Most current building thermal codes impose upper limits to the predicted annual building energy demand for heating, ventilation and air conditioning. In the building design phase these predictions are obtained using thermal simulations with variable complexity. The simplest approach uses a single lumped thermal capacitance to model the high thermal mass building elements, combined with five thermal resistances (known as the 5R1C model proposed in EN ISO 13790 standard). This model is used by many European countries as the reference simplified methodology to assess overheating risk and calculate yearly building energy demand. This paper presents a successful extension of this model that allows for its application to the prediction of the internal air temperature of free-running buildings with double skin facades. The extension consists in an increased number of thermal resistances used to model the double skin facade zone. The extended model is validated using a set of detailed thermal measurements obtained in a free running double skin test cell. For the case analysed the simplifications used in the RC model do not reduce the overall accuracy: the mean absolute error for room air temperature is approximately 1 degrees C, the same order of magnitude of more detailed EnergyPlus simulations (1.2 degrees C). (C) 2016 Elsevier B.V. All rights reserved.