Mitigating Climate Change in the Cultural Built Heritage Sector

Climate change mitigation targets have put pressure to reduce the carbon footprint of cultural heritage buildings. Commonly adopted measures to decrease the greenhouse gas (GHG) emissions of historical buildings are targeted at improving their energy efficiency through insulating the building envelope, and upgrading their heating, cooling and lighting systems. However, there are complex issues that arise when mitigating climate change in the cultural built heritage sector. For instance, preserving the authenticity of heritage buildings, maintaining their traditional passive behaviours, and choosing adaptive solutions compatible with the characteristics of heritage materials to avoid an acceleration of decay processes. It is thus important to understand what the enablers, or the barriers, are to reduce the carbon footprint of cultural heritage buildings to meet climate change mitigation targets. This paper investigates how climate change mitigation is considered in the management and preservation of the built heritage through semi-structured interviews with cultural heritage experts from the UK, Italy and Norway. Best-practice approaches for the refurbishment of historical buildings with the aim of decreasing their energy consumption are presented, as perceived by the interviewees, as well as the identification of the enablers and barriers in mitigating climate change in the cultural built heritage sector. The findings emphasise that adapting the cultural built heritage to reduce GHG emissions is challenging, but possible if strong and concerted action involving research and government can be undertaken to overcome the barriers identified in this paper.


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

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 CO 2 . There is also limited research on energy saving measures induced by changes in user behaviours, and on the challenges associated with improving the energy efficiency 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 efficient (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][12][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 effectiveness, 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 great 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 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][21][22], demand-controlled ventilation and trigeneration technologies [18], efficient 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 effectively reduce their energy consumption. This is because there is a risk of incompatibility, as the use photovoltaic systems, for example, can affect the aesthetic value of a historical building. These examples, amongst others, paint a complex picture; it can be difficult to identify a solution due to the subjectivity of different 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. Effectively, 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-efficient 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 CO 2 , 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][40][41][42][43], which they estimated to range from 62 to 86% [40]. Human behaviour, however, can lead to a rebound effect in energy usage. Hens et al., [44] showed an example of such a rebound effect 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-efficiency 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 engagement 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 efficiency 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, offsetting carbon emissions, managing waste and using water more efficiently, both from a technical point of view e.g., [8,51,52] and for informing buildings' owners and the public see [53][54][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 insufficiently 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.

Methodology
The methodology used in this study consisted of qualitative semi-structured interviews with experts in the preservation and management of cultural heritage. In total, 45 interviews were conducted in the UK, Italy and Norway; three European countries with different climates and heritage typologies. The selection of case study sites in those three countries allowed for triangulation of information. These sites were also selected as they form part of a larger project led by the first author with research on vulnerability and adaptation to climate change risks reported elsewhere [56,57]. The number of interviews was based on the principle of saturation in qualitative research, i.e., when it became evident that there was redundancy in the interviewees' answers and no new theme emerged, no additional interviews were conducted. The interviewees were academics and researchers working in different universities and research centres, including experts involved in EU-funded projects focusing on climate change and cultural heritage (42%); practitioners working in organizations and institutions with a focus on the preservation of cultural heritage (27%), and managers, coordinators and professionals involved with UNESCO World Heritage Sites (WHS) (31%). Eighty percent of interviewees have more than 10 years of experience working on preserving cultural heritage and there were more males than women interviewees (Table 1). The interviewees were from diverse backgrounds and specializations, including anthropologists, archaeologists, architects, conservation scientists, geologists, biologists, managers and coordinators of heritage sites, sustainability officers and urban planners. The structure of the interviews was prepared in advance, but during the interviews the order and number of questions varied according to the interviewees' expertise and answers (i.e., the interviews were semi-structured). Introductory questions on the professional background of the interviewees were followed by a list of questions focusing on themes related to climate change mitigation in the cultural heritage sector: GHG emission reductions at heritage sites, improvement in the energy performance of historical buildings, sustainability of materials, and methods used during conservation practices. Ethical approval for this research was obtained through the University of the West of Scotland procedure. The interviews were audio recorded and then transcribed, analysed and coded using the NVivo software (Version 11, QSR International (UK) Limited, Daresbury, Cheshire, UK). The interviews were conducted in English in the UK and Norway and in Italian in Italy. The Italian interviews were not translated during the coding process but some quotes were translated for the purpose of displaying samples of interview quotations in Tables 2-4. "The state or municipality should provide incentives (and give funding) (...) (also) for non-listed buildings. (...) The gap between economic, cultural, historical and social value should be bridged." (Academic) "When things cost more to do in the right way we have to have the willingness to pay more." (Heritage site coordinator) "Recovering money (...) through energy saving." (Academic) "It is better to give work to people that live in the place and that can do maintenance, instead of bringing low quality windows from abroad." (Academic)

