ALT Gentoo penguins (Pygoscelis papua) encounter insulation disturbances during the yearly catastrophic moult. Lewden et al. show that the plumage provides higher skin insulation, showing a lower surface temperature during the early stage of moult, when it accumulates two layers of feathers. In contrast, the surface temperature of thermal windows (bill, flipper and foot) increased during moult, highlighting the likely need for extra heat dissipation in moulting penguins. The captive condition of these individuals demonstrates the effect of physiological and plumage changes during moult on penguin thermoregulation independently of environmental conditions.