How to Install a Vapor Barrier Under a Concrete Slab

For background on why vapor barriers matter and which type your project requires, see our complete vapor barrier guide and vapor barrier types comparison.

What You'll Need

Materials:

• Vapor barrier sheeting (10-mil minimum, 15-mil for enclosed spaces and basements — see types guide)
• Vapor barrier tape (not duct tape — use product-specific seaming tape)
• Pipe boots or sleeve seals for penetrations
• Compatible sealant for penetration boots

Tools:

• Utility knife
• Tape measure
• Marker or chalk line

Planning note: Order approximately 10–15% more material than the bare slab area to account for overlaps and edge turnup. For a 20×20 slab (400 sq ft), order 440–460 sq ft of barrier material.

Use our calculator to determine slab volume, then plan material quantities:

The Correct Position: On Top of Gravel, Under Concrete

This is the most common positioning mistake, and it matters more than most installers realize.

Correct: Subgrade → compacted gravel → vapor barrier → concrete

Wrong: Subgrade → vapor barrier → gravel → concrete

Placing the barrier under the gravel allows moisture to wick through the gravel and pool against the bottom of the slab. The gravel's capillary break function only works if the barrier is above it. ACI 302.2R—the industry standard for slab construction—recommends placing the vapor barrier in direct contact with the bottom of the concrete.

The sand debate: Some builders add a 1–2 inch sand layer between the vapor barrier and the concrete to allow bleed water to drain downward during curing. Current best practice from ACI and most testing is against this approach—the sand layer becomes a moisture reservoir that works against the barrier long-term. Modern concrete mix designs manage bleed water without a sand cushion. Leave the barrier in direct contact with the underside of the slab.

Step-by-Step Installation

Step 1: Prepare the Gravel Surface

Before the barrier goes down, the gravel base must be ready. This step is often treated as an afterthought but directly affects barrier integrity.

• Compact the gravel to the specified depth (4–6 inches for most slabs)
• Rake the surface smooth, eliminating ridges, ruts, and high-low spots
• Remove protruding stones — anything sharp enough to tear 10-mil poly gets pulled or pushed flat
• Walk the area once and check for any spots that feel soft underfoot

A smooth, compacted gravel surface means fewer punctures and a barrier that lies flat without air pockets underneath. See our guides on gravel under concrete and concrete subgrade preparation for base prep specifics.

Step 2: Lay Out the Barrier Sheets

Start at one wall and unroll the vapor barrier across the full slab width. Work toward the opposite wall in parallel strips.

• Orientation: Run strips perpendicular to the pour direction. This positions seams where they're easiest to tape and least likely to be stressed during the pour.
• Edge turnup: At every edge, leave a minimum of 6 inches of material running up the inside of form boards or foundation walls. This edge material gets trimmed after the pour.
• Handle carefully: Don't drag the barrier across the gravel—it tears more easily in tension. Unroll it in place.

Step 3: Lap and Tape All Seams

This step is where most vapor barriers fail. Overlapping sheets without taping them provides almost no benefit over a single-sheet barrier with gaps.

Overlap requirements:

• ACI 302 minimum: 6-inch overlap on all seams
• Many building codes: 12-inch minimum — check local requirement before starting
• General best practice: 12 inches. The cost of extra material is negligible.

Taping correctly:

• Use manufacturer-specified vapor barrier seaming tape — 3M 8086, Stego Tape, or equivalent
• Do not use standard duct tape, packing tape, or electrical tape — these fail over time and in the presence of moisture
• Press tape firmly from one end of the seam to the other with no gaps, bubbles, or lifted edges
• Tape the full seam length — don't spot-tape at intervals

Tricky areas: Corners, where the barrier turns up at edges, and areas around existing columns or posts all require careful attention. Cut and fold the material to lie flat, then tape all cuts.

Step 4: Seal All Penetrations

Every object that passes through the vapor barrier is a potential failure point. Common penetrations in residential slabs:

• Plumbing supply and drain pipes
• Electrical conduit
• Radon mitigation pipes
• Rebar chairs (the legs of standard metal rebar chairs puncture the poly)

For pipes: Wrap vapor barrier tape tightly around the pipe and press firmly onto the barrier, creating a sealed collar. Use a pipe boot (sleeve-style fitting) for cleaner sealing on larger penetrations.

For rebar chairs: Use chairs with flat plastic bases instead of pointed metal feet. If using standard metal chairs, place small squares of scrap barrier material under each chair foot and tape them down. Running rebar through standard metal chairs across bare poly is one of the leading causes of barrier failure.

Step 5: Pre-Pour Inspection

After rebar placement and before calling for concrete, do a complete walk of the barrier surface.

What to look for:

• Tears and punctures from construction traffic (patch with tape or a piece of extra barrier)
• Unsealed or partially sealed seams
• Penetrations that weren't fully taped
• Corners or edges that have pulled away from forms
• Areas where workers walked directly on the poly over sharp gravel — these often have hidden micro-tears

Mark all problem areas during your walk, then patch before the pour. A 5-minute pre-pour inspection prevents the kind of moisture problems that take thousands of dollars to remediate later.

Step 6: Pour Concrete

Pour concrete directly onto the vapor barrier. Concrete will briefly apply significant weight and movement pressure to the barrier during placement — this is why proper sealing matters.

• Avoid dragging screed boards or concrete rakes aggressively across the barrier edge — this can shift unsealed seams
• Tamp or vibrate concrete gently near edges where the barrier turns up — don't pull the turnup material down into the pour
• After the pour sets, trim excess barrier material flush with the slab edge using a utility knife

Common Installation Mistakes

Placing the barrier under the gravel. Moisture collects between the gravel and the poly and makes direct contact with the slab bottom.

Not taping seams. The most common failure mode. An overlapped-but-untaped seam is a gap in the barrier.

Using standard duct tape on seams. Duct tape fails in sustained moisture contact and loses adhesion over time. Use vapor barrier-specific seaming tape.

Skipping penetrations. Each unsealed pipe or conduit penetration is an active moisture pathway.

Using 6-mil poly. Tears too easily during construction and may not meet current code. Use 10-mil minimum.

Tearing and not patching. Construction traffic inevitably tears poly in spots. These must be patched before the pour — not ignored.

After the Pour

Once the slab cures, trim excess barrier material at the edges. The barrier is now permanently encapsulated between the gravel and the concrete and will function for the life of the slab.

If a slab was poured without a vapor barrier and moisture problems emerge later, topside mitigation systems exist (epoxy moisture primers, sheet membranes) but cost $1–3 per square foot and require surface preparation. Installing correctly the first time is far cheaper.

For guidance on what happens if you skip a vapor barrier and what "required vs optional" actually means by application type, see vapor barrier under concrete: when you need one.