Vapor Barriers for Concrete Slabs

Find Vapor Barrier Manufacturers

Why is there water vapor in concrete?

What problems does moisture create?

How do we test moisture vapor transmission?

What are vapor barriers?

Choosing a vapor barrier

How (and where) to install a vapor barrier

Related Information:

Sullivan's Corner - SEALER SERIES Part 3: Preventing Moisture Problems

Reducing Bond Failures Caused by Moisture Vapor Transmission

Other Resources:

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Water and concrete have a complicated relationship—sort of a marriage of convenience. We need water to mix and place the concrete (often called the water of convenience) and we need water for the concrete to gain strength. Once the honeymoon is over, though, we would like the water to leave and leave fast! But ending this relationship can be painful—the water just hangs around, creating problems. Eventually, most of the water will leave on its own, evaporating from the slab surface, but only if we aren't letting its ugly sister, water vapor, move into the slab from the bottom.

The last thing you want your customers to envision when thinking of a concrete floor is a damp, cold basement slab. One of the reasons those old basement floors were like that was because they had no vapor barrier beneath them, leaving an easy path for water vapor from the soil to migrate into the slab, assuring that the cold clammy damp feeling never went away. And dampness is only part of the problem, water vapor moving through a concrete floor can delaminate sealers and overlays, discolor acid stains, produce efflorescence on the surface, damage aggregates, cause the slab to curl, and condense beneath objects on the floor.

But it doesn't have to be that way. On new interior slabs, moisture can be easily controlled and mostly eliminated from the system. Think of a concrete floor as an essential part of the building envelope. "Just as we do not tolerate water leaks in roofs or walls," wrote CTL Group's Howard Kanare in a recent article in Concrete Construction, "we should not accept structures built so that moisture can infiltrate floor slabs."

Before we run out and hire a divorce lawyer, let's see if we can save this marriage. A little understanding of how moisture moves in a slab and the role of vapor barriers might show us why there's a problem and how to prevent it.