Photo - Resists Vapor Transmission

Vapor Transmission

Moisture vapor transmission rate is a measure of the passage of water vapor through a substance and the measure of its permeability. Water vapor transmission in concrete floors is simply liquid water that rises through the pores in the concrete by capillary action through a process called diffusion.

Hydrostatic pressure

Hydrostatic pressure is pressure created by standing or resting water (usually ground water surrounding a foundation) and concerns slab on grade and below grade concrete. The pressure is caused by moisture close to the bottom of the slab where a vapor barrier was not used allowing water to rise through the pores in the concrete. When enough water accumulates, it starts to exert pressure against the concrete foundation. This can result in a damp, musty smell, moldy odor, high humidity and water leakage.

Why is water damaging to concrete

Although concrete appears hard and strong, it is actually similar to a sponge with open pores and voids in which moisture can travel. Concrete is composed of thousands of microscopic pores that fill with water as concrete is being poured and set. Concrete starts to harden and moisture near the surface of the concrete slab begins evaporating and the tiny pores dry out. Once water has completely dried in a curing concrete floor, moisture vapor continues to diffuse through the slab from the adjacent soil’s moisture content. Small pathways created during the hydration process remain open and a pathway for moisture. If no vapor barrier exists between the soil and the concrete, moisture will build up. If a flooring was installed without a vapor barrier underneath, this moisture will cause it to buckle, delaminate and will fuse mold and mildew growth.

How do you reduce vapor transmission

Installation of a concrete vapor barrier coating is the best way to control moisture vapor transmission. Most floor covering manufacturers and the ASTM state the maximum allowable moisture vapor emission rate is 4lb/24hr/1000sf for successful flooring installations. The moisture emission rate can be determined through a Calcium Chloride Test. Anything greater and remedial action is recommended to reduce the rate of vapor transmission before flooring installations. Vapor barrier coatings are able to control moisture emission rates up to 20lb/24hr/1000sf.

Calcium chloride test

In order to determine the moisture vapor emission rate of your concrete perform a calcium chloride test. To perform the test, a small amount of calcium chloride (salt) is placed under plastic on a clean piece of concrete. The calcium chloride (salt) absorbs the moisture coming from the concrete slab over a period of about 72 hours. The weight gain of the calcium chloride is then measured and the moisture vapor emission rate MVER is calculated. If a high amount of moisture is present in the concrete (over 3 lb according to a calcium chloride test) it is best to use a moisture vapor barrier coating.

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