Vapor Barrier Under a Slab: Thickness and Placement

Vapor barrier under slab construction places a sheet of plastic between the ground and the concrete to block moisture from wicking up through the slab. Moisture vapor can ruin flooring adhesives, cause mold and musty odors, and make basements damp. Building codes commonly require a vapor retarder under slabs for heated living spaces, and it is cheap insurance for garages and workshops too. Outdoor patios and driveways usually do not need one.

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Vapor barrier vs vapor retarder

Technically, most plastic sheeting is a vapor retarder: it slows moisture rather than stopping it completely. The terms are used interchangeably in practice. Codes and product standards describe performance classes based on permeance, strength, and puncture resistance.

Common thicknesses

Thickness	Typical use
6 mil polyethylene	Common code minimum for many residential slabs
10 mil	Better puncture resistance; popular upgrade
15 mil	Premium; often used under floors with moisture-sensitive finishes

Thicker, reinforced products resist tears from foot traffic, rebar chairs, and wheelbarrows during the pour. Products meeting ASTM E1745 classes are commonly specified on higher-end work.

Where it goes in the layers

A typical sequence from the bottom up:

Compacted subgrade.
Compacted gravel base, often about 4 inches. See gravel base under a concrete slab.
Vapor barrier laid directly on the base.
Rigid insulation, if specified, sometimes above or below the barrier.
Concrete slab.

Some designers have placed a layer of sand or gravel over the barrier to help concrete cure evenly, but many current recommendations favor placing concrete directly on the barrier to avoid trapping water in a blotter layer. Follow your plans or local practice.

Installing it right

Overlap seams by at least 6 inches, often more, and tape them with compatible tape.
Seal around pipes and penetrations with tape or boots.
Run it up the walls or footings to the top of the slab at the edges where specified.
Patch tears before the pour.
Avoid puncturing with stakes; use rebar chairs with wide feet.

When you can skip it

Outdoor patios, sidewalks, and driveways, where moisture can escape from the top surface.
Unenclosed sheds or carports, unless you plan to store moisture-sensitive items.
Certain designs where an engineer specifies otherwise.

For hot tub pads, garages, and workshops, a barrier is often worthwhile even when not required. See concrete slab for a hot tub.

Cost

Polyethylene sheeting is inexpensive compared with the concrete, usually a small fraction of the slab’s cost. Tape and penetration boots add a bit more. Upgrading from 6 mil to 10 or 15 mil adds modest cost but significant durability.

Bottom line

A vapor barrier under a slab blocks ground moisture from damaging floors and indoor air. Use at least the code-required thickness, often 6 mil, with 10 or 15 mil for better durability, lay it over compacted base, overlap and tape seams, and seal penetrations. Outdoor slabs usually skip it. Figure base tonnage with the base calculator.

Questions people ask

Do I need a vapor barrier under a concrete slab?

For heated living spaces, codes commonly require one. Garages and workshops benefit too. Outdoor slabs usually skip it.

What thickness vapor barrier goes under a slab?

6 mil is a common minimum, with 10 or 15 mil offering better puncture resistance.

Does the vapor barrier go over or under the gravel?

Typically over the compacted gravel base, directly under the concrete, though designs vary.

Should vapor barrier seams be taped?

Yes. Overlap seams at least 6 inches and tape them, sealing around pipes as well.

Do patios need a vapor barrier?

Usually not. Moisture can escape through the top of outdoor slabs.

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