Vapor Barrier Under a Concrete Slab

What Does a Vapor Barrier Under a Concrete Slab Actually Do?

A vapor barrier interrupts capillary action and vapor diffusion, the two mechanisms by which ground moisture travels upward into a concrete slab. Capillary action pulls liquid water through the pore network of the concrete matrix. Vapor diffusion moves water molecules through the slab as gas, driven by the difference in relative humidity between the soil, which typically holds 90 to 100 percent relative humidity, and the conditioned space above, which typically holds 30 to 60 percent relative humidity. The barrier eliminates both pathways at the interface between the subgrade and the slab.

The practical result is a drier slab surface. A slab poured without a vapor barrier over damp soil can test above 80 percent relative humidity internally, which exceeds the adhesive manufacturer threshold of 75 to 80 percent RH for most flooring installations. A slab poured over a properly installed 10-mil barrier consistently tests at lower internal RH values, allowing flooring to be installed sooner and with less risk of adhesive failure or cupping.

What Thickness and Type of Vapor Barrier Is Required?

The minimum specification for a vapor barrier under an interior concrete slab is 6-mil polyethylene with a permeance of 0.1 perms or lower, measured per ASTM E1745. For occupied spaces, residential slabs on grade, and any slab that will receive adhesive-set flooring, 10-mil or 15-mil polyethylene is standard practice because the thicker film survives concrete placement and compaction without puncture. The permeance rating matters more than thickness alone. A product labeled vapor retarder with a rating above 0.1 perms does not meet the threshold for a true vapor barrier.

Three classes exist under ASTM E1745:

• Class A: 0.1 perms or lower, rated for worst-case conditions, required under slabs in contact with high-moisture subgrade
• Class B: 0.1 perms or lower, same permeance threshold, lighter puncture resistance, suited to slabs with a granular base layer providing physical protection
• Class C: 0.3 perms or lower, a vapor retarder rather than a true barrier, not appropriate beneath flooring adhesives or moisture-sensitive coatings

Verify the product data sheet against these values before purchase. Marketing terms like heavy-duty or commercial-grade carry no standard definition.

Where Exactly Does the Vapor Barrier Go?

The vapor barrier goes directly beneath the concrete slab, not beneath the gravel or crushed-stone base. Placing the barrier below the base layer was a common practice for decades, justified by the argument that sharp aggregate would puncture the film. Research and field experience have moved the industry toward the sub-slab position because placing the barrier under the gravel allows the gravel layer itself to absorb and hold moisture, which then migrates into the slab from below. The gravel becomes a moisture reservoir rather than a drainage layer.

The correct sequence from bottom to top is: compacted subgrade, 4 inches of compacted crushed stone or clean gravel, vapor barrier lapped and taped, concrete slab. The gravel provides drainage for any liquid water and a level bed for the film. The film sits directly against the concrete so that no moisture reservoir exists between the barrier and the slab.

On slabs receiving radiant heating tubes or hydronic systems embedded at 1 to 2 inches below the slab surface, the vapor barrier position does not change. The barrier still sits directly below the concrete, and the tubing rests on top of the barrier before the pour.

How Do You Install a Vapor Barrier Without Voiding Its Function?

A vapor barrier fails at its seams if the seams are not properly lapped and taped. Lap each sheet over the adjacent sheet by a minimum of 12 inches. Tape every seam with a pressure-sensitive seaming tape rated for below-slab use. Standard duct tape degrades in the alkaline environment beneath concrete and fails within months. Use a tape specified for polyethylene film seaming.

Run the film up the interior face of the foundation wall or stemwall by 6 inches and secure it before the pour. This closes the gap at the perimeter where vapor most commonly bypasses a barrier that was simply trimmed flush to the edge. Penetrations for plumbing stubs, conduit, and anchor bolts must be sealed individually. Cut the film in an X pattern around each penetration, fold the flaps down against the pipe or conduit, and tape the perimeter of the penetration with seaming tape. A single unsealed 2-inch pipe penetration can allow as much vapor transmission as several square feet of unprotected slab, depending on soil conditions.

