The study of moisture migration from the surface of building materials and construction elements is of great importance for the characterization of their hygrothermal performance. In particular, transient calculation of moisture migration from the surface requires knowledge of the surface transfer coefficient and the moisture buffering properties of the building materials. In this paper the influence of the natural convection effect on the convective heat and mass transfer coefficient between the warmer air and a horizontal upward facing colder water surface is explored under atmospheric pressure. The air temperature and the water surface temperature in water-filled square cups are obtained by measurements with type T thermocouples connected to a data-logger. As far as we know there is no public literature about estimation of the natural convective mass transfer coefficient as a function of Rayleigh numbers lower than 104, which was the primary motivation source in this work. The effects of different characteristic lengths as well as temperature differences are discussed in the paper.