The thermal diffusivity of building materials is an extremely important parameter influencing the subsequent thermal comfort of building users. By definition, thermal diffusivity describes how quickly heat from a hot source can flow through a material. Therefore, this parameter includes both the thermal conductivity and the heat capacity of the material. This parameter is often neglected in heat-related calculations which, in the case of dynamic problems, leads to unreliable results. It should be taken into account that heat flows through all materials at a finite speed. On the other hand, knowing the correct thermal diffusivity value of building materials, it is possible to accurately determine the internal parameters in rooms over time. There are several methods for determining thermal diffusivity, most of which are destined to determine this property in laboratories. The aim of the present research is to show how the thermal diffusivity of materials can be determined in existing buildings. The presented method can be used to determine more real thermal parameters used for thermal calculations in buildings, for example, during energy audits or when calculating the demand for cooling for air conditioning or heat for space heating. This research presents the results for a 60 cm brick wall. Thermal diffusivity was determined for specific summer and winter days—most representative of the whole year. This research has shown that the applied method should be used in the summer period, due to the fact that the wall has greater temperature fluctuations. The obtained results are comparable with the previously mentioned laboratory methods. However, due to the fact that the materials analyzed on the spot, the results are more reliable, and also take into account changes in the value of thermal diffusivity resulting from the use of binders, inaccuracies in joining and external layers made of other materials.
As a result of changes in the boundary conditions, a heat exchange in the building envelope is not always in a steady state. Heat fluxes penetrating through the wall may be several times greater than the steady state. Thus, the building can have a much higher demand for energy and requires the use of more powerful heater than it results from the standard calculation. This article dealt with estimating the length of the transient state after changing the boundary conditions. The protective celling plate of dimensions 35 m × 1 m and a thickness of 0.5 m has been analysed. The plate, on the one hand, is cooled in air at a temperature of 8.6°C, on the other hand is inside the shelter at 20°C. A period of cooling, based on the theoretical and numerical calculations, was obtained, then the differences were discussed and the conclusions drawn.
Keywords: building, building physics, heat flows in building envelopes
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