Building energy assessment is usually performed based on fundamentals of the First Law of Thermodynamics, which is especially concerned on quantitative energy aspects. However, this approach does not provide a faithful thermodynamic evaluation of the overall energy conversion processes that occur in buildings, and a more robust approach should be followed. The exergy analysis is a useful method that combines First Law and Second Law perspectives and has been applied in many related engineering fields, power plants analysis, CHP systems, heat pumps or building energy systems. In this study, the overall energy and exergy performance of eight space heating options is compared for different outdoor environmental conditions. The methodology follows an approach from demand (at building envelope) to supply side (primary energy supplied), assuming that each energy supply network (ESN) or heating option is divided into the following sub-systems: room, emission, heat generator and power plant. The related energy and exergy performance of each ESN is evaluated through the following indicators: primary energy ratio (PER) and primary exergy ratio (PExR). The results show that for similar primary energy performance, PExR may assume distinct values depending on outdoor environmental conditions. The most energy efficient ESN has a PER of 2.2, while the related PExR changes from 7% to 16% for Lisbon and Berlin, respectively. Furthermore, the assessment of irreversibility rate associated to each ESN sub-system reveals the sources of inefficiencies could be pinpointed and measured, leading to tangible suggestions for further improvements. (c) 2013 Elsevier B.V. All rights reserved.