A number of organizations has develop booklets [20, 22, 26, 27, 28, 30, 32] or practical sheets [19] to answer the request of users and professionals on how intervene. Their scope is to be as ‘ready-to-use manuals’: best practices and technical suggestions are provided to help users to renovate single building components of traditional buildings, without allowing to evaluate the renovation effects on the whole building. These studies are also specific for climatic context: most of the examples are from Northern Europe or USA. Hence, these products are not replicable in other contexts or on other case studies.
Other researches have been financed by European Union as long-terms programs and represent an advance research in this topic. The result of these projects is in the form of project summaries [21, 25, 29, 31], like guidance documents on a specific topic, moving from 'How to intervene?' to "How do I decide which intervention implement?".
Although these documents have a different purpose (from the evaluation of possible retrofit strategies applicable on historical district to the possible insertion of the PV in city centre, etc.), they present the same structure: an initial background on conservation principles and energy efficiency aims, a second part with a list of possible retrofit solutions, a third part with case studies application and monitoring. However, being experimental studies and research developed by experts, they are excellent bases of study on the subject, difficult to be adopted by all stakeholders.
In the majority of cases, the measures selection phase is developed as Multiple-Criteria Decision Analysis (MCDA), based on multiple objectives and an analytic hierarchical process [21, 25, 31]. The multi-criteria analysis is sometimes followed by the development of a Decision Support System (DSS) [21, 31], an interactive software system that supports decision-making activities: after having established evaluation criteria and importance weights, the final choice is done on the scores that have been attributed to each retrofit option.
Beyond the exposed studies, there is a lack of definitive consensus about precisely which criteria should be considered, and how they should be categorized. Although it is possible to make considerations on different case studies and to assess different retrofit options, the evaluation criteria are mainly limited to heritage significance and energy efficiency, omitting other aspects such as social and management aspects, connected with the building users. Multi-criteria tools, such as DSS, and methods are not suitable to be easily managed by not experts stakeholders: the risk of using these systems is in the weighting process of criteria, because the hierarchization is subjective and not repeatable. Just a multidisciplinary team could adopt these methods to have well-balanced solutions, that take into account all research fields (restoration, energy, sustainability, etc.). It is also relevant to notice that the 85% of the products is developed to study a specific context, mostly cold or temperate climate, as a result of research efforts in northern Europe not expandable in other contexts: hence, they do not correspond to instruments of international and official driving value, because not fitted for all stakeholders, nope for several climatic areas.
The expressed considerations leave the need to define a retrofit assessment instrument, like a guidance procedure predisposed towards other climates. For this reason, recently European Guidelines EN 16883 [18] have been approved as standard, while in Italy the local Ministry of Cultural Heritage has published the national ‘Guidelines for the improvement of energy efficiency in cultural heritage’ (2015) [23].Guidelines, that corresponds to another typology of assessment instrument, are provided as practical, appropriate and easy to understand advice and guidance on how to upgrade historic constructions, reducing energy costs whilst ensuring that no damage occurs to the building [30].
The two mentioned Guidelines do not indicate mandatory methods and measures to be applied directly - because the building heritage complexity and the heritage constraints – but suggest procedures to help the actors involved in the decision-making process (whether designers or Protection Authorities) to discuss a variety of possible retrofit options.
Although they have the same purpose - to assess a plurality of possible solutions, before their implementation - the two Guidelines are certainly different: the European document is synthetic and aims to guide the lecturer in the proposed procedure, with a final multi-criteria risk assessment; the Italian one is very long with the same features of a volume with some suggestions on possible retrofit options and a list of best practices, but with a shorter suggested procedure.
The different approach of the two procedures has a different outcome as a direct result; according to that, a more detailed evaluation of these two tools will be necessary to understand their validity, limits and potential and to be able to suggest any corrective measures.

2.1 European Guidelines EN 16883  and Italian Guidelines analysis

The intent to analyze the European Guidelines EN 16883  and the Italian ‘Guidelines for the improvement of energy efficiency in cultural heritage’ is to check how these tools address the design process, defining suitable interventions for the retrofitting of historic buildings.
This two documents are not mandatory, but they are worth knowing and analyzing: they represent an instrument to help public authorities and designers to follow an iterative retrofit process; however, in both cases it is possible to notice some difficulties in their technical application and the need to be implemented: one challenge is to analyse them, to improve the two procedures through an in-depth study. The main problems checked during this analysis are those concerning the two procedures proposed and the performance assessment methodologies, as will be shown below.
To compare the two Guidelines and understand their potential, it has been tried to apply both on three representative case studies, different for climatic conditions, installations and use. The first two cases (Verga’s House and Pepoli Museum) are in Sicily. The third one (Rebecco house) is situated in the historic centre of a mountain village in Lombardy. All of them are protected with different legal constraints. More details in Fig. 1.