## 1 Introduction  
Three-quarters of the total energy consumption in the building sector is residential, where great potential to improve energy efficiency exists [1]. The buildings consume about 40% of the overall energy demand worldwide and are responsible for carbon emissions [2]. In the US, the energy consumption for the residential sector surpasses the commercial sector, as reported in 2017 by the US Department of Energy [3], with both accounting to 38.9% of the total energy consumption. Among all sectors, the residential sector plays a pivotal role in global energy demand. The percentage of total residential consumption in different regions of the world fluctuates from an average of 20% in developed countries to more than 35% in developing countries [4]. Residential energy demand is prominent because of its present size and potential growth [5]. In developed countries, the energy consumption for the residential and commercial sectors has increased steadily, reaching between 20% and 40%, respectively, and has exceeded other major sectors, namely the industrial and transportation sectors [6]. In cities with developed economies, space heating is provided through oil- or gas-fired boiler plants and space cooling relies mainly on electricity [7]. As cooling loads increase due to global warming, a shift towards electrical power will also occur. A building envelope refers to the external portions or skin of a building where heat or thermal energy is transferred and governs the interior comfort level and energy usage of the building space as defined by the Malaysian Standard MS 2680:2014 [8]. Building envelope components as mentioned by [PERSON] et al. [9] are the most important feature that can maximize energy efficiency and reduce energy consumption for a building because the envelope is where the largest heat loss or gain occurs. A study published by the US Department of Energy [2] showed that 73% of the total heat/gain loss is contributed by the building envelope. In the United States, the building sector accounted for about 41% of primary energy consumption in 2010, 44% more than the transportation sector and 36% more than the industrial sector. Space heating, space cooling and lighting were the dominant end uses in 2010, accounting for close to 70% of site energy consumption. In the past decades, efforts have been made to reduce CO2 emissions through energy conversation and efficient measures. The finite availability of fossil fuels and the highly transient nature of renewable energy sources have enhanced significant energy efficiency and conservation in various sectors. The top four energy consumption in residential and commercial buildings are space heating, space cooling, water heating and lighting, all of which accounted for approximately 70% of site consumption. Other end uses such as appliances and ventilation make up the remaining 30% [10]. Lightweight aggregate concrete (LWAC) has good thermal properties. However, its mechanical properties are generally diminished with an increase in thermal efficiency. Mitigating this problem with the use of mono and hybrid fibres in LWAC is crucial if LWAC is used successfully as an integral structural insulation concrete in the building industry. [PERSON] et al. [11] found that structural LWAC contributes positively to increasing energy efficiency by improving the thermal comfort in European countries regardless of the diverse outdoor conditions. Heating energy needs is 15% lower for LWAC as compared with normal weight concrete (NWC), while cooling energy needs do not experience significant variations in the type of concrete in Europe. Hence, designing a building envelope using LWAC would make a significant contribution to energy savings. The brittleness behaviour of structural LWAC is a weakness of this type of concrete. Fibre is added to the concrete mix to overcome this drawback. A review on literature indicated the lack of studies on the thermal performance of fibre-reinforced LWAC and limited data on the thermal properties of these types of concrete are available.