During placement, workers walk on the film. Use knee boards and plywood scraps to distribute weight and avoid tears. Inspect the film for visible tears or punctures before ordering the concrete truck. Patch any damage with a 12-inch overlap of new film taped on all four sides.

What Goes Wrong When a Vapor Barrier Is Missing or Installed Incorrectly?

Four failure patterns appear repeatedly when vapor barriers are omitted or poorly installed.

First, flooring adhesive failure. Most vinyl, hardwood, and carpet adhesives are rated for slab surfaces at or below 75 to 80 percent relative humidity. A slab over wet soil without a barrier regularly exceeds that threshold. The adhesive softens, the flooring lifts, and the repair requires removing all flooring, grinding the adhesive residue, installing a topical moisture mitigation system at $3 to $8 per square foot, and re-laying the flooring.

Second, efflorescence and surface scaling. Water carrying dissolved calcium hydroxide migrates through the slab and evaporates at the surface, depositing a white crystalline residue called efflorescence. The efflorescence itself is cosmetic, but the moisture movement that causes it also dissolves the calcium silicate hydrate matrix that gives concrete its strength, leading to surface scaling over time.

Third, mold under carpet and pad. Carpet and pad trap vapor at the slab surface and hold relative humidity above 80 percent continuously, which is sufficient for mold growth within 24 to 72 hours of saturation. Remediation costs for mold in a slab-on-grade residence run $2,000 to $15,000 depending on extent.

Fourth, coating delamination. Epoxy and polyurea coatings applied to slabs with elevated moisture vapor emission fail by blistering. A 100-square-foot blister field is a complete coating failure requiring full strip-and-recoat. Moisture vapor emission above 3 pounds per 1,000 square feet per 24 hours is the general industry threshold for coating application; unprotected slabs over saturated soil routinely exceed 8 to 12 pounds.

Does the High Water Table in Greater New Orleans Change the Specification?

In the Greater New Orleans area, the water table sits within 1 to 3 feet of the surface across most of the south shore, the west bank, and the surrounding parishes. Soil is predominantly soft alluvial clay and organic silt, both of which hold water at near-saturation levels year-round. This is not a marginal moisture condition. A standard 6-mil barrier is the code floor, not the practical recommendation. Field practice here moves immediately to 15-mil Class A film, fully lapped and taped, with particular attention to perimeter sealing at the stemwall because foundation walls in this region also admit lateral moisture.

Any slab in Orleans, Jefferson, St. Bernard, or St. Tammany Parish that will receive flooring, coating, or interior finish should be treated as a high-moisture subgrade condition regardless of the time of year or current soil appearance. Dry-weather soil borings in this region are misleading. The soil reverts to near-saturation after any significant rain event, and the slab above it will respond accordingly if the barrier is inadequate.

Is Installing a Vapor Barrier a DIY Task or a Contractor Job?

Installing a vapor barrier before a new pour is within the range of a careful DIYer on a small flatwork project such as a detached garage slab or a small patio. The film is inexpensive, 15-mil polyethylene runs $0.05 to $0.12 per square foot, and the installation requires only a utility knife, seaming tape, and attention to overlap and penetration sealing. The difficulty is not the barrier itself but maintaining the barrier's integrity through the concrete pour, which involves a pump truck, vibration, and multiple workers walking the slab.

On any interior slab, slab with embedded utilities, or slab that will receive moisture-sensitive flooring or coatings, use a concrete contractor who has installed the barrier before and knows how to protect it during placement. A torn or untaped barrier on an interior slab is worse than a well-documented absence, because the barrier creates a false assumption of protection that delays identifying the problem until flooring or coatings have already failed.

If a slab already exists and shows moisture problems, a topical moisture mitigation system or a new topping slab over a retrofit barrier are both options, but the right choice depends on the slab thickness, the flooring plan, and the actual moisture vapor emission rate, which requires testing before any product is specified. A concrete contractor with moisture mitigation experience can test the slab, measure the emission rate, and recommend a remediation path that fits the flooring schedule and budget.