###### Abstract
An experimental study found that heat losses in the winter depend entirely on the physical condition of the constructive elements and thus intensive use of heating system is needed to maintain indoor thermal comfort even in the mild winters of Southern Spain [13]. Therefore, constructive improvements will still be needed in the future since deficient envelopes have great impacts on indoor comfort than climate severity. Due to climate change, studies in different countries observed a decreasing trend in heating load in the winter, and the dilemma of an increasing cooling load or uncomfortable conditions in the summer [14]. Although climate change reduces the heating loads in historic buildings, active systems are mostly inevitable to ensure thermal comfort of occupants and the preservation of historic buildings, increasing the total energy use [15]. The relative increases in energy demand will require an increase in energy generation capacity of traditional energy systems which has small flexibility [16]. This points to an urgent need for proper energy retrofit strategies set for historic buildings in the future. Energy retrofit is an effective decarbonization strategy to mitigate global warming, since the construction sector contributes with 39% of energy and process-related anthropogenic GHG (greenhouse gas) emissions [17]. Historic buildings, as a considerable part of the building stock, have a significant energy-saving potential. It is estimated that one-third of the EU building stock is over 50 years old, and 75% of them are energy-inefficient [18]. Adapting historic buildings to modern standards of energy use and users' comfort could be achieved while respecting and safeguarding the heritage significance of these buildings. For instance, internal insulation is broadly used in energy retrofit to keep the original appearance of historic buildings. IEA-SHC Task 59/Annex 76 [19], a collaborative research project, identified and assessed 16 replicable internal insulation solutions, to overcome the barrier between conservation and energy efficiency. [PERSON] et al. [20] reviewed 71 applications of internal insulation and summarized thermal insulating plasters and mineral wool are the most common solutions. However, the addition of internal insulation may minimize the positive effect of passive cooling system of historic buildings. Because of the intrinsic characteristics of historic envelopes, the thermal lag between indoor and outdoor temperatures could reach up to one month [21]. The use of internal insulation could reduce the benefits of thermal inertia to a large extent, potentially increasing thermal discomfort in the summer [22; 23; 24], and with that aggravating the risk of overheating. With the future temperature rising due to climate change, the risk of overheating in retrofitted historic buildings without any mechanical cooling system is likely to increase. Thus, the consequences, in terms of thermal comfort and the energy conservation requirements, of energy retrofitting of historic buildings in a changing climate are relevant research gap that this study attempts to reduce. Beyond the impact of climate change on renovated buildings, this study also explores adaptive solutions to prevent overheating in historic buildings. Suitable shading and natural ventilation could deliver a thermally comfortable environment or decrease mechanical system operation even in hot climate [25; 26]. Whether common adaptive measures are adequate in a retrofitted historic building that also suffer the sever future climate conditions should be investigated. Considering potential climate challenges, mitigation and adaptation measures are important in ensuring the future-proofing of historic buildings and to minimize both their vulnerability and the discomfort for occupants.