**4. Conclusions**

The European community has outlined its objectives in the field of environmental sustainability, indicating ambitious targets for reducing energy consumption, reducing emissions, and increasing the use of renewable energy sources. In this context, the study of energy retrofitting of existing buildings is of fundamental importance, as the building sector is responsible for more than 40% of total consumption. These interventions must be inserted in a new energy scenario, in which greater electrification of consumption and greater flexibility of demand will allow a wider integration of renewable energies. In this study, referring to the Italian situation, energy retrofitting interventions for the residential sector were analyzed, using a set of four Key Performance Indicators, i.e., primary energy consumption, renewable energy use, local polluting emissions, and flexible electrical loads. The interventions here analyzed are on the building envelope, on systems upgrading and, finally, a combination of them.

Considering the current situation, it was observed that:


With regard to the system upgrading, a clear difference was observed between gas-fed systems for space heating and DHW and electrified services by means of heat pump installation. In this case, a considerable increase in flexible loads and the renewable energy use occur with a strong reduction in local emissions due the increase of the power grid supply.

The results of the simulations carried out to evaluate the effects of a combined building envelope and system improvement are as summarized below:


It is noteworthy how the results of this study can offer a further KPI to evaluate the best retrofitting intervention, from an economic perspective, accounting for new electricity business models in the era of prosumers where flexibility is becoming the new performance of the built environment.

**Author Contributions:** The authors equally contributed to this paper.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