Legislation and regulations
"(It) would be really helpful, particularly with tenements, if there was more legislation." (Heritage site manager) "There has to be work on the development of guidelines in collaboration with the municipalities." (Academic) "You have to get it written in management plans. (...) We need (...) tools, action and activities to help them." (Academic) "(Regarding) making the buildings more energy efficient, (enforce) policy requirements when it is a public building." (Academic) "To report the carbon usage of the building ( . . . ) on how the building is energy efficient." (Academic) Normally buildings consume more energy compared (with) how they were designed. Post-evaluation needs to be performed and needs a regulatory structure." (Academic) "(Historical buildings) will not be passive houses nor be A+++. We need to understand what we mean by sustainability. If we change everything, losing the material and the energy used to produce it, we will have a better building in terms of thermal performance, but we lost much more grey energy. We need to evaluate the sustainability through 360 degrees." (Academic)

Energy compensation strategies
"Face (the problem) at district or city level. (...) (Historical) buildings can be put in a grid of energy distribution at district level (and receive) the energy surplus" (Academic) "The concept of trade-off. (...) New design with thermal inefficiency in some parts but with improved efficiency in others." (Academic) Table 3. Barriers in mitigating climate change in the cultural built heritage sector.

Theme Quote
Economic factors "There is very little incentive for certain building owners." (Academic) "Sometimes we are asking people to do things in a more expensive way." (Architect)  Table 3. Cont.

Theme Quote
User behaviour "You make (a building) more energy efficient and theoretically you reduce the carbon emissions, then you find that they are using it in a way that it is not proper. Carbon emissions are made also from the users." (Academic) "There is a cultural resistance to lighting from windows. ( . . . ) Electric artificial systems used when not necessary it is a big issue. People still think that lighting is low in consumption, but it is not." (Academic) "Everybody wears t-shirts (in offices). ( . . . ) We do not try to save the energy as hard as we could." (Heritage site manager) "That would be a question to raise in each building and the answer would be different from building to building. It is hard to generalize." (Academic)  Lighting "Good light management process."

Lighting and insulation
"Changing light bulbs, repairing windows, secondary glazing, new lamping, loft insulation, cavity wall insulation." Heating "Do not heat the building but heat the people." Heating, lighting, monitoring. The interviews transcripts were coded using the categories: "enablers", "barriers" and "best practice". Subsequent to coding, the transcripts were used to search for patterns in the data. This search for patterns in the coded interview transcript led to the identification of themes representing the factors enabling and those impeding climate change mitigation in the built heritage sector was then determined by analysing the coded interview transcripts and are displayed in Figure 1.
Examples of quotes from the interviewees in relation to each of the identified themes are shown in Tables 2-4 to explain the selection of specific themes and the categorisation of the information provided by the interviewees. These representative quotes were carefully selected and shortened to reduce the word count without compromising their meaning. The use of direct quotations allows the reader to have a better understanding of the respondents' perceptions about climate change mitigation. The results do not present commonalities or differences amongst the interviewees, as the purpose of this paper is to highlight the factors enabling or constraining climate change mitigation in the cultural heritage sector, including examples of best practice for its implementation, as well as to inform decision-makers and to identify research needs. Figure 2  Examples of quotes from the interviewees in relation to each of the identified themes are shown in Tables 2, 3 and 4 to explain the selection of specific themes and the categorisation of the information provided by the interviewees. These representative quotes were carefully selected and shortened to reduce the word count without compromising their meaning. The use of direct quotations allows the reader to have a better understanding of the respondents' perceptions about climate change mitigation. The results do not present commonalities or differences amongst the interviewees, as the purpose of this paper is to highlight the factors enabling or constraining climate change mitigation in the cultural heritage sector, including examples of best practice for its implementation, as well as to inform decisionmakers and to identify research needs. Figure 2 summarises the steps involved in this qualitative methodology:

Results and Discussion
The investigation first examined the interviewees' perceptions to identify the factors that enable and those that act as barriers to climate change mitigation in the cultural built heritage sector. During the interviews, the participants addressed issues such as reducing carbon emissions and the transition to a low carbon economy, improvements in the energy performance of historical buildings, the preservation of heritage values, the sustainability of materials and methods used for refurbishing and restoring heritage assets.

