Under-slab vapor barrier and slab moisture field guide

What the under-slab vapor barrier does, and why

An under-slab vapor barrier is a plastic sheet laid on the ground under a concrete slab to keep soil moisture from moving up into the slab. The ground under a slab is almost never dry. Water sits in the soil and the granular base, and the difference in vapor pressure between the wet ground and the drier conditioned space above pushes that moisture up as water vapor. The slab does not stop it. Concrete is porous, and over months and years that vapor drives straight through the slab and arrives at the bottom of whatever flooring or coating you put down. The barrier is the layer that stops the drive at the source.

This is the part people get backwards. They think the slab itself is the moisture barrier and the sheet under it is insurance. It is the other way around. A bare slab on damp ground is a wick, not a seal. The vapor barrier is the working layer and the slab is the thing it protects, along with the flooring on top.

Slab thickness, the subgrade, and the base course are their own design, and we cover them in the slab on grade design guide by topic. The vapor barrier is a separate decision that rides on top of that base, and it gets specified for the flooring above, not for the structure of the slab.

What slab moisture does to the flooring

Moisture coming up through a slab does not show up the day the floor goes in. It shows up months later, after the crew is long gone and someone else owns the callback. That delay is exactly why it gets skipped and exactly why it is expensive.

The adhesive goes first. Vapor rising through the slab carries dissolved alkali to the surface, the pH at the bottom of the flooring climbs, and the high pH breaks down the glue. Tile lets go and rocks underfoot. Vinyl plank pops loose and the seams telegraph. On wood, the moisture swells the boards from the bottom and they cup, every plank curling up at the edges in a washboard you can feel through your shoes. Sheet goods blister. Coatings, including epoxy, lose bond and peel in sheets, and we cover coating and self-leveling moisture limits by topic in the related material.

Then there is the mold. Trapped moisture under impermeable flooring is a dark, wet space, and that is where mold grows, which turns a flooring problem into an indoor-air problem. Flooring failures from slab moisture run into the billions across North America every year, and the fix is almost always tearing the floor back out, mitigating the slab, and laying it again. You pay for the barrier once or you pay for the floor twice.

What is the difference between a vapor barrier and a vapor retarder?

A vapor barrier and a vapor retarder are the same kind of product graded by how much vapor they let through, measured in perms. A perm is grains of water vapor per hour per square foot per inch of mercury of pressure difference. Lower is tighter. The line the industry draws is 0.1 perms: a vapor barrier runs at or below 0.1 perms and lets almost nothing through, while a vapor retarder sits between 0.1 and 1.0 and only slows the moisture down.

The names get used loosely on the jobsite and in the specs, which causes real trouble. A 'retarder' that passes a tenth of a perm is fine under a slab that will stay bare or take a forgiving finish. Under moisture-sensitive flooring it is not enough, because the flooring manufacturer wrote the warranty around a true barrier. Some manufacturers push the number lower still, talking about 0.01 perms for the sheet they want to see under glued-down goods.

The practical rule: read the perm number, not the word on the label. An ASTM E1745 Class A sheet at or below 0.1 perms is what most commercial specs mean when they say barrier, and it is what the flooring manufacturer is usually counting on whether the drawing calls it a barrier or a retarder.

ASTM E1745 and the Class A, B, C grades

ASTM E1745 is the standard specification for the plastic sheet used as a vapor barrier in contact with soil or granular fill under a concrete slab. It is the number to know, because it is the one the codes and specs point at. The standard tests three things: water vapor permeance, tensile strength, and puncture resistance. A sheet has to pass all three to be classed.

The permeance bar is the same across every class. The sheet has to come in at or below 0.1 perms to carry an E1745 classification at all, so any compliant product is already at the barrier line on vapor. What separates the classes is toughness. Class A is the strongest, Class C the weakest, with B in between, and the split is about surviving the jobsite, not about stopping more vapor.

The numbers are worth carrying. Class A is roughly 45 lbf per inch of tensile strength and about 2200 grams of puncture resistance. Class B drops to about 30 lbf per inch and 1700 grams. Class C is about 13.6 lbf per inch and 475 grams. Under moisture-sensitive flooring, the guidance points hard at Class A, and most commercial specs and the residential code now call for Class A specifically. The reason is simple: the tougher sheet is the one that still has no holes in it after the rebar and the crew have been on top of it.

Is 6-mil poly a vapor barrier?

Six-mil construction poly is not the under-slab vapor barrier you want under moisture-sensitive flooring, and treating it like one is one of the most common and costly slab mistakes in the trade. It is cheap, it tears on the base, it has a higher perm rating than a real barrier, and it does not meet the toughness of ASTM E1745 Class A. The roll on the truck that says vapor barrier on the sticker is usually the wrong product.

