Contributing sources of moisture
Outside sources of water which contribute to a moisture condition can be accidental, or a normal function in nature. Accidental sources such as failed plumbing, over watered plant beds, sloping hillsides and poor trench drain systems are all examples of problems that should be carefully regarded, independent of the vapor emission rate of the slab.
Natural sources are those in which moisture cannot be stopped, but controlled. Natural soil capillarity can allow a tremendous volume of moisture to come into contact with a slab. Properly formed capillary breaks within the soil foundation system, combined with a high performance vapor retarder, are the first lines of defense. However, even properly formed capillary breaks and vapor retarders can not control water sources entering from above, or residual water of convenience left over from the placement of the slab.
Regardless of the careful control of the outside sources of water intrusion upon a slab, there can still be a high vapor emission from the slab surface from residual, inherent moisture. It is said that in one cubic foot of seemingly dry sand there is about 5 pounds of contained moisture. With floor requirements, we are concerned with that much moisture over 1,000 square feet. Since healthy concrete maintains a minimum of 80% relative humidity, the moisture in slabs will never truly disappear.
In 1965, brewer’s study on concrete moisture inflow demonstrated that vapor retarders of any degree, had little or no effect on moisture inflow to a slab with a high water/cement ratio. This is because slab porosity allows moisture volume to easily escape when drawn out by the vapor pressure differential acting upon the slab. Concrete by its very nature attracts available moisture from the air, when conditions permit, and stores it within the porous structure. Despite the contributory source, vapor emission can still present a problem. Moisture being drawn from natural forces is allowed to emit in a high volume with highly porous concrete.