Results and Discussion
The investigation first examined the interviewees' perceptions to identify the factors that enable and those that act as barriers to climate change mitigation in the cultural built heritage sector. During the interviews, the participants addressed issues such as reducing carbon emissions and the transition to a low carbon economy, improvements in the energy performance of historical buildings, the preservation of heritage values, the sustainability of materials and methods used for refurbishing and restoring heritage assets.

Enablers of Climate Change Mitigation in the Cultural Built Heritage Sector
The interviewees were positive about adapting the cultural built heritage to mitigate climate change and identified a number of factors that enable this. These factors were grouped into six themes: 'economic factors', 'legislation and regulations', 'sustainable refurbishment strategies', 'sustainable transport strategies', 'user behaviour', 'knowledge' and 'energy compensation strategies' (see Table 2). Figure 3 shows the number of interviewees mentioning each of the identified enablers. For example, 13 interviewees, i.e., the 29% of the total number of interviewees, mentioned 'economic factors' as an enabling factor to climate change mitigation in the cultural heritage sector.

Results and Discussion
The investigation first examined the interviewees' perceptions to identify the factors that enable and those that act as barriers to climate change mitigation in the cultural built heritage sector. During the interviews, the participants addressed issues such as reducing carbon emissions and the transition to a low carbon economy, improvements in the energy performance of historical buildings, the preservation of heritage values, the sustainability of materials and methods used for refurbishing and restoring heritage assets.

Enablers of Climate Change Mitigation in the Cultural Built Heritage Sector
The interviewees were positive about adapting the cultural built heritage to mitigate climate change and identified a number of factors that enable this. These factors were grouped into six themes: 'economic factors', 'legislation and regulations', 'sustainable refurbishment strategies', 'sustainable transport strategies', 'user behaviour', 'knowledge' and 'energy compensation strategies' (see Table 2). Figure 3 shows the number of interviewees mentioning each of the identified enablers. For example, 13 interviewees, i.e. the 29% of the total number of interviewees, mentioned 'economic factors' as an enabling factor to climate change mitigation in the cultural heritage sector.

Economic Factors
The interviewees perceived as important the economic factors that enable the implementation of strategies to mitigate climate change. Government incentives was the most mentioned measure by the interviewees. Such incentives are used to encourage people to make interventions compatible with the heritage assets instead of choosing market-forced solutions that are often not compatible with the heritage values. The interviewees found that the wider (non-expert) public can be reluctant to restore traditional windows due to the high costs involved and the belief that the restored windows will be less energy efficient than alternative new double-glazed windows. However, as mentioned above, the results of previous research concluded that window replacement is not necessarily a better energy saving measure. Considering the energy used in the life cycle of the windows, restoring, instead of replacing the windows, can be a better choice [34]; in addition, the energy cost related to the transport of the new elements is avoided. The interviewees also emphasised that restoration efforts compatible with the heritage building should be encouraged with the help of incentives so that people are willing to pay more for them. This is particularly the case when the money invested in the refurbishment efforts can be recovered in the long-term from the energy saved subsequent to the refurbishment efforts. The interviewees suggested restoring and maintaining the original historical materials as an opportunity for improving local economic sustainability against the importation of foreign materials and for maintaining as well as increasing traditional expertise locally.

