HEAT-TRANSFER BY CONDUCTION AND RADIATION IN BUILDING-MATERIALS - REVIEW AND NEW DEVELOPMENTS

In the first part of this paper, the mechanisms of heat transfer throu gh insulating materials are presented. Expanded PolyStyrene (EPS) foam is investigated in more details. As observed by optical an electron m icroscopy,: EPS exhibits a double-scale microstructure. It can be cons idered as a dense packing of cellular beads with three phases : the ma cropores between beads, the shell and the core of the pellets. Heat tr ansfer is modelled taking into account conduction and radiation within cells and through cell-walls according to the model of Glicksman (1). Heat transfer through the packing of cellular pellets is described us ing the model of De Vries (2). Results taking into account the effect of foam density, cell-size and proportion of each phases are presented . The thermal conductivity of cellular materials exhibits an optimum v alue with regards to the cell sizes and the density. Actually as the c ell struts act as barriers to radiation, when smaller cells are built with the same amount of polymer, the size of the struts are shrunk and they are no more efficient. The model of De Vries is also applied to lightweight concrete (Concrete + EPS beads). In the second part, heat transfer through a very low conductivity material is studied : monolit hic organic aerogel. This material has a lower thermal conductivity th an all other thermal insulants at ambient conditions (3) : around 0.02 3 W/mK without using any heavy gas. This low conductivity originates i n : very high porosity (95 - 98 %) and thus small solid conductivity; extremely small pore size (10 - 100 nm lower than the mean free path o f air), that causes a very low gaseous thermal conductivity due to Knu dsen effect. Moreover, the gas conductivity can be minimized by loweri ng the gas pressure within the aerogel. A typical thermal conductivity of 0.008 W/mk can be reached at 0.05 hPa. At low temperature, the rad iation is minimized and a value of 0.002 W/mK was obtained at 110 K an d 0.05 hPa.