Abstract
A theoretical background relating to radiative heat transfer in porous materials, and the effect on thermal conductivity, is presented. Simplified models are considered as well as a more rigorous analysis provided by the Hamaker two-flux model which takes into account both radiation scattering and absorption. Conventional equations derived from Hamaker assume radiation scattering only, and predict that the radiative conductivity increases with thickness to a limiting value—the 'thickness effect'. A modified expression for radiative conductivity, which includes absorption, has been derived from the Hamaker model. This might be expected to apply to insulants where absorption effects are small compared with those of scattering and which are subject to temperature conditions normally found in the built environment. The radiative conductivity still increases with increasing thickness of insulant. However as the amount of absorption is increased the radiative conductivity is reduced and the limiting value occurs at smaller thicknesses.
Get full access to this article
View all access options for this article.