Legislation and Regulations
According to the interviewees, having more legislation, regulations and guidelines would facilitate climate change mitigation in the cultural heritage sector. New regulations were requested not only at the international, national and regional level, but also in municipalities and at cultural heritage sites. In fact, it was even suggested to include climate change mitigation in site management plans, but also to increase policy requirements for mitigation actions in public buildings. A specific request by the interviewees was the need to improve the regulatory structure behind carbon usage reports and post-performance evaluations in order to better monitor the energy consumed in refurbished buildings during the use-phase and to compare it with the energy usage that was predicted prior the refurbishment. Consistent with this information gathered from the interviewees, there is agreement in the literature that ambitious policies can significantly reduce the energy use in buildings [58], and that for successful mitigation in the cultural heritage sector, policy actions at both the regional and local levels are needed [7,59]. In addition, Yarrow [48] highlighted a potential conflict between legislation promoting preservation of cultural heritage and that requiring a reduction of energy consumption in buildings, hence the difficulty of applying general policies to historical buildings.

Sustainable Refurbishment Strategies
The interviewees suggested a number of actions aimed at improving sustainability and thus at reducing the GHGs emitted by the cultural built heritage sector. Examples of sustainable actions in relation to the refurbishment of historical buildings include the selection of environmentally friendly and natural materials, especially if originating from local sources, recycling and reusing materials, avoiding the use of harmful chemical substances and minimising the generation of building waste. However, sometimes it is difficult to understand whether one way to refurbish a historical building is more sustainable than another is. For this reason, the sustainability of the materials and refurbishment actions should be assessed using the LCA methodology prior to the implementation of the refurbishment efforts. This would provide a better understanding of the energy and CO 2 emissions associated with a specific intervention. Such an approach, however, should also consider the embodied energy in the historical materials that are proposed to be replaced. The latter is consistent with Webb [43] who mentioned the lack of consideration of the embodied energy of historical buildings during retrofitting. Hence, further work is required on the evaluation of the sustainability of the materials and methods used during refurbishment in the built heritage sector, as the LCA approach has been applied mostly on new construction rather than on historical buildings [48], a gap in knowledge which Bertolin and Loli [37] also had previously highlighted. In addition, when applying the LCA approach to the heritage sector, the historical and cultural values need to be considered and added as a parameter in the evaluation of the solutions to be adopted. These values are often neglected and priority is often given to new technologies and to materials targeted at energy reduction [37], without considering the sustainability of the cultural built heritage sector in a holistic way as mentioned by an interview in the following quote: "We need to evaluate (...) sustainability through 360 degrees." (Table 1).

Sustainable Transportation Strategies
The interviewees also emphasised the need to improve environmental sustainability at cultural heritage sites by reducing CO 2 emissions through the promotion of sustainable transport to and from the sites as well as within the sites. This can be accomplished, for example, by promoting cycling and public transportation, as well as by closing historic town and city centres to domestic vehicles. The latter can also contribute to decreasing air pollution and thus improving air quality, as well as reducing the associated decay on monuments and the facades of historical buildings [60].

Change in User Behaviour
User behaviour has a significant impact on the effectiveness of mitigation measures adopted on historical buildings. The interviewees recommended to encourage users, e.g., occupants and visitors, to behave in a more sustainable way and mentioned the need to raise awareness through engagement to ensure that mitigation is effective. Users should be educated in energy and water saving measures, reusing and recycling materials, and decreasing their waste and emissions. This can start through simple things such as wearing a jumper inside the building in the winter and thus reducing the use of heating, instead of wearing only a t-shirt, for instance. Berg et al. [38] emphasised the importance and potential of user-driven energy efficiency measures in historical buildings and that user behaviour should always be taken into account, as the occupants play a central role in the day-to-day management of the building. The Intergovernmental Panel on Climate Change (IPCC) [58] further highlighted that in developed countries, behavioural change could decrease energy use by up to 20% in the short-term and by up to 50% by the middle of the 21 st century.

Knowledge
Some interviewees suggested more knowledge through research and dissemination as a factor enabling mitigation in the cultural heritage sector. This is consistent with the literature, which emphasises the role of new knowledge and skills to develop new sustainable strategies for heritage conservation to face climate change uncertainty [51].

