A dynamic parameter to describe the thermal response of buildings to radiant heat gains

•The surface factor (SF) describes the response of a wall to a radiant heat flux.•The SF of single-layered slabs is strictly related to the thermal effusivity.•The SF is highly affected by the internal film thermal resistance.•The time shift of the SF does not exceed 3.5 h for slabs with massive materials.•Any material placed behind an insulation layer has little influence on the SF.

This papers deals with the surface factor, a dynamic parameter that describes the thermal response of a wall to a cyclic radiant heat flux absorbed on its inner surface. Despite it was available since the Seventies in the framework of the admittance procedure, the surface factor has not been fully exploited and, unlike other dynamic thermal properties, it has been very rarely dealt with in the scientific literature.After a concise presentation of the mathematical background, the paper proposes the calculation of the surface factor for common single-layered walls, in order to investigate its sensitivity to the thickness and the thermophysical properties of the slab. Two graphs are presented for an easy and precise determination of the amplitude and the time shift of the surface factor, as a function of the properties of the material. The effect of an insulating layer is also addressed, by varying both its position and thickness. Furthermore, the investigation shows that the values of the film thermal resistance suggested by current Standards are not always appropriate, and may introduce significant inaccuracy.The results provide very useful information for optimizing the response of a wall to periodic heat fluxes acting on its internal surface.