of other aspects of the building could be analyzed with the same methodology. For example, the insulation upgrade might be compared to that of renewable energy systems. Adding a photovoltaic installation would increase the embodied energy (AEE) and would decrease the annual energy use (AEU). The NER of a domestic photovoltaic solar installation can be estimated to be a constant value of 4 for this given climate [13,54,55]. It could then be seen that the first insulation upgrade (first additional 50 mm of polystyrene) would be a better option since it has a higher NER of 5.8. The PV system would be a preferred option than further increases of insulation thickness which would correspond to the NER values of 3.1 and 1.6. This method is also applicable to aid in the selection of solutions that decrease embodied energy but increase the energy use. In these cases NER between two different options 1 and 2 can be defined as the ratio of the decrease of annualized embodied energy to the increase of annual energy use