Vapor barrier for a basement floor: what each floor type needs

Use one vapor barrier in a basement floor, and only over a slab that tests dry. The best spot is under the concrete, where sheet plastic sits right against the slab.¹ Over an existing slab, the floor type decides the layer. Laminate and engineered wood float on an underlayment with a built-in moisture barrier.² Vinyl plank already blocks vapor.¹ Carpet needs the driest slab of all. If the plastic sheet test shows water, fix the water first.

TL;DR

• Building Science Corporation puts the floor's vapor barrier under the slab, in direct contact with the concrete and over crushed stone or rigid foam.¹
• Two stacked barriers hold any moisture between them where it can't dry. Pick one layer for your floor and stop there.
• Underlayments with a built-in moisture barrier are sold for laminate, vinyl plank and engineered wood. Skip the separate plastic sheet when your underlayment already has one.²
• Water under the test sheet means moisture is coming up through the slab. Water on top of the sheet means humid room air is meeting cold concrete.
• A slab that gets wet needs drainage or waterproofing before any floor goes down. Plastic laid over a wet slab traps the water and can feed mold.¹

Should you put a vapor barrier on a basement floor?

Yes. A basement floor should have a vapor barrier, and the best place for it is under the concrete slab. Building Science Corporation's basement guidance puts sheet polyethylene in direct contact with the slab, over a crushed stone capillary break.¹ What you add on top of an existing slab depends on the floor you choose.

The reasoning comes from how a basement dries. The traditional approach keeps liquid, capillary and vapor water out from the outside. The basement then dries inward, where ventilation or a dehumidifier removes the moisture.¹ A barrier under the slab fits that plan. It stops ground moisture below the concrete and leaves the top of the slab free to dry into the room.

The stone layer does its own job. Crushed stone or coarse gravel under the slab acts as a capillary break, so water can't wick up from wet soil into the concrete.¹ The same digest warns against a sand layer between the plastic and the slab. Sand there can become saturated, and that water can only dry upward through the slab. The usual result is damaged floor finishes.¹

The digest dates from 2006 and describes how a slab should be built. Most homeowners are working with a slab that's already poured. You usually can't see what's under it. Ask the builder if you can. Check any plans you have. Look at the cut edge of the slab at a sump pit or floor drain. If you find no sign of plastic, treat the slab as one that may pass moisture and test it before you pick a floor.

How do you do the plastic sheet test on a basement slab?

The plastic sheet test takes three steps. Tape a square of clear plastic tight to bare concrete on all four edges. Leave it in place. Then lift it and look at both sides. Damp concrete or droplets under the plastic mean moisture is coming up through the slab.

Set it up

1. Pick bare concrete. Paint, sealer or glue residue will skew the result.
2. Clean the spot and let it dry.
3. Lay a square of clear plastic flat on the slab.
4. Tape all four edges with duct tape so no room air gets under the sheet.
5. Repeat in a few places. Include any spot that has ever looked damp and a spot near an outside wall.
6. Leave the sheets for the time your contractor or your flooring's installation instructions call for.

Read the result

Darkened concrete or droplets on the underside of the plastic point to the slab. Moisture is moving up through it, and any floor you lay will sit on top of that moisture.

Droplets on the top of the plastic point to the room. Warm, humid air is condensing on a surface that sits at slab temperature. Building Science Corporation says the main problem for below-grade walls comes in summer, when warm moist air meets cold basement surfaces below its dew point.¹ The fix for that result is humidity control in the room.

Both sides dry is a good sign for that spot in that season.

Know what it can't tell you

The test is a quick screen. It doesn't give you a number, and one dry result in a dry month doesn't prove the slab stays dry all year. If your flooring's installation instructions name a moisture test or a moisture limit, that test is the one that counts. A slab that passes in winter can still sweat in August, so ask your contractor to check it again closer to install day if months have passed.

Which vapor barrier goes with each floor type?

