The aim of the paper is to investigate the hygrothermal properties of a newly developed ultra-lightweight polystyrene concrete, based on laboratory measurements. It describes the measuring process of thermal conductivities, and determines the declared thermal conductivity. The temperature and moisture conversion coefficients are determined, and new approximate functions are introduced. The paper describes the sorption and desorption isotherms, and gives polynomial approximate functions. The paper also investigates the temperature dependency of sorption curves. It determines the water absorption coefficient and the free water saturation. Furthermore, it describes the measuring process of the water vapor permeability. The water vapor resistance factor and water vapor diffusion-equivalent air layer thickness are calculated.