بهره وری اقتصادی از ارتقاء حرارتی مسکن اجتماعی در آب و هوای مدیترانه

The age of European existing building stock and the very low growth rate make energy retrofit a critical issue at the moment. The spreading of good practices is strongly hampered by their cost effectiveness. National legislation thermal requirements for retrofitting works play a fundamental role. If they are too strict, the initial economic investment could be too high if compared with energy savings achievable. The paper refers to a case study of improving fabric thermal performance of stone masonry buildings located in Cagliari – Italy. They were built in 1950s. The economic feasibility of different energy efficiency retrofits is evaluated. Two different scenarios are analyzed: the former according to the earlier Italian legislation (valid from 2006 to 2008), the latter according to the latest and strictest measures (valid from 2010). The results show that heavy thermal performance upgrades, even though compulsory, are not completely cost-effective in a mild Mediterranean climate area as far as payback time is concerned. National subsidy policies can improve the economic return on the investment. The impact of present Italian incentives has also been evaluated.

Statistical studies show that nearly 76% of Italian dwellings were built before 1981, and 49% are more than 50 years old. The average annual increase in new dwellings in the 1981–2011 period was only 1% [1]. These values are above the European average. The existing residential building stock in Europe has more or less the same annual growth trend which is estimated at 1.3%, 70% are more than 30 years old and 35% are more than 50 years old [2] and [3]. The data highlight how much the Italian building stock has aged. Although the first building legislation on energy performance was adopted in 1976 [4] and an update was proposed in 1991 [5], energy consumption in the residential sector did not decrease. In 2005 the Directive EPBD 2002/91 was adopted [6] and [7]. In the same period a decrease in the growth trend of Energy consumption from 0.8% to 0.4% [8]. It is evident that the low growth of new high performance dwellings is not enough to invert the growing consumption trend; it is instead necessary to adopt effective retrofit solutions for existing buildings. As other authors have already point out, there are many obstacles to the diffusion of good practices. One of the main issues is the cost-effectiveness of home energy retrofits [9]. In particular, the literature review highlights some critical issues: • national legislation can be too strict and prescribe energy efficiency requirements that make retrofits cost-ineffective for homeowners [10] and [11]; • the price of energy is artificially low because the social costs of production are often hidden and not charged directly on users’ bills [9], [12], [13] and [14]; • uncertainties concerning the future price of energy make the economic feasibility analysis of works quite difficult [15] and [16]; and • public subsidies are necessary to reduce payback time and increase economic benefits for investors [9] and [14]. These issues are extremely important and also the recent EPBD 2010/31 [17] highlights that “Member States shall take the necessary measures to ensure that minimum energy performance requirements for buildings or building units are set with a view to achieving cost-optimal levels. (omissis) Member States shall take the necessary measures to ensure that when buildings undergo major renovation, the energy performance of the building or the renovated part thereof is upgraded in order to meet minimum energy performance requirements in so far as this is technically, functionally and economically feasible. (omissis)”. Different requirements are established by Italian Decree Law 192/05 to reduce energy consumption of existing retrofitted buildings. Among these the upgrade of building fabric thermal transmittance has undoubtedly the highest economic impact. Even stricter standards have been adopted since 2006, as is shown in Table 1.The paper refers to a case study of improving fabric thermal performance of stone masonry buildings located in Cagliari – Italy. The economic feasibility of different energy efficiency retrofits is evaluated. Two different scenarios are analyzed: the former according to the earlier Italian legislation [18] (valid from 2006 to 2008), the latter according to the latest measures (valid from 2010). The results show that heavy thermal performance upgrades, even though compulsory, are not completely cost-effective in a mild Mediterranean climate area as far as payback time is concerned. National subsidy policies can improve the economic return on the investment. Their impact has also been evaluated in the following. The results can also have interesting applications in other European areas. As shown in [19] Cagliari's climate is similar to the whole Mediterranean coastal area from Gibraltar to Cyprus. This region is characterized by heating degree days <1500 and cooling degree days ≥500. Main Mediterranean European Countries, such as Spain and Greece, have been pursuing similar incentives policies in the last years [20] and [21]. Even if the subsides are different from Italian ones (that consist mainly in tax incentives), the methodology proposed in the paper can be transposed also in different contexts, once construction costs and discount rates are known.

The age of the existing building stock and the very low growth rate make energy retrofit a critical issue at the moment. However, the spreading of good practices can be strongly hampered also by their cost effectiveness. It is strongly affected by national legislation requirements and public subsidies. The analysis carried out on the 1950 stone masonry public dwellings compares two regimes of thermal transmittance upgrade, the former according to the first Italian legislation that came into force and the latter to the heavier limits now compulsory. The following considerations can be advanced based on the calculations described above: • The IROR of NPV difference without incentives is comprised between 4% and 8%. Thus the choice of the constant discount rate, which usually ranges between 3% and 7%, is extremely important in defining which of the two scenarios is more profitable. • If Italian incentives are introduced (55%) the IROR increases of about 2% regardless of work combination and building type. • The NPV (5%, 30) analysis highlights that without incentives windows replacement is never profitable. • Complete retrofit works should be preferred to partial ones since the total cash flow at the end of the life cycle is always the highest, even though the initial investment is superior. Data show that life cycle cash of the complete retrofit combination flow is twice the windows replacement's one. • Investments needed to comply with the strictest legislative requirements have a higher NPV, but also a longer PBT. As far as roofs and windows retrofit is concerned the difference is meaningless, however for complete retrofit the data show an average PBT increase of 3 years. If a quicker return on the investment is required the lighter interventions should be preferred. • The introduction of subsides makes heavier works more profitable. It enhances NPV gaps between the two scenarios up to 10 times and reduces PBT difference by about half. These results are valid only if the case study hypotheses are fulfilled, i.e. for similar buildings in mild climatic conditions, with a similar energy price and works cost. However, it also emphasizes that more attention must be paid by national legislation on retrofit requirements. The transposition of the last EPBD could be a good chance to take these issues into account. A further research objective is to inquire into how summer cooling energy expenses affect the economic feasibility of retrofits, thus implementing the present study based only on heating consumptions. Also a more detailed analysis of thermal bridges influence would be a further development of the study.