Match the barrier to the floor, and use one layer only. Floating laminate and engineered wood go on an underlayment with a built-in moisture barrier.² Vinyl plank is already a low-permeance layer, so its own installation instructions decide what goes under it.¹ Carpet and pad hold water, so carpet needs the driest slab of the four.

The installed costs are HomeGuide's national ranges for each material, and HomeGuide describes laminate as difficult to repair and not suitable for humid spaces.³ The vinyl row comes from Building Science Corporation, which lists vinyl floor coverings among impermeable interior finishes.¹ For a wider comparison of looks and price, see the guide to basement flooring options.

Luxury vinyl plank (LVP)

Vinyl plank already blocks vapor. Building Science Corporation lists vinyl floor coverings among the impermeable interior finishes a basement should avoid. It says they inhibit inward drying and typically lead to mold growth and other moisture problems.¹ LVP is thicker and more durable than standard vinyl.³ The planks still stop water vapor.

With vinyl plank in a basement, the slab's moisture decides everything. On a dry slab with a barrier underneath, the vinyl has little moisture to trap. On a slab that passes moisture, the moisture collects at the concrete surface under the vinyl with nowhere to go.

A plastic sheet under the planks adds a second low-permeance layer. Moisture still collects at the concrete, under the lowest layer. So the extra plastic changes little on a dry slab and fixes nothing on a wet one. Underlayments with a moisture barrier are sold for vinyl plank.² Use one where the plank's installation instructions call for it.

Laminate

Laminate needs a dry basement. HomeGuide calls it easy to install and scratch resistant, but difficult to repair and not suitable for humid spaces.³ It floats over an underlayment. Underlayments sold for laminate include a vapor barrier film, some with an overlap and a pre-attached tape strip for sealing the seams.²

Pick one barrier. If the underlayment has a moisture barrier built in, lay it straight on the slab and skip the separate plastic sheet. If your installer prefers a plain foam pad, the plastic sheet goes under it with taped seams. Either way you end up with one barrier layer.

Engineered wood

Engineered wood has a composite core topped with a thin layer of solid wood veneer.³ Wood swells and shrinks with moisture. Some flooring needs to sit in the room for a day or more before installation so it can adjust to the room's temperature and humidity.³

Floating engineered wood uses the same kind of moisture-barrier underlayment as laminate. Some products are sold for engineered hardwood, laminate and vinyl plank with a click lock system.² If your installer plans to glue the floor down, the adhesive and flooring instructions set the moisture rules.

Carpet

Carpet is still a common choice for finished basements.³ It's also the floor that suffers most from a damp slab. Carpet and pad soak up water and dry slowly, and HomeGuide's advice is that basement flooring should be waterproof or water-resistant unless the space is finished and sealed.³

Pad costs $0.75 to $1.75 per square foot installed.³ It insulates and protects what's beneath it.³ Run the plastic sheet test in several spots before you choose carpet. If any spot shows moisture from below, pick a floor that handles it better or raise the carpet on a subfloor.

What is the best vapor barrier for a concrete basement floor?

The best vapor barrier for a concrete basement floor is sheet polyethylene under the slab, in direct contact with the concrete. Building Science Corporation places it over crushed stone or rigid foam insulation.¹ Over an existing slab, the best barrier is the single layer your flooring's installation instructions call for.

Under the slab, when a slab is poured or replaced

Building Science Corporation recommends rigid foam under a basement slab, with extruded or expanded polystyrene both widely used with success. The polyethylene goes over the foam and directly against the concrete. Sand never goes between them.¹ Sub-slab foam saves less energy than wall insulation. It still gives a significant improvement in comfort and in moisture damage resistance, including against summertime condensation.¹ The same digest has the slab sealed to the perimeter wall so the concrete also works as an air barrier against soil gas.¹ Soil gas includes radon, so test the house and use a licensed radon mitigation pro if the result calls for one. Whether a new slab needs a vapor retarder by code depends on the building code edition your area adopted and its local amendments, so your contractor confirms it with the building department when pulling the permit.

