AbstractThe influence of surface wetness on the thermal performance of a red brick wall has been investigated using a small temperature controlled test chamber within a larger controlled‐environment laboratory. The results confirm that the effect of surface wetness can be significant and that the processes involved are complex.Two distinct drying phases have been identified, one in which water evaporates directly from the wall surface, and another which involves diffusion of the water from within the body of the brick. The influence of the first effect is large (up to 30 per cent increase in heat loss, but typically about 15 per cent), but it is of limited duration (5–10 h). The second has a smaller magnitude, but can persist for up to 40 h.One disturbing feature has appeared in the results of the present study. This is the fact that the experimental values reported here for thermal conductivities, whether the wall is wet or dry, are significantly higher than the CIBS Guide data. The present measurements are consistent in this respect with more general observations made recently by other workers, and give rise to considerable concern over the applicability of generally accepted data. It is apparent that a considerable amount of investigation will be necessary before many of these discrepancies will be resol