Flow and heat transfer in a porous medium filled with an ideal gas of 100% humidity are strongly coupled. The transitions between the conductive and convective regimes can be found by stability analysis of the governing equations. A dimensionless Rayleigh number controls the heat and flow regime. Stability conditions obtained by perturbation analysis show that the critical Rayleigh number depends heavily on the vapor pressure. The moist gas studied here is much less stable than a dry ideal gas, because the latent heat carried by a warm moist gas is much greater than the sensible heat.