Over the slab: underlayment with a built-in barrier

This layer suits floating laminate and engineered wood, and vinyl plank where its instructions allow. Underlayment costs $0.50 to $3.50 per square foot installed.³ HomeGuide says it's typically needed under linoleum, laminate, vinyl and hardwood.³ Look for one that names your floor type and has taped or overlapping seams.²

Over the slab: plastic under a raised subfloor

A raised subfloor puts space and insulation between the slab and the finished floor. One build described on Green Building Advisor uses a sheet of 6-mil polyethylene glued to the slab, an inch of EPS foam on top and a plywood or OSB subfloor screwed through.⁴ Dimpled plastic mats are also sold as waterproof floor underlayment for basements.² Both raise the floor, which affects doors, stairs and ceiling height. Your contractor prices that at your estimate.

What to leave out

Building Science Corporation doesn't recommend radiant barrier bubble insulation under a slab, because it doesn't provide enough insulation or value.¹ Leave out sand between plastic and concrete. Leave out a second barrier when the first one is already in place.

When should you not use a vapor barrier on a basement floor?

Skip a new vapor barrier when the floor already has one layer, when the slab gets wet, or when the plastic would go on basement walls. In each case the plastic blocks drying and holds moisture against concrete or wood. Building Science Corporation links interior plastic that blocks drying on basement walls to odor, mold, decay and corrosion problems.¹

When the floor already has a barrier

Moisture caught between two low-permeance layers can't dry in either direction. Building Science Corporation's sand-layer warning is the clearest example. Water held above the under-slab plastic can only escape upward through the slab, and the usual result is damaged floor finishes.¹ A plastic sheet stacked under a barrier underlayment or under vinyl builds a smaller version of the same trap.

When the slab gets wet

Plastic over a slab only covers the water that reaches the concrete, and it hides that water from view. Under a sheet, the water stays against the concrete and the underside of your floor.

Groundwater control starts outside. Building Science Corporation's list begins with gutters that carry roof water away from the foundation and ground that slopes away from the house. It continues with a perimeter drain and a crushed stone drainage layer under the slab connected to it.¹ HomeGuide puts basement waterproofing at $6,000 to $18,000.³ If your test came back wet or you've seen water on the floor, sort out basement waterproofing before the floor goes down.

Mold is the expensive outcome. HomeGuide puts mold removal at $15 to $30 per square foot.³ If you find mold on the slab or under old flooring, have a mold remediation professional assess it before any new floor goes in.

On basement walls

Building Science Corporation says no interior vapor barriers should go on basement walls, so the wall can dry inward. It recommends foam insulation that lets the wall dry.¹ Some inspectors still ask for interior polyethylene, as one Canadian builder found.⁵ In that case, a Green Building Advisor editor suggested a "smart" vapor retarder is better than polyethylene.⁵ Local codes differ, so your contractor should confirm what your building department expects.

What to ask your contractor before the floor goes down

Ask your contractor five questions before any flooring goes over the slab. The answers show whether the moisture plan fits the floor you chose. A good quote lists the moisture test, the barrier layer and the underlayment, so you can see what each one costs.

1. Does my slab have a vapor barrier under it, and how do you know?
2. Which moisture test will you run, in how many spots, and for how long?
3. Which single barrier layer goes with the floor I picked, and what do its installation instructions require?
4. If the test shows moisture, what do you recommend first: drainage, waterproofing or a different floor?
5. Is the underlayment its own line in the quote? HomeGuide's checklist for flooring estimates names the flooring, the underlayment and the installation.³

Basement Remodeling Co. matches you with one vetted, licensed basement contractor. You get a free on-site estimate and a written, itemized quote. The contractor checks moisture at that visit. If you're planning a first build-out, start with basement finishing. If the space is already finished and the old floor is coming up, see basement remodeling. You can find the contractor who covers your ZIP code, including for basement finishing in metro Atlanta and basement finishing in metro Denver.