BUILDING  ENERGY PERFORMANCE ISSUES & THERMAL COMFORT REQUIREMENTS

It is believed that enhancing thermal comfort and energy performance of historic buildings contribute positively in enhancing the global climate.
Prior the presence of the Heating, Ventilation and Air Conditioning systems (HVAC), people designed their buildings to be heated and cooled naturally. These methods are called passive design systems that include mainly building orientation, building envelope, natural ventilation and daylight. These systems have been used for thousands of years \citep{Chen_2017}.
In the beginning of twentieth century, mechanical cooling systems were invented. So, people started to move away from the passive design using HVAC as an active system. This shift caused rapidly increasing in energy demand and consumption  in building sector. In addition, designers ignored climate condition factors because HVAC system offered comfortable indoor environment regardless building location \citep{fathy1986natural}. Since that time, the building energy efficiency was negatively affected.
After the 1970 energy crisis clearly  appeared. So, building codes began to rise through the Energy Policy Act (EPA) to  improve building efficiency. These codes discussed energy efficiency in  buildings and in mechanical equipment.
In the last few years, the design approaches tended to combine passive and active design systems together to achieve maximum energy  efficiency in buildings. Also, the concept of  psychometrics to design HVAC system became common because it considers the factors  of local climate, human occupancy, comfort, and varying temperature and  humidity controls.
Moreover, energy efficiency and thermal comfort in existing buildings specially the historic ones became high-interest topics among scholars. Many researches confirmed that retrofitted buildings significantly contribute in reducing energy demand in building sector, since it is essential for maintaining built heritage of historic structures \citep{martinez2016energy}. In order to estimate energy efficiency, energy performance and thermal comfort calculation are essential.
At EU level, there are two main directives that are responsible for reducing the energy consumption in the building sector. They are (DIRECTIVE 2010/31/EU) & (DIRECTIVE 2002/91/EC) \citep{Mazzarella_2015}.  They defined the Energy performance of a  building, and Building Envelope as following:
- Energy performance of a building It means the calculated or measured amount of energy needed to meet the energy demand associated with a typical use of the building, which includes, inter alia, energy used for heating, cooling, ventilation, hot water and lighting”- (By DIRECTIVE 2010/31/EU).
- Building Envelope “It means the integrated elements of a building which separate its  interior from the outdoor environment”- (By DIRECTIVE 2010/31/EU).
Different simulation tools can be used to calculate or measure the building performance such as Ecotect, Computational Fluid Dynamics (CFD), Radiance, and Energy Plus \citep{Xie_2017}.
Thermal  Comfort: It is defined as “that condition of  mind that expresses satisfaction with the thermal environment” (ASHRAE, 2004). Maintaining this standard of thermal comfort for occupants of buildings or  other enclosures is one of the important goals of HVAC (heating, ventilation,  and air conditioning) design engineers.
There are two models to assess the thermal  comfort in a building, as following:
- The  Predicted Mean Vote (PMV) model predicts the thermal comfort temperature in a  controlled environment, so, it is suitable to a building in a cold climate.  Using PMV model requires six parameters: air temperature, mean radiant  temperature, relative humidity, air movement, clothing insulation, and  metabolic rate \citep{Djamila_2017}. This approach called analytical  approach because it relies on monitoring. In this approach, people are put in a  temperature-controlled environment and their responses are monitored. That is  why this method appropriates highly in controlled environments and the results  are used to develop a model that can be used to predict optimum comfort.
- The adaptive model was developed based on hundreds of field studies with the idea that occupants dynamically interact with their environment. Occupants control their thermal environment by means of clothing, operable windows, fans, personal heaters, and sun shades \citep{Djamila_2017}. This approach called behavioural  approach because people are monitored in their normal environments and their  responses are related to the conditions they experience. The results are analysed  statistically to develop an understanding of the interaction between people and  buildings. It is more suitable to a building in a hot climate.  
There are other factors in the both models  can affect the thermal comfort level in a building such as cultural, religious,  social, and economic factors. 
Thermal comfort factor as well as energy performance require carefully attention during designing or retrofitting the buildings \citep{Djamila_2017}. They are strongly connecting and they are  related to climate change. For instance, the impact of climate change, which  means higher temperature, on a building is: First, the higher temperature will  affect the indoor temperature and it will cause discomfort thermal conditions  in the building. On the other hand, cooling energy requirements will increase  due to temperature rising.

DISCUSSION FOR THE RELATIONSHIP BETWEEN CONSERVATION AND THERMAL COMFORT ISSUES IN  HISTORIC BUILDINGS

The previous overview introduced the  concept of conservation and thermal comfort. Hence, it is supposed that dealing  with thermal comfort in historic building is affected by several variables,  as per the following table in figure (10):