Moisture Vapor Barrier Coating

Essentially, a moisture vapor barrier coating controls the movement of moisture vapor through it. At a certain temperature, water vapor is in thermodynamic equilibrium with its condensed state. At higher pressure, water will condense. An equilibrium gas mixture in equilibrium with solid or liquid water has a water vapor pressure equal to the partial pressure of water vapor in that mixture.

FX-409 Buy Here

A moisture vapor barrier cannot solve water problems associated with osmosis or hydrostatic water or other conditions such as improper surface preparation, water vapor transmission which can deteriorate concrete resulting in concrete cohesive failure, or alkaline silica reaction, iconic compounds or soluble salts in the concrete. Unreacted alkaline silicate compounds within the concrete can result in osmotic action that will channel water soluble compounds to the surface where they can effectively break the bond. The major problem for coatings to overcome is not necessarily the water, but the rate of movement or migration of water within the slab. Water pressure can be referred to as hydrostatic, capillary or osmotic in nature. Capillary and osmotic forces are difficult to predict and can be higher than hydrostatic forces. To better understand osmosis and hydrostatic water conditions, below is some information pertaining to this subject.

Osmosis

The process of osmosis occurs when solvent molecules (including water) move spontaneously through partially permeable membranes to regions with higher solute concentrations in such a way that the solute concentrations on both sides are equalized. Alternatively, it can be used to describe the separation of two solutions of different concentrations by means of the movement of any solvent across a semipermeable membrane (permeable to the solvent, but not to the solute).

Osmotic pressure

Osmosis may be opposed by increasing the pressure in the region of high solute concentration with respect to that in the low solute concentration region. The force per unit area, or pressure, required to prevent the passage of water through a selectively permeable membrane and into a solution of greater concentration is equivalent to the osmotic pressure of the solution. Osmotic pressures can be far greater than the bond strength of coatings to the concrete.

Hydrostatic water pressure

“Hydrostatic Pressure” means water pressure, and it is a major cause of water problems. During wet weather, the soil outside your floor or wall becomes saturated with water. With no place to go, hydrostatic pressure continues to build. Sixty pounds per cubic foot puts tremendous pressure on your floor or wall. Water weighs a little more than 60lbs. per cubic foot and if the soil around your floor or wall is saturated with water, there could be tens of thousands of pounds in hydrostatic pressure on your floor or wall. With this much pressure, even the smallest crack or gap can admit plenty of water. Any discussion of hydrostatic pressure must start with some information on the water table. Simply put, the water table is the level of the water in the ground on which your structure sits. There are a number of factors that affect the water table, with the most common being proximity to large bodies of water. Even if you live miles from the lakeshore, Lake Michigan keeps the water table high in the Northeastern Illinois, Wisconsin, and Northwestern Indiana areas. The frequency and intensity of rains also play a large role in determining the water table. And, if your structure is built on the dense clay soil that is very common in the Chicago area, you are more prone to higher water levels due to clay’s lack of absorption. So we have this body of water in the ground…and now it’s raining every day for a week; as the rain seeps into the ground, the water table rises because the ground is unable to soak up all the water. This rising water table creates hydrostatic pressure against your structure.

Accordingly, although a moisture vapor barrier coating will control the rate of moisture vapor, and effectively help protect subsequent applied systems above the barrier, it cannot overcome excessive forces created by water movement and pressure or other concrete deficiencies as described above.