The paper deals with the onset of convection in a porous layer heated from below, by considering the case when the fluid and solid phases are not in local thermal equilibrium and when form-drag and boundary effects are included in the analysis. Analytical progress is facilitated by taking stress-free boundaries conditions. Asymptotic solutions for both small and large values of the scaled inter-phase heat transfer coeffcient, H, are presented and comparisons with the numerical solutions are performed. Excellent agreement is obtained between the asymptotic and the numerical results. Copyright (C) 2003 John Wiley Sons, Ltd.