Moisture Barriers Under Slab Foundations: What Louisiana Homes Need

Moisture Barriers Under Slab Foundations: What Louisiana Homes Need

A moisture barrier under a slab foundation is a heavy plastic sheet blocking ground moisture from rising into your concrete — among the most overlooked parts of a Louisiana slab.

What does a moisture barrier under a slab foundation do for a Louisiana home? It stops damp earth beneath your house from pushing water vapor up through the concrete, protecting your flooring, indoor air, and the slab itself. Newer homes usually have one; many older Shreveport homes have nothing at all between slab and soil.

Here is why that matters more here than in drier parts of the country. Shreveport gets roughly 50 inches of rain a year, our summers run hot and humid, and the ground under most of the metro is expansive clay that holds water like a sponge. All of that moisture sits right against the underside of your slab, 24 hours a day, for the life of the house. Without a barrier, physics does the rest: water vapor migrates upward through the porous concrete, looking for the drier air inside your home.

Most homeowners never think about this layer because they never see it. You notice the symptoms instead — the musty smell in a back bedroom, the laminate floor that cups at the edges, the white powdery stains creeping across the garage slab. This post explains what a moisture barrier is, what happens when a slab does not have one, and what your options are whether you are building new or dealing with an existing home.

What a Moisture Barrier Actually Is

Despite the technical-sounding name, a moisture barrier — usually called a vapor barrier or vapor retarder in the trade — is refreshingly simple: a sheet of heavy polyethylene plastic laid over the prepared ground before the concrete is poured. Current best practice calls for 6-mil to 10-mil polyethylene (a “mil” is a thousandth of an inch), with seams overlapped and taped so the sheet acts as one continuous membrane. Better installations use a puncture-resistant product meeting the ASTM E1745 standard, which is the industry benchmark for under-slab vapor barriers.

It sits in a simple sandwich: compacted soil or gravel base on the bottom, the plastic sheet in the middle, concrete on top. Once the slab cures, that plastic is the only thing standing between tons of damp Louisiana clay and the inside of your home. Concrete looks solid, but it is porous — water vapor passes through it the way air passes through a screen door, slowly but constantly. The barrier breaks that path.

One point of confusion worth clearing up: a moisture barrier is not waterproofing. It does not stop liquid water flooding under a slab during a heavy rain, and it is not a substitute for drainage. It stops vapor — the invisible moisture migration that happens every day, rain or shine. Liquid water problems need drainage correction; vapor problems need the barrier. Many Shreveport homes need attention to both.

Why Louisiana Slabs Need One More Than Most

Three local conditions gang up on slabs here, and a moisture barrier is the defense against all three.

First, the rain. Northwest Louisiana averages around 50 inches of rainfall a year, spread across every season — spring thunderstorms, tropical systems in late summer, steady winter rains. The soil around and beneath Shreveport-area slabs in neighborhoods like Southern Hills, Broadmoor, and Ellerbe rarely dries out deeply. That is a constant moisture source pressing against the slab from below.

Second, the clay. Our native soil is expansive shrink-swell clay, and clay holds water far longer than sandy soil does. After a wet spring, the ground under a home in 71106 or 71118 can stay saturated for weeks. All that stored moisture becomes vapor drive — the pressure pushing water vapor upward through the concrete. In sandier regions, moisture drains away and the problem is smaller. Here, the clay keeps it coming.

Third, the humidity. Shreveport summers are famously humid, and air conditioning creates a temperature difference across the slab: cool interior air above, warm damp soil below. That temperature gradient actually increases vapor drive, pulling moisture upward faster in July and August than in January. It is the same physics that fogs a cold glass on a hot day, playing out silently under your flooring for months at a time.

The U.S. Department of Energy’s guidance on moisture control in homes confirms the principle: controlling vapor at the foundation is one of the most effective ways to protect a house in humid climates like ours.

What Happens When a Slab Has No Moisture Barrier

Plenty of Shreveport-area homes — especially ranches built from the 1950s through the 1970s in areas like Highland, Queensborough, and parts of Bossier City — were poured directly on grade with no vapor barrier at all. It simply was not standard practice yet. Here is what decades of unblocked vapor drive can do:

• Flooring failures. This is the most expensive symptom. Moisture vapor breaks down adhesives under vinyl and laminate, causing bubbling, cupping, and lifting. Hardwood floors cup at the edges and develop gaps. Homeowners often replace flooring two or three times before anyone looks at the slab underneath.
• Musty odors and mold-friendly conditions. Constant low-level moisture feeds mildew in carpeting, baseboards, and drywall bottoms. That persistent musty smell in a back bedroom or converted garage is a classic sign.
• Efflorescence. Those white, powdery mineral deposits on garage or patio slabs are dissolved salts carried up by migrating water and left behind when it evaporates. Cosmetic on the surface, but proof that vapor is moving freely through the concrete.
• Higher indoor humidity. A slab without a barrier can contribute meaningful moisture to indoor air, making the AC work harder and leaving the house feeling clammy even at 72 degrees.
• Concrete deterioration over time. Decades of moisture cycling can weaken the slab surface, causing dusting, scaling, and spalling — and in our expansive clay, a chronically wet subgrade can contribute to the soil movement behind slab foundation settlement.