Energy Compensation Strategies
Some interviewees suggested the application of energy compensation schemes. This is where parts of a system that are inefficient in their use of energy can benefit from the energy surplus produced by the efficient parts. In relation to cultural heritage, this can be done at different scales: city district and building. In the first case, different buildings in the same district can share energy. For instance, the energy needed for historical buildings can be supplied by the surplus energy produced by new energy efficient buildings or from off-site renewable sources of energy. GHG emission reductions through such a trading allowance is a cost-saving measure that was proposed by Cassar [51]. In the second case, one interviewee mentioned the possibility to meet the energy requirements for parts of a historical building where no refurbishment is allowed with the energy saved by improving the thermal performance of other parts of the buildings where modifications are permitted.

Barriers to Climate Change Mitigation In The Cultural Built Heritage Sector
In addition to the factors found to enable climate change mitigation in the cultural built heritage sector, the interviewees also identified a number of barriers. These barriers were grouped into nine themes: 'economic factors', 'lack of regulation', 'value', 'material procurement and sustainability certification', 'user behaviour', 'loss of traditional skills', 'lack of knowledge', 'incompatible solutions' and 'diversity'. Figure 4 shows the number of interviewees mentioning each of the identified enablers as well as their percentages. Eleven interviewees, i.e., the 24% of the total number of interviewees, mentioned 'economic factors' as a barrier to climate change mitigation in cultural heritage sector. Table 3 provides examples of quotes from the interviewees in relation to each enabling factor. themes: 'economic factors', 'lack of regulation', 'value', 'material procurement and sustainability certification', 'user behaviour', 'loss of traditional skills', 'lack of knowledge', 'incompatible solutions' and 'diversity'. Figure 4 shows the number of interviewees mentioning each of the identified enablers as well as their percentages. Eleven interviewees, i.e. the 24% of the total number of interviewees, mentioned 'economic factors' as a barrier to climate change mitigation in cultural heritage sector. Table  3 provides examples of quotes from the interviewees in relation to each enabling factor.

Economic Factors
The interviewees perceived economic factors as an important barrier to climate change mitigation in the cultural built heritage sector as restoring historical buildings is expensive. Thus, financial resources can help with conservation efforts when they are limited or not available. The lack of money to restore cultural heritage can also have negative consequences on the inherent historical, cultural and social values embedded in heritage sites. For example, an interviewee stated: "There is an economic aspect that prevails over the social, historical and cultural value." (

Economic Factors
The interviewees perceived economic factors as an important barrier to climate change mitigation in the cultural built heritage sector as restoring historical buildings is expensive. Thus, financial resources can help with conservation efforts when they are limited or not available. The lack of money to restore cultural heritage can also have negative consequences on the inherent historical, cultural and social values embedded in heritage sites. For example, an interviewee stated: "There is an economic aspect that prevails over the social, historical and cultural value." ( Table 2). The adoption of cheaper solutions and in particular the loss of original material can influence the historical and cultural values of the site, as well as weaken the identity of a place. The economics of conservation is therefore an important criterion to consider when retrofitting historical buildings [43].

Lack of Regulation
A significant barrier perceived by the interviewees is the lack of regulation when adapting cultural heritage to mitigate climate change. The interviewees highlighted that stronger regulations are required to force the owners of heritage assets to adapt properly. An interviewee mentioned the example of a UNESCO WHS, where detailed guidelines were carefully designed and provided to the local community, but people did not follow them due to lack of enforcement. An interviewee from another WHS mentioned misinterpretation of the regulations as an issue, where the components of historical buildings were replaced with similar substitutes, albeit made of different materials characteristics, varying in shape and colour from the original components. Such substitution can decrease the authenticity of a heritage site. There are parallel examples between the information provided by the interviewees and case study examples reported in the literature. Yarrow [48], for instance, reported the concern of a conservation officer regarding a retrofitting where the building owners substituted the historical components for new ones claiming that they 'looked the same'. Devolution of the regulations to the local level was also mentioned as a problem by one interviewee, because, in some cases, a lack of financial and human resources put cultural heritage preservation in second place. The lack of protocols and guidelines to help heritage managers adapt cultural heritage assets to climate change was also highlighted by Sabbioni et al. [8].

