Dynamic thermal features of insulated blocks: Actual behavior and myths

The latest updates in the European directive on energy performance of buildings have introduced the fundamental “nearly zero-energy building (NZEB)” concept. Thus, a special focus needs to be addressed to the thermal performance of building envelopes, especially concerning the role played by thermal inertia in the energy requirements for cooling applications. In fact, a high thermal inertia of the outer walls results in a mitigation of the daily heat wave, which reduces the cooling peak load and the related energy demand. The common assumption that high mass means high thermal inertia typically leads to the use of high-mass blocks. Numerical and experimental studies on thermal inertia of hollow envelope components have not confirmed this general assumption, even though no systematic analysis is readily available in the open literature. Yet, the usually employed methods for the calculation of unsteady heat transfer through walls are based on the hypothesis that such walls are composed of homogeneous layers. In this framework, a study of the dynamic thermal performance of insulated blocks is brought forth in the present paper. A finite-volume method is used to solve the two-dimensional equation of conduction heat transfer, using a triangular-pulse temperature excitation to analyze the heat flux response. The effects of both the type of clay and the insulating filler are investigated and discussed at length. The results obtained show that the wall front mass is not the basic independent variable, since clay and insulating filler thermal diffusivities are more important controlling parameters.