Simulation of roof retrofitting strategies to mitigate attic overheating issues of the terrace house under hot and humid climate.

. The purpose of this study is to investigate the potential mitigation strategies to overcome the overheating issue at the attic of the landed residential house located in a hot and humid climate area. An existing terrace house with 178 m 2 was selected for this research. A building simulation model was created and calibrated by using the previous site measurement data collected by Ng et.al. in the year 2018. The calibrated model was subsequently used to evaluate various retrofitting methods and strategies, including roof insulation, and passive radiative cooling methods. The study shows that high albedo cool roof paint is the most effective roof retrofitting strategy compared to other approaches.


Introduction
There is an urgency for us to implement energy saving in the building because of the high energy consumption and CO2 emission. Previous research shows approximately 30-40% of total global energy consumption and significant greenhouse gas emission is contributed by the building sector, with HVAC systems being the biggest contributors (Pérez-Lombard et al., 2008). In Malaysia, HVAC system is inevitable for every residential unit because of our hot and humid climate. According to the data collected by the Malaysian Meteorological Department, it shows that in Malaysia there is rarely have a stretch of no-sunshine days except during the northeast monsoon season (General Climate Malaysia, 2017). As a result, the heats penetrate through the building envelope mostly in form of solar radiation, rather than conduction & convection.

Terrace house in the hot humid climate
Malaysia is a tropical country characterized as warm and humid located within the Tropic of Cancer and Capricorn. The climatic elements are categorized as high temperatures and uniform diurnal patterns throughout the year. The annual mean temperature is 26.4°C with an average daily maximu m temperature is 34°C and an average daily minimum of 23°C (Al Tamimi & Syed Fadzil, 2011).
Terraced houses have been popular in Malaysia due to the increasing demand for housing. According to the data from the Department of Statistic Malaysia in the year 2019, 17% of the housing in KL (83,907 units) and 42% of the housing in Selangor (655,269 units) are terraced houses.
As the annual maximu m intensities of solar radiation falling on horizontal and vertical surfaces are about 1000 and 850 W/m2 respectively for east and westfacing surfaces (Ossen, 2005) in Malaysia, the large roof surface of the terrace housing is the main factor that contributes to the overheating attic issue. Most of the terrace housing in Malaysia are using dark-coloured clay roof tile which allows most of the heat to conduct down through the roofing materials. The surface temperature of the roof tile will increase drastically and the heat at the roofing materials will then radiates to the interior spaces.

Roof insulation
The thermal insulation of the membrane materials is characterized by their thermal resistance values (B. Pause, 2015). Hence it can reduce heat loss and heat gain through the building envelope. It helps to keep the indoor temperature stable as well. Mass insulation is generally used and features very low thermal conductivity, of the order of 0.05 W m-1 K-1. Therefore, heat transfer due to conduction is reduced. However, this type of thermal insulation was proven that not effectively reduce heat transfer through radiation.
There is various type of insulation products in the market, such as Rockwool, fibreglass, polyurethane foam, rigid foam, etc. It is normally applied underneath the roof structure or above the ceiling. Mineral wool includes a variety of inorganic insulation materials such as rock wool, glass wool, and slag wool. The average range of thermal conductivity for mineral wool is between 0.03 and 0.04 W/(m.K) and the typical -values of glass wool and rock wool are 0.03 0.046 W/(m.K) and 0.033 0.046 W/(m.K). Low thermal conductivity value, non-flammable, and highly resistant to moisture damage are the common properties of all these materials. (Hung Anh and ZoltánPásztory, 2021).

Passive Radiative cooling
Previous studies show that building with high roof-towall ratios, such as terrace housing, roof surfaces are the main receivers of solar heat gain, leading to 5%-10% of total building energy consumption and more than 40% of energy usage for the higher floors (Gao, 2017). Therefore, radiative roof cooling strategies are proven to be effective in hot climates with high sunlight hours.
Radiative roof cooling is a passive cooling strategy that aims to reduce the effect of heat gain on building roofs during sunny days (Akbari et al., 2006). It works as a reflector of invisible electromagnetic radiation (short-wave and long-wave) and emits heat (infrared radiation), therefore the thermal absorbance from the roof is reduced (Urban and Roth, 2010).

Research Objectives
The objectives of the research are: (a) To study the different roof retrofitting strategies to mitigate the attic heat flux issues.
(b) To compare the efficiency of the passive radiant cooling strategies such as high albedo roof paint and radiant barrier to the different insulation strategies including spray foam, rigid foam and phase change materials.
(b) To conclude the best mitigation strategies to reduce the indoor temperature of the attic space by using the building simulation method.

Methodology
In this study, the data collected by Tuck et al., (2020) from their previous research will be used as to calibrate the energy model as a benchmark (control group) for this study. The calibrated model will then be modified according to the parameters set and compared with the previous data to evaluate their performance in reducing the indoor temperature of the attic space.
The calibrated model will be used for parametric studies of various mitigation strategies. The performance of the mitigation strategies will be evaluated using the data simulated. The lower the indoor air temperature as compared to the control group (previous research data), the more effective the mitigation strategies. Referring to Table 2, the construction of the house was completed in 2004 with brick walls on the reinforced concrete frame structure. The floor slabs on both floors are reinforced concrete slabs. The first floor was covered with cement board ceiling and concrete roof tiles on the roof level. No heat insulation was installed in the roof attic and wall. Table 1 shows detailed information on orientation, floor area, ratios of wall and window area over room volume and specification on materials of the investigated house.

