The thickness of the still air layer adhering to perforated plastic plates and a fabric was investigated in terms of the water vapor pressure gradient in the zone of diffusion above the samples. The water vapor pressures above the samples were determined from measurements using a temperature and relative humidity sensor. To analyze the thickness of the still air layer for a wide range of evaporation rates, the hole diameter and the number of holes per unit area were changed and the measurements were under different temperature and humidity/gradient conditions. The thickness of the still air layer corresponded to a distance of 10 mm from the sample surface and was not related to the surface construction and materials.