Value
Some interviewees perceived preserving the values of cultural heritage as a barrier to climate change mitigation, because it limits the refurbishment options to make historical buildings more energy efficient. This is because the measures adopted when retrofitting heritage assets can affect the heritage value, notably when historical elements are removed and replaced. Preserving the values of cultural heritage on the one hand and the implementation of measures to reduce the carbon footprint of heritage assets on the other hand should always be carefully balanced in any refurbishment project and, in this respect, buildings of diverse ages and listing category should be considered differently. The interviewees highlighted that preserving heritage value, authenticity and integrity should be prioritized when considering adaptation measures and strategies, as once the historical materials are removed and destroyed they will be lost forever. For this reason, the interviewees suggested alternatives to refurbishing buildings of high heritage value, such as integrating them as part of district energy networks, as described in Section 4.1.6, with the curbed CO 2 emissions from retrofitted buildings used to offset the higher energy used in historical buildings.
One interviewee further added that it would be beneficial to have examples of public building refurbishment that preserve the value, integrity and authenticity of the heritage as models for others to follow: "Long-term adaptation planning has to be done first in public buildings to be an example for the private." (Table 3). Another interviewee representing a government organisation recommended selecting adaptation measures that would not conflict with the heritage value: "Where we had (...) a potential for losing the values, we just do not go there. We would rather do something else." An example of the latter is restoring historical wooden windows by inserting a secondary glazing with magnets, which can be removed during the warmer season, rather than replacing the windows with new ones. When refurbishing historical buildings, evidence is needed to justify retrofitting changes that influence the value, integrity and authenticity of the building. If restoration impacts on the value of those building, their historical meaning could be lost [51]. Historic Environment Scotland (HES), a governmental institution in charge of preserving cultural heritage in Scotland advises on the unrealistic expectation that historical buildings can reach energy efficiency levels similar to new constructions but also advises for adaptive solutions that preserve the value, identity and significance of those assets to the community [61]. Furthermore, it should be emphasized that if the refurbishment of historical buildings can be seen as a threat to the heritage values, retrofitting can also constitute an opportunity to protect buildings, for example to heritage buildings in disuse or to those with minimal maintenance carried out [43].

Material Procurement and Sustainability Certification
The interviewees identified material procurement used during refurbishment of cultural heritage buildings as a barrier to climate change mitigation, as well as weaknesses in building sustainability certifications. With globalization, building materials are increasingly imported from other countries and, during the production process, they often travel from various locations around the world before reaching the construction site. The more a material travels, the more GHG emissions are derived from its transportation. However, the interviewees highlighted that in some cases, energy calculations and sustainability certifications do not consider the energy associated with transportation out of the country where the building is located. This can put foreign building materials at an advantage with regard to costs and certification, and this can influence the lack of locally sourced material during the refurbishment of historical buildings. The interviewees emphasized that the use of foreign materials not only increases GHG emissions, but may also not have the same characteristics as the original materials. There is also a lack of consideration of the energy embodied in the historical materials in energy evaluations. In fact, it was stated that historical buildings are not considered in performance qualification and building standards and that energy evaluation software are not accurate in calculating the energy used by historical buildings.
The interviewees also reported a lack of consideration of the physical behaviour of heritage buildings and of the vapour permeability of historical material and an inaccuracy of Energy Performance Certificates (EPC) that make assumptions about energy use but do not reflect the actual performance of historical buildings. Incomplete Environmental Product Declarations (EPD) can also contribute to this issue [62], and, consistent with this, Cassar [51] stressed that some sustainability certification tools do not use a holistic approach; they are based on relative energy targets and they exclude social and economic sustainability aspects. In addition, Webb [43] emphasised the lack of consideration of embodied energy during the retrofitting of historical buildings, stressing the need for improved simulation tools and software to reduce the gap between simulations data and real building performance.

User Behaviour
User behaviour is considered a barrier to climate change mitigation because certain attitudes can minimise or even nullify the refurbishment efforts aimed at decreasing the use of energy by the building. The interviewees identified resistance to change and to the adoption of sustainable behaviours, for example, leaving the lights on during the day when there is high luminosity from the window, wearing light clothes in the winter and turning the thermostat up to a level beyond comfort level, to name a few. An interviewee also mentioned the rebound effect, which is a decrease in the expected energy savings from a specific building alteration because of behavioural change leading to an increase in energy consumption. For instance, in buildings were measures were introduced to use less electricity by installing heat pumps, people further increased their energy consumption by setting room temperature to a higher level on their thermostat. Sabbioni et al. [8] even proposed a change on a regulatory level to induce a modification in user behaviour.

