Investigation on the sensivity to temperature variations has been achieved on a full-scale experimental dairy wastewater treatment plant including an unheated but insulated upflow anaerobic sludge blanket. A simple steady-state heat transfer model based on energy balance has been designed to forecast the biogas production depending on ambient air and dairy wastewater temperatures variations. Energy balance has been described for any part of the digestion plant. Calculated heat losses were in the same range than observed losses with an uncertainty of about 10%. From the equalization tank to the digester the average heat loss under cold period was close to 10 degrees C due to convection and conduction. Mesophilic conditions are not respected for couples of ambient air and wastewater temperatures ranging respectively from 8-35 to 35-29 degrees C. Technical solutions are suggested to increase the biogas production. (C) 2012 Elsevier Ltd. All rights reserved.