Vapor Barrier for a Basement Floor | Options, Installation and Cost

Do I Need A Vapor Barrier In My Basement Floor and What Will it Cost?

$0.30 – $1.75 per square foot installed

To find out if you need a vapor barrier between your basement concrete slab and floor covering consult with local contractors and check local and provincial/state building codes. As a rule of thumb, if there is any hint of moisture in your basement then installing a vapor barrier below grade is recommended. The price you pay for your vapor barrier will increase in line with the thickness and quality to your chosen barrier.

Vapor barriers aren’t always necessary and can, in some circumstances, do more harm than good. If you live in a particularly hot and dry climate you may not need a vapor barrier. However, more often than not, installing a moisture barrier is good practise and not a step you want to skip just to save yourself a few bucks. Read on to find out why.

WHAT IS A VAPOR BARRIER? WHAT DOES IT DO?

A vapor barrier is a material that prevents moisture from passing through it. Types are described below, but the simplest vapor barrier basement floor option is a sheet of polyethylene plastic.

Technically speaking, when used on a basement floor, the material is called a moisture barrier. But the terms are synonymous since the materials are designed to stop or slow the transfer of moisture and they can also be effective against the flow of ground gases like radon.

WHY A BASEMENT FLOOR VAPOR BARRIER IS NECESSARY?

Because a vapor barrier is cheap and basement flooring is expensive!

Here are the details.

Concrete allows moisture to pass through it. Seems unlikely, since the stuff is so hard and basement floor slabs are normally 4 inches thick. But it’s true.

Rainwater seeps into the ground and can travel beneath your concrete slab. Some groundwater also rises beneath the concrete.

And moisture seeks to “spread out,” so it travels from a place of high humidity to a place of lower humidity. Meaning what? If a quality vapor barrier isn’t in place, water will travel from the soil under and around your foundation into the interior of your basement. Concrete will wick the water from below and carry it to the surface.

There’s no need to detail what would happen next in the form of mold, musty smells and wet, ruined basement flooring. All for lack of an affordable, easy-to-install basement floor vapor barrier.

Climate plays a part: In dry climates, contractors often skip the vapor barrier. But in average to humid climates, and anywhere ground water is abundant, an impermeable vapor barrier is essential.

But there’s a vapor barrier beneath my basement slab! First, that’s a rarity for residential construction, though common for commercial buildings. Secondly, are you sure it’s in good condition and still keeps out moisture? Again, because vapor barriers are cheap and flooring isn’t, there’s little expense and no harm in adding one inside.

Try this test: This might convince you of the need for a vapor barrier. Cut a piece of clear plastic/poly about 2 feet square. Clean an area of the concrete floor, and tape down the plastic on all sides. Give it 24-48 hours. Is there moisture on the concrete side of the plastic? If so, it shows moisture migration up through the slab, and you surely need a basement moisture barrier. If it’s dry, we refer again to the principle that a vapor barrier is cheap, and flooring isn’t. Our recommendation is to install one anyway, especially if you test during a time of the year when there is little rain or melting snow.

BASEMENT FLOOR VAPOR BARRIER OPTIONS AND INSTALLATION

There are 3 good vapor barrier basement floor options. Here they are in order of use, from most to least common.

How do you install these vapor barrier basement floor materials? We let you know for each. In our opinion, DIY is something you should consider if so inclined.

Plastic Vapor Barrier Rolls

This is the top choice for contractors and DIY homeowners. Rolls of plastic (polyethylene or “poly”) are available from 6 mils to 20 mils. Rolls are typically about 3 feet to 10 feet wide. Start at an edge/corner, and roll out the barrier, and cut it at the far wall. Overlap the next piece by about 6 inches and tape the seams with quality seam tape (not cheap plastic “duct” tape).

Or choose a self-sealing roll vapor barrier. Quite a few roll vapor barriers have peel-and-stick adhesive strips incorporated into them. Overlap the pieces, peel the backing, and stick the top layer to the lower layer. Edges are taped at walls too.

Underlayment with Attached Vapor Barrier

There is a range of these including those made from rubber, synthetic rubber, foam and cork. They have a thin, often 6 mil, poly vapor barrier attached. Underlayment with an attached vapor barrier is a good choice for a floating floor basement installation.

Most are rolls. Lay out the roll, and cut to length. Roll out and cut the adjoining piece. Butt and tape the seams. A few do have self-sealing adhesive strips. The result is a floating underlayment ideal for use on concrete.

Note on Features: Some underlayment with vapor barrier adds cushioning and sound control to the flooring. A few have some R-value, R-3 is common, meaning they are insulated, so they make the floor a bit warmer. Find out more about insulating basement flooring options.

Carpet Pad

This is a sub-group of the “underlayment with vapor barrier attached” category, but it’s unique enough to merit mention. Products like Shaw Courage Antimicrobial pad have moisture barrier on both sides. Seams should be taped, as usual. Most carpet padding is 6’ wide.

Installation Tips

These will help with the performance of the vapor barrier and flooring.

Direction: When using roll underlayment or glue-down vapor barrier, lay it perpendicular to how you intend to install the floor.

Check and address slab/wall joints: If there are gaps between the slab (floor) and wall, they should be filled with waterproof caulk and/or covered with vapor barrier. If the gaps go all the way through the slab, consider having them professionally sealed or, at minimum, wrap the gaps with 6 mil or thicker plastic and tape the edges on the wall and floor.

COST OF BASEMENT VAPOR BARRIER

Basement vapor barrier cost is $0.10 to $1.35 per square foot. Since that’s not very helpful, here’s a chart with cost by type.

Vapor Barrier
Materials/sq. ft.
Installed Cost/sq. ft.
6 Mil Poly $0.10 – $0.15 $0.30 – $0.45
12 Mil Poly $0.12 – $0.20 $0.32 – $0.50
20 Mil Poly $0.18 – $0.25 $0.35 – $0.55
Self-adhering Poly $0.15 – $0.30 $0.30 – $0.55
Underlayment w/ Vapor Barrier $0.55 – $1.25 $0.70 – $1.65
Carpet Pad w/ Vapor Barrier $0.75 – $1.35 $1.00 – $1.75

 

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