Deep energy retrofit of the existing public stock is a crucial strategy to promote environmental targets at national and EU private levels, as required by EU Directives. The article presents a deep energy renovation strategy focused on high school buildings. The research is carried out in a sample of school buildings in Catalonia in which their possibilities of energy rehabilitation are analysed, developing a toolkit for prioritizing the buildings to intervene and the standardization of possible energy improvement measures for high school buildings. Such methodology is specifically developed to promote the replicability of deep renovation in Mediterranean schools, the model is applied to a specific case enquiry focused on the climatic zone Cfa according to Köppen (and equivalent to the Zone D3 at the Spanish level according to Spanish Technical Building Code), in accordance with a volume and constructive characteristics replicable in large schools built at the same period (70’s). Despite being schools that are over 50 years old, their demolition is not considered appropriate but it is preferred to gradually improve their construction elements and facilities system throughout their lifespan. It is worth noticing current thermal discomfort and low air quality in schools, particularly during the summer months when overheating can be a significant issue, given the absence of an active cooling system. It should be noted that the technical aspects evaluated in this first phase are those that meet the definition of an nZEB building under the cost optimal methodological framework: (1) prioritize demand reduction (minimizing heating demand with the aim of not penalizing cooling demand decisively enough to require the new implementation of refrigeration equipment to avoid overheating), (2) followed by the energy optimization of the installations’ consumption in relation to their performance and, finally, (3) promoting the implementation of renewable energies. Next, a methodology is proposed to discriminate and prioritize the intervention measures to be carried out, in a cost optimal deep energy renovation strategy, considering environmental aspects such as the life cycle of the building. This methodology is validated and its application in a representative case study is shown. Four key strategies are followed: (1) inspection and evaluation of the reference scenario, (2) establishing the strategic vectors for deep renovation according to the official definition of nZEB, (3) Considering the life cycle analysis of the interventions carried out to assess their environmental impact beyond the use phase, and (4) evaluation of the economic performance between investment and economic benefit of the energy bill with respect to the environmental benefit, in order to guarantee a cost optimal deep energy renovation. The research aims to show that energy rehabilitation solutions, seen from an environmental impact point of view during the use phase of the building, cease to be relevant when the environmental impact of all phases of the building (LCA) is introduced. In this case we can see how the environmental impacts generated by the improvement interventions are compensated at most after 3 years of use of the building. It can be concluded that the reduction in demand reaches values around 40%, versus the slight increase in cooling demand (less than 10%). However, the study has managed to reduce the cooling
