Radiative Heat Transfer in Buildings

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In the field of building physics, heat and moisture transports are often studied. These transports represent core problem in building performances. The heat transport is often investigated as heat conduction through building envelopes but the heat transfer inside buildings usually remains overlooked. The heat transfer in closed spaces may consist of conduction, convection and radiation. In thermal equilibrium, these transports compensate not only heat losses going through building envelopes but they may influence the temperatures of interior surfaces that occasionally suffer from the condensation of water vapors. So far, the thermal building technology has investigated heat losses prevalently as simple heat conduction through building envelopes along with ventilation (infiltration or exfiltration). Such an approximation avoids considering an alternative procedure taking into account direct radiative and convective heat flows from interior heating sources towards exterior spaces. For this purpose, it is necessary to have a computational formalism capable of determining the radiative heat flows established in interiors of buildings. The recent monographs provide such formalism only for gray or black bodies but not for their combinations. In this contribution, the general formalism for the computation of radiative heat flows between black and grey bodies is derived. The formalism consists of the system of equations specifying radiosities, heat fluxes and heat flows related to each surface of the interior. It is shown that this general system of equations may be reduced to two particular systems holding separately for black and grey bodies. In this way, the universality of the developed computational formalism is documented.