**1. Introduction**

By ratifying the Kyoto Protocol, the European Union (EU) committed to reducing its emissions of greenhouse gases (GHGs). Such a commitment was further reinforced by the 2015 Paris Agreement in which the EU agreed to a 40% reduction in GHG emissions by 2030 [1]. This can be accomplished, in part, by improving the energy efficiency of buildings as they are responsible for 40% of the total energy consumption in Europe [2,3]. However, a large percentage of the European building stock is composed of historical buildings, with 35% of them over 50 years old and 75% inefficient in their use of energy [2,4]. The potential contribution of the cultural built heritage sector to GHG emission reduction targets is therefore significant and requires action.

The United Nations Educational, Scientific and Cultural Organization (UNESCO), the International Centre for the Study of the Preservation and Restoration of Cultural Property (ICCROM) and the International Council on Monuments and Sites (ICOMOS) have pledged for the development of mitigation strategies applied to cultural heritage. This is particularly relevant to Europe where a large percentage of the heritage properties inscribed in the UNESCO World Heritage List are located [5]. They also encourage managers to reduce GHG emissions at site level, especially within the World Heritage network [6]. Strategies to mitigate climate change in the built heritage sector include the implementation of energy e fficiency measures, for instance, decreasing the use of energy for lighting, heating, cooling and ventilation, or reducing the energy used for transporting building materials. Other measures which can contribute to mitigation e fforts include waste reduction, reusing and recycling materials, using sustainable materials and processes and decreasing water use [7,8]. The carbon footprint of the cultural built heritage can also be reduced through energy e fficiency planning and interventions to decrease emissions within its own managemen<sup>t</sup> [9].

The cultural built heritage sector can therefore significantly contribute to climate change mitigation. The aim of this paper is to understand how climate change mitigation is currently considered in the managemen<sup>t</sup> and preservation of the cultural built heritage in Europe. Specifically, the objectives are to determine the perspectives of experts in cultural heritage preservation on the enablers - and the barriers - to mitigate climate change within their sector, and to identify best-practice approaches for the refurbishment of historical buildings with the aim of decreasing their energy consumption.

### **2. Mitigating Climate Change in the Cultural Built Heritage Sector**

There is an increasing body of research on climate change mitigation in the built heritage sector, with most studies focusing on reducing the energy used in heritage buildings through retrofitting efforts, i.e., improvements in the thermal performance of the building envelope, and upgrading the heating, ventilation and air conditioning systems. Less research has been accomplished on the use of traditional passive measures in historical buildings as strategies to reduce energy consumption, and on the use of the Life Cycle Assessment (LCA) methodology for the selection of materials requiring less energy to produce, and thus emitting less CO2. There is also limited research on energy saving measures induced by changes in user behaviours, and on the challenges associated with improving the energy e fficiency of heritage buildings in relation to the impact of the refurbishment on their historical value.

Several studies investigated possible retrofitting solutions for historical buildings to reduce their energy consumption. The proposed measures include improved thermal insulation of floors and roofs, external wall insulation through the use of highly insulating plaster [10], the installation of more e fficient (and draught-proofing) windows, the improvement of heating, cooling, ventilation and lighting systems (e.g. installation of light emitting diodes [LEDs]), the installation of photovoltaic tiles, and even the elimination of rising damp [11–13]. Hence, previous studies have mainly presented examples of retrofitting of historical buildings that have been successful in decreasing the building energy consumption.

Improvements in the insulation of the building envelope is a major theme in climate change mitigation. There are several options and evidence for their varying e ffectiveness, including recently advanced options. For example, Berardi [14] investigated the properties of aerogel systems with plasters, concrete tiles/panels and fibre blankets, emphasizing that these materials have grea<sup>t</sup> thermal performance, but they are too expensive to be used for a sustainable economic return. Also, Zhou et al. [15] investigated the performance of internally insulated walls with aerogel-based high insulating plaster and renders such as lime mortar and mineral plaster, indicating that internal retrofitting using such materials can alter the hygrothermal performance of walls and, for this reason, recommended caution in their use. Novel but more traditional methods of insulation can also be used, e.g., Nardi et al. [16] investigated the upgrading of the internal vertical envelope using insulating panels made of hemp fibre, which resulted in increased thermal performance. There are, nonetheless, possible disadvantages resulting from these building alterations, such as an increase in decay caused by changes in hygrothermal performance and vapour movement when new materials are introduced to increase thermal performance.

