“All natural systems tend towards equilibrium. A hot cup of coffee transfers heat to the air in the room until they are both at the same temperature (this is related to entropy and the second law of thermodynamics). In chemistry, higher concentrations of a chemical move towards areas of lower concentration (known as Le Chatelier’s principle). The same is true for areas of higher or lower relative humidity (which is actually a measure of the vapor pressure of water vapor in air). This movement of vapor is called diffusion.
These principles mean that if the relative humidity in the concrete slab is different than the relative humidity of the air above the slab, then moisture is going to try to move in or out of the slab. Without a vapor barrier, the relative humidity in the slab just below the surface can often be 100%. Since the air is seldom that humid, moisture is going to move from the slab into the air and as the surface dries a bit it will draw moisture up from the bottom…”
Learn more about moisture issues by clicking the link below:
“What Problems Does Moisture Create”