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Habitat loss and forest fragmentation are currently substantially reducing the availability of natural bat roosts worldwide. However, since bat populations have been recognized as important ecosystem service providers, bat boxes have become one of the most popular measures employed to protect them. Evidence exists that in arid regions bat boxes act as ecological traps due to abrupt rises in summer temperatures. Several reported mortality events highlight the lack of appropriate guidelines in temperate areas, which might be putting bat conservation in jeopardy. We aimed to explore which bat box features might cause mortality so we compare temperatures in bat boxes, modelling the influence of the orientation and model, and quantifying the risk of extreme heat episodes. A total of 797 overheating events were recorded during a bat-box monitoring program in Catalunya (2014–2015). In 2016, we compared temperatures in up to 15 models side-by-side in the Ebro Delta Natural Park. Bat-box model and orientation clearly affected the number of overheating events. Black-coloured and south-facing boxes recorded the highest temperatures, including the most popular models, with temperatures over 40 °C. The number of overheating events was clearly dependent on the bat-box model. For example, a new model manufactured from rice chaff did not experience any overheating event in contrast with the 2F model that recorded over 50. We do not recommend the use of black boxes in south-facing sites in warm areas. In fact, observed internal temperature gradients suggest that complex boxes may help counteract the effects of overheating. We highlight the importance of taking microclimate characteristics into consideration when setting up a bat-box scheme.