How can we provide better thermal conditions inside bat boxes?
Artificial roosts for bats, including bat boxes, can pose a risk to bat safety and health in certain environments. Some roost designs overheat (internal temperatures > 104°F), leading to bats experiencing heat stress or dying. When artificial roosts are poorly designed or placed, they may offer less suitable microclimates than the natural tree hollows and crevices to which bats are adapted. Global warming may compound the risk of heat stress to bats in artificial roosts.
Bat boxes are typically designed for use by maternity colonies of bats, which usually include tens to hundreds of adult females and their pups. Pregnant and lactating adult females prefer warm roosts that offer conditions conducive to growth during pregnancy and infant development in the initial few weeks after birth. While adult females hunt for insect prey, pups are left in the roost, making them vulnerable to cool nighttime temperatures. Even though artificial roosts can be quite warm during the day, most do not retain heat into the night due to a lack of insulation and low thermal mass. Designing artificial roosts that retain heat later into the night should promote pup development and enhance survival rates. Ideally, artificial roosts will offer microclimate conditions more like those found in natural roosts—typically cavities and crevices in trees—which are larger and more insulated.
Below, we provide a list of suggestions and strategies to improve thermal conditions in artificial bat roosts, backed by empirical research, with the goal of enhancing their safety for bat species worldwide.
Design boxes for heat storage
Small artificial roosts with low thermal mass will overheat more readily. We recommend tall, insulated artificial roosts with multiple sides or accessible roost chambers offering large temperature gradients and allowing bats to move to avoid high temperatures.
The simplest way to increase thermal mass is to build a bigger box. Structures with more mass are slower to warm and cool, so they are less likely to overheat and will retain heat at night. Larger roosts also house more bats, allowing for more social thermoregulation (i.e., many warm bodies heating the roost) during cold spells. Because warm air rises, tall boxes offer larger thermal gradients. Roosts that are tall (> 3 feet or 1 meter) or offer multiple chambers give bats the option to move about to find their preferred temperatures. Placing an artificial roost in contact with a larger object, such as a building, is another way to increase thermal mass. However, deploying one massive roost, like a bat condo, may result in aggregations of hundreds or thousands of bats, more than would usually be found in natural structures, and this may attract predators or heighten the risk of injury from natural catastrophes. A sensible alternative is to deploy several tall or structurally complex boxes in one area.
A bat box typically has walls made of wood, with air spaces in between layers. A simple wooden design will track the outside air temperature at a site, gaining heat throughout the day. Building boxes with layers that store more heat will increase the time it takes for a box to heat and overheat. Water is a relatively lightweight material with a high capacity for heat storage. Following research that showed the benefits of heated bat boxes, we worked with Dr. George Bakken to conceptualize our water-jacket design, aiming to decrease energy expenditures for reproductive bats or those recovering from white-nose syndrome. Water packets inside the box walls reduce the risk of overheating at the top of the box and retain heat at night. Pups will benefit from the absence of injuriously high daytime temperatures at the top of the boxes. This may be particularly important during heat waves, given the tendency of pregnant and lactating mothers and their pups to occupy the warmest part of the box where crowding may impede escape from lethal conditions. Our field tests and computer modeling indicate that the extra cost and effort needed to construct the water jacket rocket box is justified.
Adding insulating layers (e.g., polystyrene) to the outside of an artificial roost will slow the gain and loss of heat further, enhancing the performance of any design prone to overheating and rapid cooling. Placing insulation over the heat storage layer (e.g., over a water layer) will provide the strongest buffering effect.
For roosts situated in particularly warm climates, adding extra vents or a larger roof that provides midday shade could further reduce the risk of overheating.
Know that color alone won’t make a box suitable
Painting a box a dark color is not sufficient to produce the thermal conditions bats need. Using a dark color on the exterior of a simple bat box substantially increases the chance the box will reach lethal temperatures when placed in direct sunlight. When given the choice, bats prefer dark artificial roosts over lighter ones and they’re unable to predict the risk of lethally hot temperatures on warm, sunny days. At night, dark roosts cool just as quickly as light roosts, if made from the same material. Because paint alone doesn’t create optimal conditions, we must choose paint colors in concert with safer roost designs. For smaller boxes or those that are uninsulated, we recommend painting the box a color that is 40% dark or less (i.e., a medium-low to light brown), as these lighter colors will absorb less solar radiation and be safer on warm, sunny days. However, such a box may be chilly on cool, sunny days. To overcome this issue, we suggest building a box with high thermal mass and insulation in the outer walls and then painting this box a darker color — even black paint may be appropriate in places with cooler climates and shorter days.
Think location, location, location
Though most bat box users are adapted to using trees, this doesn’t mean bats prefer shade. Maternal females tend to prefer roosting in large trees – often dead trees — receiving a lot of sunlight, especially during the peak time for pup development (late spring to early summer), as warm roosts help adult females to save energy. Still, given their tendency to overheat, we recommend artificial roosts receive some shade during the hottest part of the day. We suggest installing roosts on posts along the east or southeast side of a tree line, so they receive morning or midday sun but not intense afternoon sun. When it is possible to install multiple roosts, we recommend placing some in full shade, which gives bats more options during spring and fall, when they may alternate using sunny and shaded roosts.
Offer more complex designs
Profile: two-chambered rocket box
Biologists Dan Dourson and John MacGregor first conceived the rocket box design in 1994, with the idea that this roost would mimic the conditions under the sloughing bark of a dead tree, a common roost choice for many forest-dwelling bat species. The rocket box design offers roosting space on four sides with differing solar exposures, and bats can choose from cool conditions near the bottom or warm conditions near the top of the 1-meter-tall chamber. A rocket box offers a larger temperature gradient and more volume than a traditional flat-fronted box with only one chamber. We build rocket boxes with two chambers and a pass-through hole on each side so that bats can more readily move between chambers inside the box. The rocket box attracts Myotis bat species adapted for roosting in tree crevices and cavities. In a temperate climate, even the plain rocket box design does not buffer extreme outside temperatures, but adding a “water jacket” improves thermal conditions for bats. Adding insulation outside a water layer would further delay overheating and cooling; increasing the height to greater than 1 meter would enhance the thermal gradient.
About this page
This is version 2.0 of this website, which was originally launched in 2021. Information on this site is partly drawn from Joy O’Keefe’s research program on bat boxes, ongoing since 2015. Thanks to Dr. Reed Crawford and Dr. George Bakken for written and intellectual contributions, Johnny Baakliny for the water jacket schematic, and Isabella Hubrich for the 2025 website re-design. Updated in July 2025.
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References
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