppings will fall from the open bottom of the bat house, avoid locating it above windows or doors, or in areas frequently traveled by people. Sheds, garages, and barns are generally better locations than houses, though each homeowner will need to assess their own property to determine the optimal location. As an added benefit, bat droppings (guano) are a sought-after fertilizer, and can be collected in shallow pans to add to your compost pile or flower beds!
If you have a location near water (a lake, stream, pond, or river), you have a better chance of attracting a nursery colony. Mother bats prefer not to range too far from their young, so they prefer a location within a quarter of a mile of some reliable water source. Female bats searching out locations for nursery colonies are often the first to discover and inhabit bat houses, and are soon followed by the males.
What materials to use
The optimal wood for use in bat houses is something naturally weather-resistant, such as cedar, black locust, white oak, or old barn wood. These materials are much more durable than a soft wood, such as the commonly-accessible pine or plywood, and will stand up to harsh weather conditions for a longer period of time. If you do choose to use plywood, be sure to use exterior-grade plywood. Avoid treated lumber at all costs, as the chemicals used are toxic to bats.
What paint to use
The color of paint used varies by your geographic location. In general, hotter climates need lighter colors, such as light gray or tan. As you move further north, the color of the paint should get progressively darker, with northern states requiring dark brown, dark green, or dark gray paint. Start with a coat of primer (not Kilz primer, as the fungicides are repellent to bats), then paint with one or two coats of non-toxic, water-based exterior paint or stain. This will not only preserve the wood and extend the useful life of your bat house, but will also help to maximize the natural solar energy of the sunlight, and heat the box to a comfortable temperature for its winged residents. Paint only the exterior of the bat house, leaving unprimed and unpainted bare wood on the interior surfaces, to allow the bats to grip the wood while roosting.
Coating the exterior surfaces only with melted wax is another alternative, suggested by Dave Miller in our correspondence about the bat houses he built for Steigerwald Lake National Wildlife Refuge near Vancouver, Washington. This would be appropriate in cooler climates, but not in the hotter areas of the country. He explained that it soaks deeply into the wood, protecting and conditioning it, and is a very natural finish that blends into the environment, rather than drawing attention as a man-made structure. It is also long-lasting, and never needs re-applying.
Especially if you live in a colder climate, it is important to seal the bat house against drafts. A non-toxic, exterior grade caulk, such as the commonly available latex caulk, helps retain heat inside the bat house and exclude rain and moisture.
Where to find plans
Plans for bat houses are available readily on the web. One of the best sources is Bat Conservation International. They have done extensive experimenting and tests of bat house designs nationwide for over 30 years, and have tailored their plans to reflect the most successful bat houses. Plans are available for everything from small, one-chambered houses, to multi-chambered houses that are ideal for nursery colonies, to large-scale bat houses that can attract thousands of bats to a community. Please see the Resources section at the end of the article for links to some free bat house plans.
There are also bat plans available through some states' Department of Natural Resources websites. Be sure to compare plans on independent websites with those from reputable sources. It would be discouraging to put the time and resources into building a bat house, only to have the bats refuse to use it due to poor design or inappropriate materials!
Be sure the plan you select has approximately 3/4" wide slots between the interior boards, with roughened wood or grooves to allow bats to cling to the slats inside. Smooth or slick painted boards are difficult to cling to, and will especially discourage use as a nursery colony. There are several ways you can roughen the wood, to help the bats grip the surfaces, including using a saw to cut shallow grooves, using a wall-paper removal tool, or attaching mesh or flattened tree bark to the surfaces. (Thanks to Dave Miller for that suggestion!)
Choose a plan that has a "landing pad" area, which is a longer board that extends from the bottom of the bat house to give them somewhere to land, then crawl up into the bat house. Designs that look like bird houses, or have holes in the front for the bat to enter, will not attract bats. Likewise, houses that are drafty or poorly sealed won't be able to maintain the temperatures necessary for bats to take up residence.
