On the thermal time constant of structural walls

In the present investigation a numerical model is employed for the calculation of the transient wall heat flux, under the effect of a step temperature change at a particular wall side, for given initial and boundary conditions. The results were employed for the evaluation of the wall thermal time constant, which is widely employed quantity in Physics and technology to describe the transient output response characteristics of a system under the influence of a step change of a particular input variable parameter. According to the derived results, it was found that the wall thermal time constant is a wall side specific quantity, something which leads to the definition of two time constants, the forward and reverse, which conventionally correspond to the room and ambient wall sides respectively. Furthermore, their ratio appears also to be a very interesting quantity, which represents the measure of the uneven distribution of heat capacity in respect of the wall plane of symmetry and becomes unity for symmetrical walls. Both thermal time constants were evaluated for a number of typical walls, broadly employed in construction practice, and the results were comparably presented and discussed in respect to their fundamental significance on the load characteristics and energy demand of buildings. 2003 Elsevier Ltd. All rights reserved.