composite walls, passive cooling, Laterite stone, Time lag and decrement factor, Optimum insulation thickness, Optimum insulation position
Passive cooling is one of the methods to keep the building cool and to reduce the load on air conditioner. Passive cooling demands the study of the thermal characteristics like admittance, transmittance, decrement factor, time lag, surface factor and surface factor time lags. These characteristics were determined for both homogeneous and composite walls. The buildings are built with composite wall. This demands the study of thermal characteristics of composite wall. In this study, five building materials like Laterite stone, Burnt brick, Mud brick, Reinforced brick and Fly ash bricks were selected as homogeneous building materials. Five local Insulating materials such as Saw dust, Rice husk, Coir board, Jute felt and Jute fiber were selected to form the composite wall along with building materials. The composite walls were framed using the combination of building materials and insulation materials. Total hundred combinations of composite walls, without insulation (Configuration-A) and with insulation material located at inner side (Configuration-D), insulation material at mid plane (Configuration-C) and insulation material at outer side (Configuration-B) of the composite wall were studied. To study the thermal characteristics of such configurations of composite wall, one dimensional heat flow diffusion equation was solved using matrix algebra under periodic boundary conditions. The penetration length of thermal waves, the phase velocity of the heat wave in the wall and optimum fabric thickness of the selected building and insulating materials were calculated. From this study it is found that the decrement factors of the fly ash brick (0.401) and jute felt (0.4) are the least and the time lags of the fly ash bricks (8.15h) and jute felt (8.75h) are the highest among all the studied homogeneous building and insulating materials. Hence these two homogeneous materials are more effective building and insulating materials at suppressing temperature swings. It is summarized that insulation located at the inner surface (Configuration D) of the composite wall gives the lower time lags and higher decrement factors. This is the worst condition of insulating the wall among all the studied configurations. It is also concluded that fly ash brick composite walls with jute felt insulation located at the mid plane of the wall give greater time lags (11.17 h) and reinforced brick composite walls with rice husk insulation located at the inner side of the wall give lower time lag values (7.37h). It was also observed that fly ash brick composite walls with coir board insulation located at the outer side of the wall give the lowest value of decrement factor (0.17) and reinforced brick composite walls with jute fiber insulation located at the inner side of the wall give higher decrement factors (0.37). Fly ash brick composite walls with jute felt insulation located at the mid center plane of the wall are the most recommended composite walls among all the studied composite walls. Henceforth this composite wall is recommended for passive cooling.