Another area of attention is the improvement of the heating, cooling, ventilation and lighting systems e.g., [17], and the cogeneration of heat and power using renewable energy sources to reduce the buildings' operational energy requirements. These can include heat pumps as heating and cooling systems, which use outdoor air, underground water e.g., [18,19] and heat stored in the ground e.g., [20–22], demand-controlled ventilation and trigeneration technologies [18], e fficient lighting systems [23] and 'hybrid' energy systems [24]. Unlike other research, Lo Basso et al. [24] considered the heritage values as a key factor to take into consideration when proposing changes to the heating and power system of historical buildings, notably in the use of photovoltaics, solar hybrid collectors, and heat pumps as solutions to e ffectively reduce their energy consumption. This is because there is a risk of incompatibility, as the use photovoltaic systems, for example, can a ffect the aesthetic value of a historical building. These examples, amongs<sup>t</sup> others, paint a complex picture; it can be di fficult to identify a solution due to the subjectivity of di fferent aspects to take into consideration when designing refurbishment measures, such as values and money.

Traditional passive measures adopted in historical buildings use renewable energy sources, notably wind and solar, for heating, cooling and ventilation. Such measures include the design of patios and courtyards to improve building ventilation [25], the use of natural ventilation [26,27], double windows [28], and coloured reflecting mortars and tiles [29,30]. On the one hand, these measures can effectively maximize the intrinsic characteristics and behaviours of historical buildings, for example by using natural ventilation in heavyweight buildings for night pre-cooling in warm climates [26]. On the other hand, these approaches should consider aspects such as heritage value preservation and the energy embodied in the materials of the building, i.e., the energy required for their extraction, manufacturing, transportation, and during construction. For instance, Rosso et al. [29] demonstrated that the energy demand for cooling a building was decreased by using newly developed coloured reflecting mortars and tiles. However, the aesthetic impact of applying these new mortars should be evaluated against the potential loss of heritage values and the energy embodied in the original mortars and tiles. One could argue that if the historical tiles are not damaged, there is no reason to replace them, as their replacement will result in GHG emissions and a loss of heritage value. E ffectively, the energy embodied in historical buildings is rarely considered in energy-retrofit strategies [31,32]. The adaptive reuse of heritage building materials can reduce GHG emission; nonetheless, historical buildings still need to be upgraded to be energy-e fficient over their full life cycles [32]. Some assessments have compared the energy embodied in historical materials with materials that are more recent. However, such comparison should not be done on its own and needs to consider the historical and cultural value of the ancient materials. The adaptive reuse of historical buildings allow for preserving both the energy embodied in the material and the heritage values of the building [33].

Also, Litti et al. [34] investigated the replacement of historical windows with new ones as an additional measure to improve thermal insulation. The findings highlight that replacing windows does not necessarily allow for the largest energy savings over their full life-cycles, while their maintenance may result in comparable or more considerable savings [34]. In fact, the LCA methodology can be used to select solutions or materials using less energy and thus emitting less CO2, however, the application of the LCA methodology is still in its infancy in the cultural heritage sector [35,36]. As a result of this gap in knowledge, Bertolin and Loli [37] developed a decision-support tool integrating a LCA approach within the framework of building conservation principles, nonetheless, they highlighted the need for further work in this direction given the complexity of this issue.

