Vapor Barrier Under a Concrete Slab: Mil, Class and Placement

Once concrete is poured, you have exactly one chance to get moisture control right under it. A concrete slab vapor barrier is the sheet of heavy polyethylene or specialty membrane placed on the prepared subgrade before the pour, and it is the main defense against ground moisture wicking up through the slab. Skip it or install it carelessly and years later you may see cupped hardwood, lifted vinyl plank, failed adhesive, musty basements, or efflorescence on the floor surface.

This guide covers what the barrier does, how thick it should be, what the ASTM classifications mean, where it goes relative to the gravel base, and how to lap, tape, and repair it. It is about new pours only, not plastic sheeting laid on top of an existing slab under new flooring.

What a Concrete Slab Vapor Barrier Does

Soil beneath a house holds moisture even in dry climates. Concrete is porous, so water vapor moves upward from damp ground toward the drier air in your home. A continuous low-permeance membrane under the slab slows that vapor drive to a trickle. The terms vapor retarder and vapor barrier are often used interchangeably in the field; technically, most sheet products are retarders with a very low permeance rating rather than absolute barriers.

Whenever the slab will be finished with moisture-sensitive flooring such as wood, laminate, vinyl, carpet, or glue-down products, or when it forms the floor of conditioned living space, a below-slab barrier is standard practice and required by many building codes. Garages, sheds, and exterior slabs have different requirements; check with your local building department.

Materials and Tools

• Vapor retarder roll (10 mil minimum, 15 mil recommended for living spaces)
• Manufacturer-approved seam tape
• Pipe boots or collars for plumbing and conduit penetrations
• Mastic or sealant compatible with the membrane
• Utility knife and straightedge
• Plate compactor, rake, and level for the subgrade and base
• Soft-soled footwear for the crew walking on the membrane

10 Mil vs 15 Mil: How Thick Should It Be?

Thickness is measured in mils (thousandths of an inch). Older practice used 6 mil polyethylene, and many codes still reference 6 mil as a minimum, but thin plastic tears easily when crews walk on it, drag hoses across it, and wheel concrete buggies over it. Once punctured, it no longer performs as a continuous barrier.

• 6 mil: often allowed by code for basic applications, but prone to damage. Not recommended when flooring will go on the slab.
• 10 mil: a common minimum for residential slabs. Good puncture resistance on a well-prepared base.
• 15 mil: a preferred choice for finished living spaces, basements, and slabs receiving moisture-sensitive floors. A 15 mil vapor barrier roll resists tears from rebar chairs, foot traffic, and stones in the base much better.

For most homes, a concrete slab vapor barrier at 15 mil is the safe default. The price difference between 10 and 15 mil is small relative to the cost of the slab and the flooring above it, which is why many builders and flooring installers default to 15 mil.

ASTM E1745 Class A, B and C

ASTM E1745 is the standard specification for plastic water vapor retarders used in contact with soil or granular fill under concrete slabs. It classifies products by tensile strength and puncture resistance, while all classes must meet a low permeance requirement.

• Class A: highest tensile strength and puncture resistance. Suited to demanding placement conditions.
• Class B: intermediate.
• Class C: the lowest of the three, but still far tougher than generic hardware-store poly that is not rated at all.

Look for a product that states its E1745 class on the roll or data sheet. Generic plastic under concrete floor areas that carries no rating may be recycled material with inconsistent thickness and permeance. Also check for ASTM E154 test data, which is the method used to evaluate these properties.

Placement: Above or Below the Granular Base?

This question has sparked debate for decades. The current prevailing guidance from concrete industry sources favors placing the vapor retarder directly beneath the slab, in contact with the concrete, when the floor will receive moisture-sensitive coverings or be in a climate-controlled space. Placing it on top of the granular base reduces the amount of moisture trapped below the slab that can later migrate upward.

The historic practice of putting a layer of sand or gravel on top of the membrane was meant to help concrete cure evenly and reduce curling. The downside is that the fill layer can become saturated from rain or wet-curing before the pour, creating a reservoir of water sealed under the slab. If you are unsure, follow the design professional’s or local code’s specification, and discuss with your concrete contractor how they will control curling and finishing timing when concrete sits directly on the membrane.

Step-by-Step Installation

1. Prepare the subgrade. Remove organic material, compact the soil, and install and compact the granular base to the specified depth and grade. Rake out sharp stones on the surface.
2. Complete below-slab work. Plumbing, radon piping, electrical conduit, and any under-slab insulation should be in place and inspected.
3. Unroll the membrane. Run sheets in the direction of the pour where practical. Keep the product clean and avoid dragging it over the base.
4. Lap seams. Overlap edges by at least 6 inches, or the amount the manufacturer specifies, and tape the full length with the approved seam tape.
5. Seal penetrations. Use pipe boots or cut the membrane closely around pipes and seal with tape or mastic. Every pipe, conduit, and post should be sealed.
6. Turn up at the perimeter. Extend the membrane up the inside of the footing or foundation wall to the top of the slab, or seal it to the footing as specified.
7. Protect it during the pour. Set rebar chairs on plates or use chairs designed for membranes. Keep workers in soft-soled boots, and do not drive equipment on the sheet.
8. Inspect and patch. Walk the entire surface right before the pour.

Puncture Repair

Small holes and tears are common, and repairing them is simple if you catch them before concrete covers them. Clean the area, cut a patch of the same material that extends at least 6 inches past the damage on every side, and tape all four edges. For larger rips, replace the section and lap and tape the seams as you would at any other joint. Missing even a few punctures can create a concentrated pathway for moisture right where a flooring installation will later show problems.

Common Mistakes

• Using unrated thin plastic that tears during rebar placement.
• Leaving seams lapped but untaped.
• Cutting a big X around plumbing and leaving the flaps open.
• Allowing rain to pool on the membrane before the pour without pumping it off.
• Not turning the membrane up the perimeter.
• Assuming the barrier eliminates the need for moisture testing before installing flooring. New slabs still release a significant amount of construction water as they dry, and flooring manufacturers require testing before installation.

Cost Considerations

Budgeting a concrete slab vapor barrier is straightforward because the membrane is a small share of a slab project.

Rated under-slab vapor retarder rolls generally cost more than generic plastic, and 15 mil products cost more than 10 mil, but for a typical residential slab the material difference is often a small line item compared with concrete, labor, and finished flooring. Seam tape and pipe boots add modestly. Labor to install the membrane properly is usually folded into the concrete contractor’s price.

When to Involve a Professional

For new homes, additions, and basements, the concrete contractor, builder, or structural designer should specify the product and placement to meet local code. Radon-resistant construction, expansive soils, high water tables, and slabs with in-floor radiant heat add design considerations that a professional should address. A properly specified and installed concrete slab vapor barrier is one of the cheapest insurance policies against future flooring failures, and the details at seams and penetrations matter as much as the thickness.