Effects of Thermal Bridges on the Thermal Performance of External Walls

Abstract

Cooling and heating become an important operational process in buildings. This process instates the consumption of energy to attain the desired conditions of comfort. This consists of adding or removing amounts of heat to or from the building depending on the surrounding climatic conditions. The created difference between internal and external conditions releases a heat flow from the hot environment to the colder environment. The flow and the amounts of heat traversing the building envelope depend on the capacity of the building envelope to obstruct the heat flow. Therefore, the thermal resistance of the building envelope is a crucial variable in the equation of energy-saving plans and sustainability strategies. The current research focuses on the thermal resistance of the external wall as a main part of the building envelope, it emphasizes the phenomena of thermal bridging in external walls. In thermodynamics, thermal bridges refer to a higher flow of heat transfer through a specific material, more conductive than its surroundings. Thermal bridges in building walls are usually caused by mortar joints used between insulated building blocks, solid blocks used as per the specifications of masonry works in the first line of blockwork and around openings, and by the concrete columns and beams within the building envelope. The present research investigates the effects of different thermal bridges on the performance of the external wall in terms of thermal transmittance and energy saving. The thermal resistance of external walls is calculated according to the combined method. The studied cases show that the ratios of different components of external walls depend on several parameters including the structural design and blockworks construction details. Mortar joints in blockworks are not regular; they vary roughly from 15 mm to 25 mm. The thermal bridging effect of mortar joints is simulated by reducing wall thermal resistance by a percentage that depends on the thickness of mortar joints between units of thermal blocks. The solid blocks used in the construction of external walls are used, due to some technical requirements, to make the first line of walls and around openings. They constitute an important thermal bridge also; in fact, they occupy an important fraction of the external wall reaching 8%. The third thermal bridge taken into consideration in this research is the exposed concrete within the building envelope. Despite the use of some solution applied to insulate exposed concrete, in all investigated cases and almost all cases, the concrete elements within the building envelope remain without thermal insulation. These percentages of thermal performance reduction due to different thermal bridges are obtained from a calculation of the thermal resistance of walls for different study cases using the combined method. The results show a huge drop in the thermal performance of external walls which results in a big loss of energy used to neutralize the transferred heat inside/outside the buildings through different thermal bridges.