A vapor barrier is a layer of material, usually plastic sheeting or a liquid-applied membrane, that blocks water vapor from migrating from the ground into a concrete slab or from the slab into a floor coating.
Where Vapor Barriers Are Installed
On new construction, a vapor barrier is typically placed under the slab before the concrete is poured, sitting directly on the compacted base material. On existing slabs where moisture is already a problem, contractors can apply a liquid-applied vapor barrier primer directly to the surface of the concrete before installing an epoxy or polyaspartic system. Both approaches aim to stop the same thing, water vapor reaching the coating from below.
Vapor Barrier vs. Moisture Mitigation System
A vapor barrier under a slab is a passive, permanent layer installed once during construction. A moisture mitigation system is an active repair applied to an existing slab that is already showing high moisture readings, usually as a specialized epoxy topping. Both accomplish a similar goal, but a mitigation system is a fix applied after the fact, while a vapor barrier is built in from the start.
When a Vapor Barrier Is Worth the Extra Step
Slabs poured on grade or below grade, slabs near a high water table, and any building without a documented vapor barrier under the concrete are all higher-risk candidates for coating failure. Testing the slab’s moisture vapor emission rate or relative humidity before installation is the best way to know whether a vapor barrier or mitigation system is needed, rather than guessing based on the age or location of the building.