Investigated house and calibration of the model
The previous research is focused on the efficiency of the High-Density Poly Ethylene (HDPE) roof cover in improving the indoor thermal environment. This study shows that the roof cover can effectively reduce air temperature in the attic by approximately 1.4°C on average for a whole day and by up to 3.5°C in the daytime.
As the previous research focuses mainly only one of the mitigation methods to lower the indoor temperature, this paper is to broaden the research scope by simulating various cooling strategies using the building simulation method.
The comparison of the field measurement data and building simulation is tabulated in Table 3 and Graph 1. The temperature difference is lesser than 2% and both data sets' standard deviation is between 1.21 and 1.22. Hence the calibrated model is proven accurate and ready for building simulation.  Window to wall ratio 0.36 External wall to floor ratio 0.53

Insulation materials and Passive Radiative Cooling Strategies
Three different types of insulations are selected for the research, including rigid foam, spray foam and phase change material. Data from various insulation materials were collected and tabulated in Table 4 to compare their performance. According to Al-Homoud et al., (2005), PU Spray foam and Extruded polystyrene have the least thermal conductivity compared to other similar insulation types, hence it is selected as the insulation materials for the simulation works.
Referring to Table 8, various types of Phase Change Materials (PCM) insulation had been studied and compared based on their material properties. Although paraffin wax has the lowest thermal conductivity among other PCMs, it's not recommended to insulate residential housing due to its flammable properties. Hence, BioPCM M25 panels are selected to represent PCMs material in this study.
There are 2 materials chosen as the passive radiant cooling strategies for this study, which are aluminiu m foil (radiant barrier) and high albedo cool roof paint. The aluminium foil will be placed under the roof tiles and serve as a radiant barrier to reflect the heat from the roof tile, hence preventing heat from entering the indoor space. While the high albedo cool roof paint with high solar reflectance can reflect solar radiation and prevent it to be absorbed by the roof tiles. The detailed material properties are stated in Table 7.

Building simulation
The energy model was built using Design Builder software according to the field measurement data including the adjacent block and the existing awning position (Figure 2). The calibrated simulation model was modified from the corner terrace house to an intermediate terrace house layout to get simulated result that represent the most typical terrace house layout in Malaysia. (Figure 2a). In this study, the indoor radiative temperature of the attic and the room below attic, which is the master bedroom after roof retrofitting will be simulated and analyzed.
The heat flux of the space will be calculated based on the formula (Figure 4), in order to study the effectiveness of the roof retrofitting strategies in lowering down the heat flux of the internal space.     Table 6: Thermal Properties of selected insulation matreials Table 7: Thermal properties of the selected radiant cooling method.

Result and Discussion
The simulation result is shown in Table 9 and Graph 2 to 4.

Indoor temperature of Master Bedroom
The average day time temperature ranges from 32.2°C to 35.6°C, with the highest in A3 and the lowest in B2. The average night time temperature ranges from 30.4°C to 33.5°C, with the highest in A2 and the lowest in B2. The max temperature ranges from 34.6°C to 40.3°C, with the highest in A1 and the lowest in B3. The min temperature ranges from 28.2°C to 31.5°C, with the highest in A2 and the lowest in B2. The standard deviation ranges from 1.5 to 4.5, with the lowest in C1-C2 and the highest in Baseline. The median ranges from 31.5°C to 33.8°C, with the lowest in B2 and the highest in A2. The mean ranges from 31.3°C to 33.7°C, with the lowest in B2 and the highest in A3.

Indoor temperature of Attic
The average day time temperature ranges from 31.7°C to 33.4°C, with a mean of 31.8°C. The average night time temperature ranges from 31.7°C to 33.6°C, with a mean of 31.9°C. The maximu m temperature ranges from 32.3°C to 34.4°C, with a mean of 33.3°C. The minimum temperature ranges from 29.4°C to 32.5°C, with a mean of 31.3°C. The standard deviation ranges from 0.9°C to 1.4°C, with a mean of 1.0°C. The median temperature ranges from 31.9°C to 33.5°C, with a mean of 33.0°C. The mean temperature ranges from 31.8°C to 33.5°C, with a mean of 32.9°C.

Heat flux of Master bedroom
The average daytime heat flux values range from 3.1 W/m2 to 5.6 W/m2 with a mean of 3.7 W/m2 and median of 3.9 W/m2. The average nighttime heat flu x values range from -3.3 W/m2 to 2.5 W/m2 with a mean of -1.2 W/m2 and median of -

Conclusion
A numerical simulation was conducted to find out the suitable roof retrofitting method to reduce the indoor temperature of the terrace house attic space. The data was extracted from the energy model with various combinations of retrofitting strategies. The significant conclusion is as below: (a) Both roof insulation and passive radiant cooling strategies can reduce the operative temperature as well as reduce the heat flux effects of the attic space.
(b) High albedo cool roof paint (B2) is the most effective roof retrofitting strategy compared to other approaches. Hence, it shows that by reflecting the sunlight from the building envelope, especially the rooftop surface, the indoor temperature drops significantly.
(c) By combining high albedo cool roof paint, radiant barrier and spray PU foam roof insulation into the roof, the indoor temperature of the attic space can be even lower. It can reduce the indoor temperature by 2 to 3°C.