There are not only material aspects to consider when retrofitting historical buildings. Users' behaviour has an important role to play in mitigating climate change, as it determines the preference and choice for room temperature and ventilation, for instance [38], and can help target specific groups in carbon reduction strategies. Many studies highlighted the high energy saving potential derived from changes in user behaviour [39–43], which they estimated to range from 62 to 86% [40]. Human behaviour, however, can lead to a rebound e ffect in energy usage. Hens et al., [44] showed an example of such a rebound e ffect with the average indoor temperature of houses increasing after improving

insulation. The increasingly energy-intensive way of life should also be considered when designing energy-e fficiency policies and strategies by promoting lifestyles compatible with carbon reduction [45]. Occupants' behaviours in their use of energy can be approximated using variables such as type of dwelling or the Heating, Ventilation and Air Conditioning (HVAC) system they use, and thus help at targeting carbon reduction strategies to specific groups. Guerra Santin et al. [46] found that occupants living in non-detached dwellings or in houses where thermostats are installed consume less energy, for instance. This shows that more engagemen<sup>t</sup> with behavioural research is needed to identify opportunities for reducing GHG emissions [47].

Yarrow [48] investigated the perspectives of building professionals, planners and home owners in relation to not only the issue of climate change mitigation when refurbishing historical buildings, but also on the challenges involved in relation to the impact of energy e fficiency improvements on the historical significance of heritage buildings. On the one hand, there is pressure to preserve the authenticity of the historic built environment, but, on the other hand, there is pressure to mitigate climate change. Considering the above options, it is clear that many mitigation choices involve physical alteration of heritage assets. This raises the question as to whether the mitigation solutions proposed in the literature are compatible with the heritage values and also with the traditional characteristics and behaviour of historical materials and structures. This is a complex issue and there are contrasting examples in the literature. For instance, Ascione et al. [49] developed a methodology to select measures to retrofit a historical building according to energy, environmental and economic indicators. Using that methodology, one of the solutions identified was the replacement of the historical windows with new double glazed ones, a solution that was proposed without considering the values of the heritage building. As an example of best practice, De Santoli et al. [50] focused on reducing the heat load of a historical building and, to deal with this issue, their proposed solution consisted of an air exchange system integrated with the existing architectural elements of the building, such as chimneys and fireplaces. By converting them to a new role, the solution remained compatible with the heritage values, and minimised changes to the building fabric. Webb [43] further stressed that energy retrofitting of historical buildings can be an opportunity to help preserve them for future generations. There is an inherent complexity in balancing the drivers and constraints of mitigation-related energy retrofitting of historical buildings [48]. There is a desire to improve internal comfort and to reduce operating costs, while the need to preserve heritage values can constrain mitigation actions; for example, Cornaro et al. [10] discounted the potential for interior wall insulation due to the presence of frescoes.

Overall, the literature on approaches to reduce GHG emissions in historical buildings focuses on presenting case studies on measures to improve the thermal performance of historical buildings, e.g., building envelope insulation and upgrading of heating and cooling systems. This is in addition to generic advice on reducing the environmental footprint of historical buildings through retrofitting, renewable energy generation on site, o ffsetting carbon emissions, managing waste and using water more e fficiently, both from a technical point of view e.g., [8,51,52] and for informing buildings' owners and the public see [53–55]. This review highlights the paucity of studies on the challenges to overcome in the cultural built heritage sector to mitigate climate change. A broader picture is needed to inform and support decision making on the priorities to consider when promoting climate change mitigation in the cultural built heritage sector. A number of questions remain insu fficiently addressed in the literature, notably, how do experts involved in the preservation of the cultural built heritage consider climate change mitigation? What are the enablers for implementing mitigation strategies, and what are the barriers to overcome? Answering those questions is essential for the development of mitigation measures and for the identification of future research directions. To date, most studies have used quantitative methods, except for Yarrow [48] who followed a qualitative approach. The current study uses a qualitative methodology involving interviews with experts, and the above literature review was conducted to provide background information for the interpretation of the interviewees' responses. To the author's knowledge, this is the first paper that identifies enabling and constraining factors as

well as examples of best practice in mitigating climate change in the built heritage sector as a result of consultations with experts.