Lack of Knowledge
The interviewees identified a lack in knowledge as a barrier to mitigate climate change in the built heritage sector. More research testing the use of modern materials during the refurbishment of historical buildings and on the traditional behaviours and characteristics of historical building is needed. Some interviewees also identified the need for more knowledge transfer to users on the topic of climate change mitigation in the field of cultural heritage in a way to make them more aware and capable of making choices compatible with the traditional behaviours and values of historical buildings.

Loss of Traditional Skills
Another significant barrier identified by the interviewees is the progressive loss of traditional skills and techniques. Given the common incompatibility of modern solutions with historical buildings and materials, this loss can threaten heritage preservation. The need to pass on knowledge and skills to the next generation of heritage professionals, and the importance of teaching traditional skills and techniques, and to understand how they relate to modern construction materials is emphasised in the literature, as well as methods on how to develop suitable conservation techniques appropriate with the nature of older buildings [51,63].

Incompatible Solutions
When refurbishing historical buildings, the adoption of solutions that are not compatible with the behaviour of historical materials can accelerate their degradation. For instance, the interstitial condensation between layers made of different materials. The traditional behaviour of the historical buildings, their thermal-balance, the natural ventilation of their indoor environments, and the ability of some historical materials to allow vapour movement are characteristics that must be taken in consideration during a refurbishment project. More research is needed on the evaluation of market-based solutions, which are usually designed for new structures but that are also applied to historical buildings. Historical buildings have more sophisticated bioclimatic properties than modern constructions, and new materials that are incompatible with the physical behaviour of the historical ones can lead to an increase in decay. Retrofitting measures can alter the moisture balance of historical buildings, affecting, for example, their breathability, leading to an increase in the deterioration of historic materials [43,48]. Those buildings are designed to have controlled thermal comfort based on the natural unconditioned climate, environmental and site conditions rather than using machine-driven systems. The development of adaptation strategies should therefore consider the balance between decay reduction and energy consumption improvement of historical buildings [64,65].

Diversity
Some interviewees mentioned that historical buildings are too diversified for adaptation solutions to be generalised and that adaptation has to be considered on a case-by-case basis. The question remains as to whether it is possible to generalize adaptation options to decrease the energy consumption of historical buildings or if each individual cultural heritage asset is too specific. More research is needed on specific classifications, or typologies, of heritage buildings to understand which possible adaptation solution can be proposed by comparing adaptation measures used in similar situations.

Best Practice Examples
The last part of the investigation focused on the identification of best-practice examples in mitigating climate change, as expressed by the interviewees. Examples of best practices include refurbishment measures to increase the energy efficiency of historical buildings in relation to upgrading the insulation, ventilation, lighting and heating, as well as the monitoring of climatic conditions ( Table 4). The majority of these measures are consistent with the solutions proposed in the literature and raised in Section 2 of this paper.

Insulation
The interviewees suggested insulating floors, ceilings, roofs and walls, and to reduce the loss of heat through windows by adding secondary glazing and using curtains, shutters and blinds. Examples of insulation systems used in literature are shown in Section 2 of the current paper e.g., [14][15][16].

Ventilation
The interviewees emphasised the need for ventilation in historical buildings to avoid condensation, which can be done by using mixed mode or hybrid ventilation systems, with mechanical devices improving natural ventilation of buildings in a controlled way, or by passive ventilation using traditional systems such as wind chimneys and louvers. Consistent with this, the literature stresses the importance of systems to control the relative humidity and the ventilation of the indoor environment of cultural heritage buildings to decrease energy consumption while also reducing the risk of degradation [17], and on the potential of chimneys for providing ventilation in traditional buildings [53].

Lighting, Heating and Monitoring
The interviewees also suggested improving lighting and heating systems while monitoring climatic conditions throughout the year. The literature presented in Section 2 backs these recommendations.

