The effect of non-steady state thermal indices of building walls on energy consumption; Case study: A residential house in hot and dry climate of Birjand

According to global primary energy consumption statistics, buildings significantly contribute to energy consumption, with an observed annual increase in this regard. As a consequence, buildings are responsible for greenhouse gas emissions and climate change. Therefore, it is essential to mitigate adverse environmental effects by reducing energy consumption. Heat transfer through the walls of buildings plays a crucial role in this context, and thermal performance significantly affects both energy consumption and the thermal comfort of indoor spaces. Since heat transfer through the walls is closely related to variations in indoor temperature, outdoor temperature, and the materials used in construction, it directly influences both cooling and heating loads. This study aims to achieve optimal performance by identifying the thermal behavior of materials to reduce energy consumption via external walls. Consequently, an understanding of the performance of materials in exterior walls is essential to measure their physical behavior in response to changes in outdoor temperature. To reduce energy consumption, two parameters—decrement factor and time lag—can create significant thermal inertia. However, there are many non-steady-state thermal indices, such as thermal transmittance, volumetric specific heat, and surface factor, and the impact of each on building energy consumption has been investigated through calculations. The non-steady-state thermal indices of walls depend on two important factors: material type and wall thickness. The study results indicate that wall thickness has a greater effect than the type of material used in the wall. The material type can lead to significant differences in the numerical values of indices such as volumetric specific heat, thermal admittance, thermal transmittance, time lag, and time lead. Increasing wall thickness reduces thermal indices such as thermal transmittance, decrement factor, and time lag (surface factor) while increasing the time lag (decrement factor). Thermal admittance, surface factor, and time lead show no significant relationship with wall thickness. Additionally, by designing a residential unit in the hot-dry climate of Birjand (Iran), the effects of each non-steady-state thermal index on energy consumption were simulated. The results indicated that indices such as thermal transmittance, thermal admittance, and volumetric specific heat have direct effects on cooling and heating energy consumption; an increase in these three parameters leads to higher cooling and heating energy consumption in buildings. Conversely, indices such as surface factor, time lag, and time lead negatively affect cooling and heating energy consumption, with their increase resulting in reduced annual energy consumption. However, the decrement factor and time lag have no significant effect on building energy consumption, with their correlations being weak and negligible. The annual energy consumption of buildings in this climate can be reduced by using materials with good thermal insulation properties. The simulation results indicate that the lowest annual heating and cooling energy consumption is associated with buildings constructed from AAC blocks, 3D concrete panels, and Perlite blocks. In contrast, the highest energy consumption occurs when using precast concrete, Liper blocks, or various types of clay bricks in the external walls.