Vapor Barriers Under Concrete: When You Need One and How to Install

A vapor barrier is a sheet of impermeable material placed between the ground and the concrete slab. Its job is to block moisture vapor from migrating upward through the concrete. This sounds simple, but the details--material choice, placement, and overlap sealing--determine whether it actually works. If you're planning a new slab, use our concrete slab calculator to size the project and plan materials.

Check local building codes before your project. Vapor barrier requirements vary by jurisdiction, and some areas mandate specific materials and installation methods.

When You Need a Vapor Barrier

Required (Don't Skip)

• Any slab that will have flooring installed over it. Carpet, vinyl, tile, hardwood--all fail when moisture vapor passes through the slab. This includes basements, finished garages, and living spaces.
• Heated slabs (radiant floor heating). Moisture vapor interferes with heat transfer and can damage heating elements.
• Below-grade slabs (basements). These are surrounded by soil moisture on all sides. Without a vapor barrier, moisture transmission is constant and often leads to the problems described in our basement concrete problems guide.
• Any slab over high water table or wet soil. If the soil is saturated, hydrostatic pressure drives moisture through the concrete aggressively.

Recommended

• Garage floors you might coat or finish later. Easier to install during construction than to mitigate moisture after.
• Slabs on clay soil. Clay retains water and releases it slowly, maintaining constant moisture contact with the slab.
• Any enclosed slab. If the space above will be enclosed (walls and roof), moisture entering through the floor has nowhere to evaporate except into the living space.

Optional

• Outdoor slabs (patios, driveways, sidewalks). These dry from the top naturally and aren't enclosed. A vapor barrier isn't harmful but provides little benefit.
• Detached shed foundations with good drainage and ventilation.

Choosing the Right Material

Not all plastic sheeting is a vapor barrier. The material must meet specific performance standards.

Material	Thickness	Perm Rating	Best For	Cost/sq ft
Polyethylene (standard)	10-mil	~0.05 perms	Budget residential, garages	$0.05-0.10
Polyethylene (reinforced)	15-mil	~0.01 perms	Standard residential, basements	$0.08-0.15
Class A vapor retarder (Stego Wrap, etc.)	15-20 mil	Under 0.01 perms	High-moisture, heated slabs, strict codes	$0.15-0.30

What NOT to Use

• 6-mil poly: Too thin. Tears during construction and doesn't meet current building code requirements in most jurisdictions. It was standard decades ago but has been superseded.
• Roofing felt / tar paper: Not a vapor barrier. It's a moisture retarder at best and degrades when in constant contact with moist soil.
• Recycled or reclaimed plastic: Unknown perm ratings and inconsistent thickness. Not worth the risk.

Installation: Step by Step

Proper installation matters as much as the material itself. A barrier with gaps, tears, or unsealed seams is a barrier that doesn't work.

Preparation

The vapor barrier sits on top of the gravel base, directly under the concrete.

Your layer stack from bottom to top:

1. Compacted subgrade (native soil, excavated and compacted)
2. Gravel base (4-6 inches of compacted crushed stone)
3. Vapor barrier (on top of gravel)
4. Concrete slab

Installation Steps

1. Grade the gravel smooth. The vapor barrier will conform to whatever it's laid on. Sharp rocks, ruts, or uneven gravel create stress points that lead to punctures. Rake the gravel smooth and compact it.

2. Roll out the barrier. Start at one wall and unroll across the full width. Leave 6 inches of extra material at all edges to turn up against foundation walls or forms.

3. Overlap seams by 6 inches minimum. Most manufacturers specify 6 to 12 inches. Some building codes require 12 inches.

4. Tape all seams. Use vapor barrier tape (not duct tape, not packing tape). Seal every overlap, every cut, and every penetration. This is the step most installers rush--and it's where most barriers fail.

5. Seal penetrations. Plumbing pipes, electrical conduit, radon mitigation pipes, and rebar chairs all penetrate the barrier. Each penetration needs to be sealed with vapor barrier tape or compatible sealant.

6. Turn up at edges. Bring the barrier up the foundation walls or form boards at least 3 inches. After the pour, trim the excess flush with the slab surface.

7. Protect during pour. Walk carefully on the barrier during rebar placement and concrete pouring. Have extra tape ready to patch any tears before concrete is placed. For the full pour process, see our guide on how to pour concrete.

Common Installation Mistakes

Placing the barrier under the gravel. This allows moisture to wick through the gravel and contact the slab directly. The barrier goes on top of the gravel, under the concrete.

Not taping seams. Untaped overlaps allow vapor to pass through. The entire point of the barrier is eliminated if seams aren't sealed.

Using rebar chairs that puncture the barrier. Standard metal rebar chairs have pointed feet that poke through the poly. Use chairs with flat bases or place small squares of plywood under each chair.

Tearing and not patching. Construction traffic on the gravel inevitably tears the barrier in spots. Walk the entire surface before pouring and tape every tear.

The Sand Layer Debate

Some builders place a thin layer of sand (1-2 inches) between the vapor barrier and the concrete. The theory is that the sand allows bleed water to escape downward during curing, preventing surface crusting.

The case against sand: Sand between the barrier and the concrete creates a reservoir that traps water against the slab bottom. This can actually increase moisture transmission over time.

Current best practice: Place the vapor barrier directly under the concrete with no sand layer. Modern concrete mix designs and proper subgrade preparation manage bleed water without needing a sand cushion. ACI 302.2R recommends direct contact between the vapor barrier and concrete.

Retrofitting Moisture Protection on Existing Slabs

If your slab was poured without a vapor barrier and you need moisture control for flooring installation:

Topside moisture-mitigating systems:

• Epoxy moisture barriers (Mapei Planiseal, Ardex MC Rapid): Applied to the slab surface, they block moisture from above. Cost: $1-3 per sq ft.
• Sheet membrane systems: Roll-on or peel-and-stick barriers between the slab and flooring. Cost: $0.50-2.00 per sq ft.

Test first: Use a calcium chloride test (ASTM F1869) or relative humidity probe (ASTM F2170) to measure actual moisture emission before choosing a mitigation system. Some flooring products tolerate moderate moisture; others fail at very low levels. If moisture is causing visible surface problems, our concrete floor problems guide covers diagnosis and repair options.

Key Takeaways

• Use minimum 10-mil polyethylene (15-mil preferred) for any slab that will have flooring, coatings, or enclosed space above it
• Place the vapor barrier on top of the gravel base, directly under the concrete--not under the gravel
• Tape every seam, overlap by 6-12 inches, and seal every penetration
• Outdoor slabs (patios, driveways) don't need a vapor barrier
• Existing slabs without barriers can be retrofitted with topside epoxy moisture systems ($1-3/sq ft)
• Check local building codes--requirements vary by jurisdiction

For more project guidance, browse our complete library of concrete guides and tutorials.