Conclusions
Mitigating climate change by reducing GHG emissions is now urgent to prevent future-and reduce current-danger to cultural heritage resources. This needs to be accomplished, in part, by reducing the energy used by cultural built heritage assets, notably through improvements in their energy efficiency. Organizations such as UNESCO, ICOMOS, ICCROM, IPCC and the European Commission promote actions to cut GHG emissions in the cultural built heritage sector. This paper analysed the perceptions of cultural heritage experts on the issue of adapting the cultural built heritage to mitigate climate change. Specifically, it reports on the factors enabling and those acting as barriers to mitigate climate change, as perceived by the managers of heritage sites and experts working on the preservation of cultural heritage in universities, research centres and governmental institutions.
A common view amongst the interviewees was that climate change mitigation in the heritage sector is necessary but challenging. Most research accomplished to date investigated measures to mitigate climate change by reducing the energy consumption of cultural heritage buildings, but there is limited research on the identification of the challenges to overcome in this regard. This paper provides a better understanding of what needs to be provided and prioritized for the mitigation of climate change in the cultural built heritage sector to take place. In summary, this study identified the following barriers constraining climate change mitigation in the field of cultural heritage: economic factors, lack of regulation, heritage values as it can limit the type and scope of refurbishment, inadequacies in material procurement and sustainability certifications, inefficient use of energy due to building occupants' behaviour, lack of knowledge, loss of traditional skills, the adoption of solutions incompatible with the assets and hence causing further damage, diversity of heritage resources and hence the difficulty to identify solutions that are fit for all. The factors enabling climate change mitigation that could help to overcome some of those identified barriers include economic resources and incentives, legislation and regulations, sustainable refurbishment strategies, sustainable transportation strategies, change in user behaviour, knowledge, and energy compensation strategies. Figure 5 provides a graphical summary of the factors enabling and constraining climate change mitigation in the field of cultural heritage.
This research emphasised that mitigating climate change in the cultural built heritage sector is a complex issue and requires a holistic approach for the identification of sustainable strategies to reduce GHG emissions. In this regard, ambiguities still remain on the consideration of heritage values and the energy embodied in historical materials, as well as the potential of traditional passive measures when adapting heritage buildings to mitigate climate change. This study improves our qualitative understanding of the key themes raised by stakeholders and can be used as a basis for further research seeking to quantify the effectiveness of best practice in climate change mitigation in the cultural heritage sector. Specifically, further research should be developed on adaptation strategies that consider the balance between reduction of decay and improving the energy consumption of historical buildings [64]. More research should also be conducted on the use of traditional solutions for improving energy efficiency, and on the compatibility (or lack) of new technologies and materials with the historical ones. Historical buildings work differently from modern buildings. Further research is required on the re-use of traditional passive measures such as wind chimneys, greenhouses, passive ventilation and heating, to name a few, and thus avoiding carbon intensive energy consumption, as well as on the promotion of sustainable natural resources, e.g., wind and sunlight. This could be done by understanding the level of energy efficiency achievable by different types of heritage (e.g. ancient castles and monuments versus non-ancient historical assets) in relation to their values, integrity and authenticity.
Monitoring before and after retrofitting and/or refurbishment works should be encouraged to estimate properly the energy consumption and the energy payback period from the use of renewable sources post intervention. This includes the need to rectify the current energy certification process, which does not consider the energy used to transport materials when historical materials are replaced by new ones in their calculations; the need for LCA evaluations before refurbishing historical buildings to understand the impact of the proposed solutions on the environment; and the need to promote the use of natural materials to avoid toxic chemicals.
A decrease in energy consumption also needs to be promoted through changes in user behaviour (e.g. wearing appropriate clothes and heating less, reducing the heating of rooms when not used, use more natural illumination and less electricity), which can be implemented with the use of information and communication technology (ICT). More knowledge needs to be disseminated and collaboration between different countries and at different levels should be encouraged, from the governmental to the end user. Including increasing awareness and cooperation between heritage organizations, governmental institutions, research centres and academia. This research emphasised that mitigating climate change in the cultural built heritage sector is a complex issue and requires a holistic approach for the identification of sustainable strategies to reduce GHG emissions. In this regard, ambiguities still remain on the consideration of heritage values and the energy embodied in historical materials, as well as the potential of traditional passive measures when adapting heritage buildings to mitigate climate change. This study improves our qualitative understanding of the key themes raised by stakeholders and can be used as a basis for further research seeking to quantify the effectiveness of best practice in climate change mitigation in the cultural heritage