Analisi numerica e sperimentale della trasmissione del calore nelle cavità d’aria dei blocchi forati

The thermal performance of hollow bricks, as elements of the building envelope, is mainly linked to the presence of air cavities. The heat propagates within those components through the three basic modes of transmission: conduction in the solid matrix, convection of the air cavity, direct radiation between surfaces that face in the cavities. The rigorous treatment of such heat exchange is very complex even in thermal conditions close to the environment with moderate temperature differences. In engineering practice a simplified calculation method is used, as incorporated in national and international standards, which reduce heat transfer into the cavity to an equivalent heat conduction that results in the transport of the same thermal power. UNI EN ISO 6946 and UNI 10355 follow this method by proposing a simplified model of computation that lead to different results. Given the importance that the correct assessment of the insulation performance of the envelop in the context of energy certification of buildings it was considered useful to undertake a detailed analysis of the reliability of the estimation methods of heat transfer in air cavities of hollow bricks. The study was conducted with the use of different tools, numerical and experimental. An experimental study was conducted in the laboratory with heat transmission measurements, in specimens with parallelepiped cavities, through the guarded hot plate apparatus according to UNI EN 12664. The analysis of the results, numerical and experimental, compared with the predictions obtained from simplified models of technical standards, have called attention to the order of magnitude of the calculation accuracy obtainable with such procedures. For certain geometric configurations and boundary conditions, the application of the standard procedures can lead to large errors of evaluation of the thermal resistance of the cavity.