This article provides the results of a study to determine the emanation coefficient of artificial construction material, namely heavy concrete produced by semi-dry pressing, depending on equilibrium sorption moisture content achieved in the desorption stage. We measured radon volumetric activity in a sealed chamber at different air relative humidity, determined the specific activity of naturally occurring radionuclides (NORs), and calculated the specific effective activity of NORs. Then we obtained mathematical models of the dependence of the emanation ratio on the degree of pores filling with water, and it was determined that the emanation coefficient increases by almost 50 % in the range of relative humidity typical for residential and public buildings, which necessitates the numerical control of this parameter in order to accurately assess the dose load on the population.