What is moisture vapor transmission?
Moisture vapor transmission (MVT), also called moisture vapor emission rate (MVER), is the rate at which water vapor moves through concrete and can pass through applied coatings, typically measured in pounds per 1,000 square feet per 24 hours using calcium chloride testing or in-situ relative humidity probes.
Before coating concrete with epoxy, polyurethane, or hybrid systems, contractors measure moisture vapor transmission to determine if the slab is dry enough. This is usually done using one of two methods: calcium chloride testing (which places a measured salt sample on the concrete surface under a dome for 24 hours) or in-situ relative humidity (RH) testing (which uses probes inserted into the concrete at depth).
High moisture vapor transmission indicates that water is moving up through the concrete slab. If this moisture reaches the interface between the concrete and the coating, it can cause adhesion failure, blistering, peeling, or delamination. Epoxy and polyurethane coatings are moisture barriers, and if applied over damp concrete, trapped vapor pressure builds beneath the coating film and pushes it away from the substrate.
Most epoxy coating manufacturers specify maximum acceptable MVT levels (commonly 3 to 5 pounds per 1,000 square feet per 24 hours for epoxy systems). Concrete that exceeds these thresholds requires additional preparation such as drying time, moisture mitigation membranes, or use of specialized moisture-tolerant coating products. Testing is done before coating work begins to prevent expensive failure and ensure warranty compliance. Many flooring professionals in Ontario rely on MVT testing as a standard step in project assessment.