The cooling performance of heat spreading layers, consisting of materials that have relatively high thermal conductivity embedded into heat-generating mediums, presents itself as a viable method of reducing peak operating temperatures in, for instance, integrated power electronic applications. In this paper the boundary condition associated with single-directional heat extraction during the cooling of a generalised rectangular, three-dimensional heat-generating volume is considered numerically. Numerically based correlations are given from which the cooling performance of a layered structure can be calculated. These correlations are based on data for large ranges in geometric dimensions, thermal conductivities, fraction of volume used for cooling purposes, and high interfacial resistance values. (C) 2008 Elsevier Ltd. All rights reserved.