DESIGN OF DOUBLE SKIN METALLIC ROOFING FOR THE PROTECTION OF BUILDINGS AGAINST THE THERMAL EFFECTS OF SUN RADIATION

摘要

Absorption of solar radiation creates large heat build-up at the surface of a metallic roofing. An increasing part of the energy consumption for MVAC in modern building is due to air conditioning. Hence, improvements of the thermal insulation of metallic roofing is required especially in very sunny areas. One of the best ways for keeping solar heat out of the buildings is to develop reflective coatings rather than further increase the thickness of the insulating material of sandwich panels. The reduction of the solar absorptivity of a pre-painted metallic roofing may contribute to solve this problem, but it does not show a sufficient efficiency in case of dark roofing colours. A new concept of double skin metallic roofing is proposed where the heat build up is dissipated by an outside protective skin, with minimized heat transfer to the underlying moderately insulated ceiling. In this hybrid system, the requirements for the radiative properties of the skin components are not the same as those of a conventional "cold roof". High solar reflectance of the outer metallic skin is still favourable, but a high absorbance of its outer surface may be compensated by a low infrared emittance of the inner side of the skin. Bare metals or metallic coatings which naturally exhibit low infrared emittance are ideal surfaces for this purpose. In this paper, a hybrid system is proposed which combines a stainless steel outer skin separated by an air gap from a ceiling made of a conventional insulated sandwich panel. The air gap between the two components is designed to carry away by natural convection most of the unwanted heat radiated by the hot skin. The system performs at its best with an un-painted mat metallic surface. In addition, it also permits the use of a painted outside surface with light or even dark shades with no or little adverse effects.