Be aware that it may take two years or more for bats to locate and take up residence in your bat house. Besides building it correctly and placing it in a prime location, there is little you can do to actively attract bats to your house. Bats are most likely to discover your bat house during the times in which they are naturally seeking out new roosts, such as when they emerge from hibernation in spring, or when they have been recently excluded from a nearby house or building where they had previously been living. If you do have plans to evict a colony of bats from your house, and provide alternative roosts in the form of bat houses, choose your timing carefully. It is best to complete this process in the autumn, when there is little risk you'll be separating baby bats from their mothers, or sealing them inside your home to starve. Bats work as a community to care for the young, with some going out to feed, while others stay behind to care for the pups. Sealing your house while you have a nursery colony inside will lead to frantic mother bats looking for an alternate entrance, and caregivers and pups trapped inside.
What to avoid
Be skeptical of any bat house plans that look more like bird houses, or have large, square, single chambers inside. Bats like houses with 3/4" spaces, so they can pack tightly together and share body heat. Avoid smooth wood or slick paints, bright lights, and windy hill-tops.
If you don't have the equipment, time, or inclination to build your own bat house, there are several reputable sources where you can purchase a pre-built bat house. I would proceed with caution, however, as a quick search online for bat houses turned up many designs that were more focused on attracting buyers than bats. Bat Conservation International is one of the best sources for well-designed bat houses, and as a bonus, you know the money you spend there is put back into bat research and education.
What species of bats should you expect to take up residence?
The answer to this varies widely, depending on what species of bats are native to your part of the country. Bat Conservation International states that there are 47 species of bats native to North America. Their decades-long survey of bat houses has identified that the bats most frequently found to inhabit bat houses are the little brown bat (Myotis lucifugus), Mexican free-tailed bat (Tadarida brasiliensis) and big brown bat (Eptesicus fuscus). No matter where you live in the lower 48 states and much of Alaska and Canada, you have the chance to attract at least one of these species. 
| Bat houses in hot, arid climate||Bat house in prairie landscape|
If you live within the range of the endangered Indiana bat ( Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, Mississippi, Missouri, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Tennessee, Vermont, Virginia, and West Virginia), you have the potential to help preserve a bat species that has been listed as endangered, and has experienced a dramatic population loss of about 50% since the early 1970's. These bats typically hibernate until early April-mid May, depending on the season, so this is an ideal time to build and mount a bat house, and provide them with a safe, warm home during the summer months! 
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| Approximate range of|
the endangered Indiana bat
| Endangered Indiana bats|
in a bat house
Bat House Plans:
Free plans from Bat Conservation International
Diagram of bat house built by Dave Miller (yaquina on Flickr.com). Included with permission. Many thanks to Dave for sharing his experience, input, and pictures for this article. Many more pictures of the assembly and mounting of his slate-fronted bat houses can be found in his Flickr Stream.
To learn more about bats and bat houses, visit:
 Bat Conservation International Bat House Homepage
Also, an excellent resource is:
The Bat House Researcher, Newsletter of the North American Bat House Research Project, Volume 12, No. 1, Spring 2004, by Bat Conservation International. It is available as a pdf document here:
 United States Fish and Wildlife Service, species profile on Indiana bats (Myotis sodalis)
Thumbnail image at start of article, of dark bat house on a pole: from Flickr Creative Commons, by Mike Prosser. Some rights reserved.
Bat houses mounted back-to-back on pole: from Flickr Creative Commons, by BXGD. Some rights reserved.
Bat house located near water: from Flickr Creative Commons, by yaquina (Dave Miller). Some rights reserved.
Bat house with bark-covered "landing pad" and removable partitions mounted on tree: from Flickr Creative Commons, by yaquina (Dave Miller). Some rights reserved.
Bat houses with cacti: from Flickr Creative Commons, by Rory D. Some rights reserved.
Bat house on the Prairie: from Flickr Creative Commons, by Andrew Smith. Some rights reserved.
Map of Indiana Bat range: fromWikimedia Commons. Map is from the Fish and Wildlife Service, and is in the Public Domain.