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Epoxy Primers and Vapor Barriers

Most projects will utilize one of our three basic primers. Typical floors that have been properly prepared and are not experiencing any moisture issues and are oil free, will use our standard Water Based TL015 Primer. If you have oil stains you should use our oil blocking primer. There is another set of primers in the industry, often referred to as moisture blocking primers or liquid vapor barriers. While they play a vital role in the industry, they will not work in all situations. Please read our information on our Vapor Barriers before using them.

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We carry a variety of primers for all your garage floor epoxy systems.

What is an Epoxy Vapor Barrier?

A moisture vapor barrier coating, is a coating designed to control the movement of moisture vapor through the coating. The vapor pressure of water is the pressure at which water vapor is in thermodynamic equilibrium with its condensed state. At higher pressure, water would condense. The water vapor pressure is the partial pressure of water vapor in any gas mixture in equilibrium with solid or liquid water.

Example – The Saturation Pressure of Water Vapor

The Saturation pressure of water vapor in moist air at dry bulb temperature 25oC can be calculated as:

pws = e( 77.3450 + 0.0057 (273 (K) + 25 (degC)) - 7235 / (273 (K) + 25 (degC)) ) / (273 (K) + 25 (degC))8.2 = 3130 (Pa) (3130 Pascals = 0.45396811993999997 PSI)

As can be seen from the example above. The actual pressure exerted from vapor is minimal and the only function of the vapor barrier is to control the flow of moisture vapor through the vapor barrier to minimize the amount of vapor migrating from one side of the vapor barrier to the other side of the vapor barrier. This in turn can help subsequent systems placed above the vapor barrier from the effects of excessive moisture vapor. 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 is the spontaneous net movement of solvent molecules (including water) through a partially permeable membrane into a region of higher solute (dissolved solid) concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations.

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.

Vapor Barrier Instructions

Vapor Barrier Data Sheet



TL409 Epoxy Vapor Barrier

Moisture issues present a major issue for coatings. While liquid vapor barriers are widely sold as the solution, it is important to understand when they will or will not work. Moisture barriers are not going to fix major issues caused by excessive hydrostatic pressure, poor slab install and other issues. They will deal with mild moisture issues. Please read this page thoroughly as it contains a ton of information. If you are having a moisture issue, have your slab tested professionally to make sure a liquid vapor barrier will resolve the issue.

TL409 is a two component 100% solids epoxy seal coat that can help control moisture vapor emission rates up to 20 lb/24hr/1000 ft2, prior to application of Vinyl sheets, Tiles, Cementitious overlays, Terrazzo, Wood veneers, Carpet or polymeric coating systems. The product meets the ASTM F3010 product requirements for vapor permeance at the recommended thickness. It is recommended for interior use over horizontal concrete — floors not walls. The product should be applied at a 17 Mil WFT/DFT (same for both) and yields a coverage of 94.4 Sq. Ft. per gallon.