Assessment of CO2 Emissions Reduction Based on Different Insulation Materials in Residential Buildings: Example from Turkey

In this study, the reduction of CO2 emissions caused by energy savings due to thermal insulation materials applied to the exterior of buildings was investigated. The study includes the evaluation of four different insulation materials including XPS, EPS, PUR and Rock Wool in terms of the reduction potential of CO2 emissions for all city centers in Turkey. Moreover, the optimum insulation thicknesses of the buildings that have both heating and cooling needs were determined for these insulation materials by means of Life Cycle Cost Analysis. The annual energy needs per unit area of the external walls of the buildings, which are insulated according to their optimum insulation thickness were determined and correspondingly the amount of fuel needed to meet this energy was calculated. In the study, CO2 emission values released from buildings to atmosphere were compared in terms of fuel amounts obtained from total energy demands during heating supplied from natural gas and cooling supplied from electricity of the residential buildings insulated with diverse insulation materials. Consequently, when CO2 reduction potentials of the buildings insulated with materials in the optimum insulation thickness are compared with those of the uninsulated buildings, insulation material providing highest potential in CO2 emission mitigation was to be found as Rock wool.


Introduction
Today, climate change resulted from greenhouse gases (GHGs), such as carbon dioxide (CO 2 ), methane (CH 4 ), chlorofluorocarbons (CFCs) and nitrous oxide (N 2 O) released into the atmosphere owing to consumption of fossil fuels is one of the most severe problems threatening to our world. When the impacts of the greenhouse gas emissions are compared, it is seen that the most contributor gas to climate change is carbon dioxide [1]. The increase in GHGs because of energy consumption based on the growth of population and improvements in industrial sectors is expected to reach by approximately 30 % by 2040 [2] and this increase will significantly trigger to increase in the atmospheric temperature giving rise to many problems, like increasing sea level rise [3]. According to the data of 2016, it has been stated that approximately 40% of the total energy consumption among the global energy consumption sectors was stemmed from the residential buildings. The carbon dioxide emissions based on the energy consumption of residential buildings consists of approximately 35% of those emitted globally [4]. Most of the energy spent for heating and cooling needs of buildings in case of that they are not insulated or well insulated is disappeared and this situation triggers to some issues, IOP Conf. Series: Earth and Environmental Science 801 (2021)  such as environmental problems [5].For this reason, insulation materials with high thermal qualities to be applied to buildings will be effective in the reduction of the energy consumption of buildings especially resulting from their air conditioning requirements and correspondingly they will contribute to decrease CO 2 emissions produced during energy consumption [6] Furthermore, 35% of Turkey's total energy is used by buildings, and 65% of this energy is used in the air conditioning of buildings [7]. Moreover, approximately 60-80% of the thermal loads that are lost in buildings is resulted from heat transfer through conduction and convection on the outer walls of the buildings [8]. Referring to these information, all city centers in Turkey were considered and the relationship between external thermal insulation on buildings and CO 2 emissions caused by the buildings was investigated in the present work. As a result, the potential reduction of CO 2 emissions through the thermal insulation of the residential buildings in Turkey was determined.

Materials and method
Natural gas has started to be used to meet the energy needs required for heating in almost all buildings in all city centers of Turkey since year of 2019. That's why, heating energy was supplied from natural gas and also this study was benefit from electrical energy for cooling needs in the residential buildings and so mitigation of CO 2 emissions based on the usage of thermal insulation materials was evaluated under these assumptions. Furthermore, another assumption was that the thermal insulation was only applied to the outer walls of the buildings and also from the outside of these walls. Thickness of the thermal insulation was determined by means of Life Cycle Cost Analysis (LCCA) considering both the heating and cooling needs of the buildings. The energy demands of the buildings were determined by regarding the insulated and uninsulated walls. Consequently, the CO 2 emission values, resulted from fuel consumption due to the total energy demands based on heating and cooling needs of residential buildings, to be released into the atmosphere were determined. Equations and parameters required for calculations are presented in the following sections.

Properties of external walls and insulation materials and required parameters
The schematic representation of the external wall and the insulated wall taken as reference in the present study is presented in Figure 1, and the necessary parameters for the calculations are presented in Table 1. The thermal insulation materials examined can be listed as extruded polystyrene foam (XPS), expanded polystyrene foam (EPS), polyurethane rigid foam (PUR) and rock wool (RW), which are mostly preferred as thermal insulation materials in the construction sector in Turkey.
In Equation 1, expresses the annual heat loss (W/m 2 ). Considering heating, DD value will be equal to HDD value and also it will be taken as CDD for cooling conditions. The U value in the equation indicates the total heat transfer coefficient, and its value can be calculated from Equation 2.
In Equation 2, R i and R o values are the heat transfer resistance of area surrounding the wall internally and externally, and R w is the heat transfer resistance of the uninsulated wall (m 2 K/W). Moreover, x is the insulation thickness (m) and k is the thermal conductivity coefficient (W/mK) of the insulation material. The annual heating and cooling energy demand for the unit area of the outer walls can be calculated using Equations 3 and 4 [12]. In addition, the formula showing the amount of fuel to meet this energy need was presented in Equation 5, in which Hu expresses the low heat value of fuel (see Table 1).
Life Cycle Cost Analysis (LCCA) was used to determine the optimum insulation thickness for the residential buildings. This analysis method is based on the energy-cost analysis of the investment over an estimated period. Accordingly, total heating costs over a period of N years are evaluated using the In the Equations 6 and 7, i is the interest rate, r is real interest rate, g is the inflation rate and N is the life time and they can be accepted as 12.5%, 0.059, 11.75% and 10 years, respectively. Correspondingly, PWF value was found as 9.640 thanks to Equations 6 and 7 and financial values used in the calculations.
As a result, the expression giving the optimum insulation thickness that will minimize the total air conditioning cost for heating and cooling purposes is given in Equation 8 [13].
In Equation 8, C fuel , c e and c y represent the unit prices of natural gas ($/m 3 ), electrical energy ($/kWh) and the insulation material ($/m 3 ), respectively.