If you are seeing several of these at once — say, cupping floors plus a musty smell plus white stains in the garage — the slab is telling you a consistent story. Our slab foundation repair evaluations always look at moisture conditions, not just movement, because the two are so often connected.

New Construction: Getting the Barrier Right

If you are building or adding on, this is the cheapest moisture protection you will ever buy — a few hundred dollars of plastic and labor that protects the slab for its entire life. Here is what a proper installation looks like:

The most common failure is the thin 6-mil sheet from the hardware store, laid with loose overlaps and no tape, then punctured by foot traffic before the pour — technically a barrier, functionally full of holes. If you are building in Caddo or Bossier Parish, specify 10-mil or an ASTM E1745 product in writing and ask to see it before the concrete truck arrives.

Existing Homes: Your Options When There Is No Barrier

Here is the hard truth first: you cannot slide a moisture barrier under an existing slab. The concrete is already there. But you can absolutely manage the moisture — the goal shifts from blocking vapor at the source to controlling it at the surfaces and reducing the moisture load in the soil.

Topical sealers and epoxy coatings are the closest thing to a retrofit barrier. A penetrating concrete sealer reduces vapor transmission through the slab surface; a full epoxy or moisture-mitigation coating system — the kind used before installing flooring over problem slabs — can cut vapor drive dramatically. These are especially useful in garages and workshops where you want a durable, good-looking floor anyway.

Dehumidification and ventilation handle the indoor side. A whole-home or dedicated dehumidifier keeps indoor relative humidity in the 40–50 percent range, which stops the musty smells and protects flooring even when some vapor still migrates. In our climate, this is good practice for slab homes regardless.

Exterior moisture management reduces the source. Proper grading away from the foundation, gutters with downspouts discharging well away from the house, and French drains where water collects all lower the moisture content of the soil against the slab. Less water in the soil means less vapor drive — simple as that. If plumbing leaks under the slab are contributing, those need fixing too, since a leak can saturate the subgrade far beyond what weather alone does.

A foundation repair professional evaluating a slab home should be checking all of this — moisture readings, drainage, grading — alongside the structural measurements. The slab’s movement and its moisture problems usually share the same root causes.

Moisture Barriers and Foundation Settlement: The Connection

Homeowners sometimes wonder whether a missing barrier can actually cause a foundation to settle. The direct answer: the barrier itself does not hold the house up, so its absence does not cause settlement the way bad soil does. But the moisture it fails to block absolutely contributes to the conditions that cause settlement in our clay.

Expansive clay moves with moisture content — that is the whole story of Shreveport foundation problems. Anything that keeps the subgrade chronically wetter on one side of the house than the other creates differential movement: the wet side swells and lifts, the dry side stays put, and the slab between them stresses and cracks. Poor drainage is the usual culprit, but unblocked vapor and chronic dampness under the slab add to the moisture imbalance. It is one more reason our services treat moisture control and structural repair as two halves of the same job.

Frequently Asked Questions

Does my slab foundation have a moisture barrier?

If your Shreveport-area home was built before roughly the 1980s, probably not — vapor barriers under slabs were not standard practice then. Homes built from the 1990s on generally have at least a 6-mil poly sheet. The only way to know for sure on an existing slab is a core sample or documentation from the original builder.

Can you add a moisture barrier under an existing slab?

No — the slab is already poured, so there is no way to get a sheet underneath it. For existing homes, the practical equivalents are topical sealers or epoxy moisture-mitigation coatings on the slab surface, interior dehumidification, and exterior drainage improvements that reduce the moisture load in the soil.

How thick should a vapor barrier be under a slab?

Current best practice is 10-mil polyethylene or a product rated to the ASTM E1745 standard, with overlapped and taped seams. Thinner 6-mil sheeting is the historical minimum but punctures easily during construction, which is why the heavier products are worth specifying on any new build.

Will a moisture barrier stop my slab from settling?

Not directly — settlement is caused by soil movement, not vapor. But by keeping the subgrade moisture more consistent, a barrier removes one contributor to the wet-dry cycling that makes our expansive clay heave and shrink. Think of it as moisture management, not structural support.

What are the signs my slab has no vapor barrier?

Cupping or bubbling floors, persistent musty smells, white efflorescence stains on exposed concrete, and indoor humidity that stays high despite the AC running. If you see several of these together, especially in an older home, a missing barrier is a likely contributor.

Is a moisture barrier required by code in Louisiana?

Modern residential building codes generally require a vapor retarder under slabs in most climate conditions, and Louisiana follows the International Residential Code framework. But codes only apply at the time of construction — older homes are grandfathered, which is why so many existing slabs here have nothing underneath.

How much does it cost to deal with slab moisture problems?

It ranges widely: a penetrating sealer for a garage might run a few hundred dollars in materials, professional epoxy moisture-mitigation systems typically run several dollars per square foot, and exterior drainage work ranges from a few hundred for regrading to several thousand for French drains. A free inspection is the way to find out which pieces your home actually needs.

Pelican Foundation Repair
6118 Fairfield Ave, Shreveport, LA 71106, USA
318-585-2003
https://foundationrepairshreveport.com/