Prestazioni energetiche degli edifici storici. Metodi di calcolo analitico e semplificato: valutazioni e limiti di applicazione/Analytical and simplified methods for the evaluation of the energy performance of historic buildings: application limits

Given the current possibility to investigate the historical building only through calculation methods developed for new or recent buildings, the research started from the analysis of these tools to verify their application validity and limits to the study of ancient architectural fabric . In particular, these instruments can be classified according two categories: analytical calculation methods, which require to properly describe all the geometrical characteristics and thermal properties of the structures and simplified methods, which facilitate some steps of the calculation using a series of pre-calculated technological solutions for standard components. The analytical calculation method is very accurate because, knowing all the components of the building envelope and the structure of the building, allows to obtain an energy performance index very close to the real data. However it is also quite detailed and complex because it requires many calculations and the identification of a variety of elements constituting the building. For this reason, a simplified calculation version was developed, and its use is now permitted by technical standards UNI TS 11300. The structure of the simplified method has been studied and developed according to criteria of simplicity and necessarily it contains within itself some approximations, due to the simplifications introduced in the calculations. If several studies have tried to estimate the approximations contained in these simplified tools, for ancient buildings it is still to be proven whether they can be acceptable and in what terms you can apply the simplified methods so that they can provide the expected correct energy performance value. Therefore, the hypothesis tested by this study is that, for historical buildings, there is a significant difference between the energy performance result obtained by the analytical method, using the rules for not historical building, and the result obtained with simplified methods, recognized by the Italian Thermo-technical Committee . The research tested the different calculation tools on chosen case studies which allow, through the comparison of the results obtained, to conduct a sufficiently reliable critical analysis of the calculation methods (Figure 1). All the buildings under study show a volumetric complexity resulting from considerable indoor heights and a complex conformation of the interiors, with the different units which are interwoven in the building. From an energy point of view, this means that the same room can have a number of different thermal exchanges, all to be described in the operations of calculation of the energy behaviour. In fact, the identification of all the thermal exchanges with the outside, with the attic or slabs on the ground and with the indoor not-heated spaces (for example doorways and stairwells) is necessary for a good calculation. Each of the case studies was simulated using three different methods of calculation: - standardized analytical method: analytical calculation, conducted in accordance with the instructions contained in UNI TS 11300, Parts 1 and 2 - Docet method: simplified calculation, recognized by the CTI, supported by the corresponding software Docet® - simplified method UNI TS 11300: simplified calculation, carried out pursuant to the simplification suggested by the official set of rules in case of the absence of specific data required for the analytical calculation . The two simplified calculation methods were chosen for their prevalence in the professional world due to their scientific correctness. In Figure 2 the results of case studies simulations are presented in a synthetic graph. Among the results of the research, it was found that, compared with the standardized analytical method, the simplified method, provided by the same legislation, overestimates the value of the energy performance index for the overall envelope (EPgl-inv) of about 20%, stating that the building consumes more than the real and that the Docet simplified method overestimates EPgl-inv of about 40%. The analysis has highlighted the main problems in the application of simplified methods to historical buildings and allowed to propose corrections to such methods in in order to define specific tools.

La ricerca parte dall’ipotesi che le tipologie di calcolo oggi disponibili (metodo analitico standardizzato e semplificato) presentino limiti di applicazione all’architettura storica, le cui caratteristiche tecnologiche e morfologiche si differenziano molto rispetto ai fabbricati nuovi e di recente costruzione.