What Is a Concrete Vapor Barrier?

A concrete vapor barrier is a low-permeability sheet placed directly beneath a concrete slab. Its purpose is to block water vapor migration from the ground into the slab. Water vapor moves upward naturally due to capillary action and vapor pressure differences, even when liquid water is not visible.

Over time, uncontrolled vapor movement can lead to flooring failures, mold growth, curling or cracking, and costly remediation. A properly selected vapor barrier addresses these risks at the source—below the slab.

Photo: Vapor barrier placed directly beneath reinforcement before concrete placement.

Why vapor barriers belong directly under the slab

Modern best practices—and industry guidance such as ASTM E1745—support placing the vapor barrier directly beneath the concrete slab, not below the granular base. This minimizes the amount of moisture that can accumulate and be driven upward.

When vapor barriers are buried too deep, moisture can collect in the base layer, increasing the amount of vapor pressure pushing toward the slab.

Why Thickness Matters: Understanding 15 Mil Vapor Barriers

Vapor barriers are commonly specified by thickness, measured in mils (1 mil = 0.001 inch). Thicker barriers generally offer better durability, puncture resistance, and long-term performance during construction and service life.

Why 15 mil is often specified

• Lower permeability: Helps restrict vapor transmission through the slab system.
• Higher puncture resistance: Better suited for foot traffic, rebar, and equipment during placement.
• Construction durability: Reduces risk of tears that compromise performance.
• Compatibility with modern slabs: Especially important under conditioned buildings and finished floors.

Photo: Seams taped and overlapped to maintain vapor continuity.

Seams, overlaps, and detailing matter

Even the best vapor barrier can fail if seams and penetrations are not handled correctly. Sheets should be overlapped according to manufacturer guidelines and sealed with compatible tape.

Around columns, footings, and penetrations, the barrier should be cut, wrapped, and sealed to maintain continuity across the entire slab footprint.

How to Install a Concrete Vapor Barrier Correctly

Installation quality is just as important as material selection. Below is a simplified, field-tested approach commonly used with 15 mil vapor barriers.

Typical installation steps

1. Prepare and compact the subgrade and granular base.
2. Unroll the vapor barrier directly over the base.
3. Overlap seams per manufacturer requirements.
4. Tape seams and repair any punctures immediately.
5. Detail around penetrations and foundations.
6. Place reinforcement and concrete with care to avoid damage.

Photo: Large-scale slab installation using a 15 mil vapor barrier.

Common installation mistakes to avoid

• Leaving unsealed seams or gaps
• Puncturing the barrier and not repairing it
• Placing the barrier too deep below the slab
• Assuming thin plastic provides equivalent protection

Case Study: Commercial Slab Protected with a 15 Mil Vapor Barrier

Project: A light-commercial building with finished flooring planned throughout the space. Moisture-sensitive adhesives were specified, making vapor control critical.

Design challenge

The site had moderately moist soils, and long-term flooring performance was a priority. Thin poly sheeting was ruled out due to puncture risk and permeability concerns.

Solution

A 15 mil vapor barrier was installed directly beneath the slab, with all seams taped and penetrations carefully detailed. The heavier material allowed crews to work without excessive tearing during reinforcement placement.

Outcome

• Concrete placement proceeded without barrier failures
• Flooring installed on schedule with no moisture-related delays
• No signs of vapor-related flooring distress observed after occupancy

Concrete Vapor Barrier FAQ

Educational note: Always verify project requirements, local codes, and manufacturer installation guidelines when specifying or installing vapor barriers.