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Abstract. The thermal environment of farrowing facilities is generally controlled at the thermo-neutral zone for sows (15°C to 19°C). This imposes thermal challenges for newborn piglets, which are thermally comfortable at temperatures 10°C to 20°C higher (32°C to 35°C). To satisfy the energetic requirements of piglets, supplemental heat is installed in creep or brooder areas. In this study, we determined optimum supplemental heat requirements (supplied by heating lamps) for piglets based on energy balance as a function of air temperature and animal body weight. We also determined the zone of least thermoregulation of piglets for a given weight when supplemental heat is not provided. Energy balance was calculated using an ensemble of mechanistic models of bio-heat transfer that predicts hair-coat temperature, skin temperature, and skin heat flux. Inputs to the ensemble of mechanistic models include air temperature, black-globe temperature, rectal temperature, and system parameters (e.g., thickness of internal tissues and thermal conductivities). Input temperatures were predicted from measured air temperature in the pen and supplemental heat using machine learning. System parameters were measured or obtained from the literature and optimized using the Monte Carlo method. Ensemble predictions of hair-coat and skin temperature agreed within 3.5% with measured data. The ensemble-predicted zone of least thermoregulation agreed well with previous reports. Predicted optimum supplemental heat showed an exponential decay trend with increasing air temperature and/or animal weight. For air temperature between 15°C and 19°C, the predicted optimum supplemental heat was 266 to 344 W and 44 to 128 W for piglets weighing 1 kg and 20 kg, respectively. The predicted optimum supplemental heat was ~200 W lower for piglets at the end of the farrowing cycle (assuming weight of 20 kg) than at birth (assuming birth weight of 1 kg).