Concrete Vapor Barrier: Complete Guide

This guide covers everything: what a vapor barrier actually does, when you need one, the main types and their performance specs, how installation works, and the specific considerations for garage floors. Each section links to a deeper guide if you need more detail on that piece.

What a Vapor Barrier Does

Ground moisture doesn't just flow as liquid—it also moves as vapor through soil and through concrete. Even a properly drained subgrade releases moisture vapor year-round. That vapor migrates upward through the concrete slab by diffusion—a slow, constant movement from high-moisture areas (the ground) to lower-moisture areas (the air above).

The vapor barrier is a sheet of impermeable or near-impermeable material placed between the ground and the slab. It interrupts this vapor transmission path before moisture reaches the concrete. Without it, concrete acts as a wick.

The layer stack from bottom to top:

1. Compacted subgrade (native soil)
2. Compacted gravel base (4–6 inches)
3. Vapor barrier (on top of the gravel)
4. Concrete slab

For a slab on grade, this sequence is the baseline. Deviations from it—such as placing the barrier under the gravel—create new problems while trying to solve old ones.

When You Need a Vapor Barrier

Required — Don't Skip

• Any slab with flooring above it. Hardwood, vinyl, tile, carpet—all are damaged or destroyed by moisture vapor transmission. This includes basements, finished garages, and living-space slabs.
• Heated (radiant) slabs. Moisture interferes with heat transfer and can damage heating elements.
• Below-grade slabs (basements). Ground moisture pressure is constant and aggressive in below-grade conditions.
• Any slab over wet soil or high water table. Hydrostatic pressure drives vapor through concrete.

Recommended

• Garage floors you might coat or finish later. Far easier to install during construction than to mitigate afterward.
• Slabs on clay. Clay holds water and releases it slowly. The moisture supply is effectively continuous.
• Any enclosed slab. If walls and a roof trap the air above the slab, moisture that enters has nowhere to go except into the interior.

Optional

• Outdoor slabs (patios, driveways, sidewalks). These dry from the top naturally. A vapor barrier doesn't hurt but provides little benefit.
• Unheated detached sheds with good drainage and air circulation.

For a full breakdown of requirements by project type, including building code context and what happens if you skip one, see our guide on when vapor barriers are required vs optional.

The Main Types

Not all plastic sheeting qualifies as a vapor barrier. The key metric is permeability—measured in perms. A true vapor barrier has a perm rating below 0.1; anything higher is technically a vapor retarder.

Type	Typical Perm	Cost/sq ft	Best For
6-mil polyethylene	~0.08 perms	$0.03–0.06	Not recommended (tears easily, fails current codes)
10-mil polyethylene	~0.05 perms	$0.05–0.10	Budget residential, garages
15-mil reinforced poly	~0.02 perms	$0.08–0.15	Standard residential, basements
Class A membrane (ASTM E1745)	Under 0.01 perms	$0.15–0.30	High-moisture, heated slabs, strict code environments

The short version: use 10-mil minimum for most slabs, 15-mil for basements and code-required projects, Class A membranes where code or your flooring manufacturer specifies. Never use 6-mil as a primary vapor barrier—it's a legacy spec that most codes have moved past.

For detailed specs on each type—including the reinforced vs standard distinction and Class A certification requirements—see vapor barrier types compared.

How Installation Works

Vapor barrier installation is straightforward, but the details matter. The three failure points that account for most moisture problems are:

1. Untaped seams. Overlapping sheets without taping them is almost no better than no barrier at all.
2. Unsealed penetrations. Pipes, conduit, rebar chairs—each one is a gap in the barrier if not wrapped and taped.
3. Wrong position. The barrier goes on top of the gravel, directly under the concrete—not under the gravel.

The ACI 302.2R standard recommends placing the barrier in direct contact with the bottom of the slab (on top of the gravel, not under it). This is the position that performs best in testing. A sand layer between the barrier and the slab—sometimes added to prevent punctures—actually creates a moisture reservoir that can work against you.

For step-by-step installation with seam overlap specs, penetration sealing methods, and pre-pour inspection checklist, see how to install a vapor barrier under a concrete slab.

Vapor Barrier vs. Vapor Retarder

These terms are used interchangeably in conversation but are not the same in codes and product specifications.

• Vapor barrier: Perm rating below 0.1. Almost no moisture passes through.
• Vapor retarder: Perm rating between 0.1 and 1.0. Slows but does not stop moisture transmission.

Flooring adhesive manufacturers typically specify a vapor barrier (under 0.1 perms) for moisture-sensitive installations. Using a vapor retarder where a barrier is specified can void your flooring warranty and lead to adhesive failure.

When a building inspector or flooring spec sheet says "vapor barrier," verify the perm rating of the product you're using. Many products sold as "vapor barriers" at hardware stores are technically vapor retarders at their listed thickness.

Full explanation of the distinction, perm classifications, and how to read product labels: vapor barrier vs. vapor retarder.

Garage Floor Considerations

Garage floors are the most commonly skipped application—and one of the most consequential. If you ever plan to apply an epoxy coating, the moisture barrier decision made at pour time determines whether that coating lasts or peels.

Moisture vapor transmission causes epoxy delamination. A coating that looks perfect the day you apply it can start bubbling within a year on a slab without vapor protection. Retrofitting a topside moisture barrier after the fact adds $1–3 per square foot and requires specific surface prep.

Code requirements differ between attached and detached garages, and whether the space will be heated changes the product specification. See vapor barrier for garage floors for the full breakdown.

Related Preparation Steps

A vapor barrier works as part of a complete subgrade system. Two related guides that cover the work that happens directly before vapor barrier installation:

• Gravel under concrete — the capillary break layer that goes beneath the vapor barrier
• Concrete subgrade preparation — excavation, base material selection, and compaction

Getting those steps right before laying the barrier is just as important as the barrier itself.

Calculate Before You Order

Use our concrete calculator to determine how much concrete your slab requires before you plan materials.

For garage floor projects specifically, our concrete garage floor calculator includes volume and bag count estimates sized to typical garage dimensions.