In a previous study, the thermal conductivity of a high-density mineral fiberboard had been experimentally determined as a function of moisture content at ambient temperature in steady-state conditions, allowing moisture to be redistributed in the product. The results obtained were significantly different from those of other authors who had performed experiments with a uniform moisture distribution. The present work is an attempt to understand the reasons for these differences. A second series of tests has been performed on the same high-density product in an attempt to re-create the results of these authors. A method of measurement has been developed, and the results obtained for mean temperatures of 10 and 24°C are presented and discussed. Special attention is given to the heat transfer caused by vapor diffusion and to the latent heat component of thermal conductivity due to this diffusion. A simple model for predicting the thermal conductivity in the conditions of the tests is presented; fairly good correlation with the experimental results was obtained. Finally, all results are compared and discussed in order to see which can be considered representative of field conditions.