Calculation of emission values
When the composition of natural gas including 85% methane (CH 4 ), 7% ethane (C 2 H 6 ), 3% propane (C 3 H 8 ) and 2% butane (C 4 H 10 ) is considered, the formula of natural gas used to meet heating needs can be expressed as in Equation 9 [13].
When it is assumed that natural gas burns with 90% efficiency, 2.05 kg of CO 2 is approximately released into the atmosphere as a result of the burning of 1 m 3 of natural gas according to the chemical formula given in Equation 9 (1 kmol of natural gas was taken as approximately 22.4 m 3 ). Data belonging to energy sources used in Turkey for generation of electricity in 2019 were used to determine the amount of CO 2 emissions resulted from the consumption of electrical energy used for cooling needs of the buildings. The distribution of electrical power generation and CO 2 emission amounts based on sources were investigated and the values are presented in Table 2 [14]. In accordance with Table 2, the amount of CO 2 emissions released to the environment from production of 1kWh of electrical energy was found as 0.461 kg.

Findings and discussion
The optimum insulation thickness of the outer walls was determined by means of the formula given in Equation 8. Comparison of the optimum thickness values of the insulation materials was presented in Figure 2. Correspondingly, when the optimum insulation thicknesses of the insulation materials that will minimize the total air conditioning cost are compared, it is seen that the thinnest insulation material is PUR while the thickest insulation material is RW. Moreover, it can be easily seen from Figure 2 that the insulation thickness increases with respect to the increase in DD values.
The amount of energy that will meet the heating in winter and cooling in summer was calculated with the help of Equation 3 and 4 as shown in Figure 3. The annual total energy amount per the unit area in case of usage of four different insulation materials applied to uninsulated external walls is exhibited in Figure 3. In accordance with Figure 3, it can be easily expressed that the buildings insulated with PUR material showed the most energy demand, while the energy saving of the buildings insulated with RW material demonstrated the best results. The amount of fuel needed for the buildings, which energy needs per unit area were determined, were calculated with the help of Equation 5. The amount of fuel to be met the energy needed according to the insulation materials shows a similar trend with the amount of energy presented in Figure 3. The    Table 3. Comparisons of CO 2 emission values based on the insulation materials and DD values are also demonstrated in Figure 4. In accordance with the variation in distribution of CO 2 emissions released from buildings during heating and cooling based on HDD+CDD values, insulation materials have a remarkable impact on the reduction of the emission amounts (see Figure 4). The relative order of potential reduction of CO 2 emissions for different insulation materials was RW > EPS > XPS > PUR (see Figure 4). Another major result obtained from Figure 4, CO 2 emission values of the buildings in the cities with DD numbers bigger than 3000 0 C-day showed a very fast increase trend than other cities with lower DD values.

Conclusion
In this study, firstly the optimum insulation thickness for thermal insulation to be applied to the outer walls of buildings in 81 city centers of Turkey was investigated by means of the life cycle cost analysis method for four different insulation materials commonly used in Turkey. Secondly, the amount of fuel based on energy savings stemming from the application of optimum insulation thickness was calculated. Thirdly, CO 2 emissions values of residential buildings were calculated from  8 energy sources including natural gas and electricity preferred for heating and cooling of the buildings, respectively. Therefore, results obtained from the present study can be summarized as follow: 1-It was observed that the insulation material thickness increased with respect to the increase in the number of Degree-Days. 2-The thinnest insulation material is found as PUR and as for the thickest insulation material, it is RW. 3-It has been determined that the buildings requiring the most energy based on their air conditioning needs are insulated with PUR material, while the buildings with the least energy need are those that are insulated with RW. 4-It was found that CO 2 emissions released from buildings showed an increase trend with increasing Degree-Day values. 5-When the CO 2 reduction potentials of the insulation materials are compared, they can be found as 78.85%, 73.65%, 66.66% and 58.89% for RW, EPS, XPS and PUR insulation materials, respectively. 6-When the annual heating and cooling needs of buildings in all cities of Turkey were evaluated together, it was determined that the total amount of CO 2 released from air conditioning of the unit area of the uninsulated external walls was approximately 1924.36 kg/m 2 . However, this value decreases owing to the usage of thermal insulation materials as found in this study: it was found as 617.83, 487.68, 764.32 and 391.06 kg/m 2 in case of use of XPS, EPS, PUR and RW, respectively. 7-Based on all these findings listed above, it was determined that Rock Wool insulation material is less harmful to the environment and more economic and so correspondingly provides more energy efficient. Referring to these results, it was concluded that the use of RW as a thermal insulation material under Turkish conditions is more suitable than the other three (XPS, EPS and PUR) thermal insulation materials studied.