Many mammals hibernate during winter, reducing energy expenditure via bouts of torpor. The majority of a hibernator’s energy reserves are used to fuel brief, but costly, arousals from torpor. Although arousals likely serve multiple functions, an important one is to restore water stores depleted during torpor. Many hibernating bat species require high humidity, presumably to reduce torpid water loss, but big brown bats (Eptesicus fuscus) appear tolerant of a wide humidity range. We tested the hypothesis that hibernating female E. fuscus use behavioural flexibility during torpor and arousals to maintain water balance and reduce energy expenditure. We predicted: (1) E. fuscus hibernating in dry conditions would exhibit more compact huddles during torpor and drink more frequently than bats in high humidity conditions; and (2) frequency and duration of torpor bouts and arousals, and thus, total loss of body mass would not differ between bats in both environments. We housed hibernating E. fuscus in temperature- and humidity-controlled incubators at 50% or 98% relative humidity (8°C, 110 days). Bats in the dry environment maintained a more compact huddle during torpor and drank more frequently during arousals. Bats in both environments had a similar number of arousals, but arousal duration was shorter in the dry environment. However, total loss of body mass over hibernation did not differ between treatments indicating that both groups used similar amounts of energy. Our results suggest that behavioural flexibility allows hibernating E. fuscus to maintain water balance and reduce energy costs across a wide range of hibernation humidities.