Thickness is a stand-in for performance, and the trade has moved up. The common under-slab barriers run 10, 15, and 20 mil, and they are engineered sheets, not the painter's plastic that shares the aisle. The 2021 residential code change makes the floor official: it raised the minimum under-slab vapor retarder from 6 mil to a minimum 10 mil conforming to ASTM E1745 Class A. So the old 6-mil habit is not just weak, on many jobs it no longer meets the adopted code.

Pick the thickness for the abuse the sheet will take, not the lowest line that passes. A 10-mil Class A sheet is a reasonable floor for light residential work. On a slab with heavy rebar, machine traffic during the pour, or a long exposure before concrete, step up to 15 or 20 mil so the sheet survives to the pour with its perm rating intact. A barrier with a puncture in it is not a barrier anymore.

Does the vapor barrier go directly under the slab?

For a slab that will take moisture-sensitive flooring, the vapor barrier goes directly under the slab, in contact with the bottom of the concrete, with no sand or granular blotter between the sheet and the slab. ACI Committee 302 changed its guidance to this years ago, around 2001, and the change settled a long argument in the field.

The old practice put a blotter course of sand or fine fill over the barrier and poured on that, the idea being the blotter would soak up bleed water and let the slab dry more evenly from both sides. In the field it did the opposite. The blotter took on water from rain before the pour, or it filled through a tear or an unsealed penetration in the sheet, and then it became a reservoir of trapped water sitting under the slab with nowhere to go but up. The slab pulled moisture out of that wet sand for years. The cure was worse than the problem it was meant to solve.

So the modern call for moisture-sensitive floors is barrier directly under the slab on top of the prepared base. There is a real tradeoff that comes with it, which is how the slab dries and whether it curls, and that is the next section. For a slab that will stay bare or take a forgiving finish, a blotter is sometimes still used, but once a moisture-sensitive floor is in the picture, the sheet goes against the concrete.

Does a vapor barrier make the slab curl?

Putting the barrier directly under the slab changes how the slab dries, and that is the honest tradeoff. With the sheet against the bottom, the slab cannot give up any moisture downward into the ground. Everything has to leave through the top. So the top dries and shrinks while the bottom stays wet, the two faces change volume by different amounts, and the slab can curl, lifting at the edges and joints. That is the mechanism people blame the barrier for.

Here is where the field belief and the testing part ways. Recent work found that slabs cast directly on the barrier actually curled less than slabs cast on an open sub-base, and that curling tracks the concrete mix, the paste content, more than it tracks the sheet under the slab. The barrier is not the villain it gets called.

The real lever is the mix and the finish, not skipping the sheet. A low water content, a moderate cement factor, and a well-graded aggregate with the largest size the job allows all cut the shrinkage that drives curling. Good curing helps. Skipping the barrier to fix curling trades a manageable mix-and-curing problem for a permanent moisture problem, which is a bad trade. Fix the curling in the mix design and keep the barrier.

Laps, seams, and sealing the penetrations

A vapor barrier only works as a continuous skin. Any gap in it, a loose seam, an open hole at a pipe, a torn edge at a wall, becomes the path the whole slab's moisture funnels through. ASTM E1643 is the installation standard that pairs with E1745, and it is the one that tells you how to keep the sheet continuous.

Lap the seams at least 6 inches and seal them with the manufacturer's tape or accessory, not whatever tape is in the gang box. Seal the sheet to its boundaries: terminate and seal it to the foundation wall, the grade beam, or the slab edge so moisture cannot sneak in around the perimeter. Every penetration through the slab gets sealed, and there are a lot of them: plumbing risers, conduit stub-ups, sleeves, and the spots where the sheet meets columns and footings. At a pipe, cut a boot or a detail patch and lap it 6 inches onto the field sheet all the way around the penetration, then seal it.

Footings and columns are where this falls apart on real jobs, because the sheet has to be cut and fit around hard geometry while everyone wants to pour. Those interruptions are exactly where moisture gets in if they are not detailed. A barrier that is perfect across the field and open around twelve column bases is not a barrier. Treat the penetrations as the job, not the cleanup.

Protecting the sheet from puncture

The barrier passes its puncture test in a lab and then meets the actual jobsite, which is harder on it than any test. Between the day it goes down and the day concrete covers it, every boot, wheelbarrow, rebar end, and chair leg is trying to put a hole in it. A sheet with holes is a retarder at best, no matter what class it started as.

Rebar support is the usual culprit. Standard chairs with sharp feet press into and through the sheet under the weight of the steel and the crew. Use chairs with broad feet or runners made to sit on a vapor barrier, or set a pad under the legs. Keep wheel traffic and material staging off the sheet where you can, and route the pour so the buggies are not grinding across bare membrane. Pulling rebar across the sheet drags the ends through it like a knife.

Walk it before the pour and repair every hole you find. The repair is a patch of the same material lapped and sealed at least 6 inches past the damage on all sides. This is not optional housekeeping. The window between placing the sheet and placing the concrete is the only time the barrier is reachable, and a hole you leave is a hole you cannot reach again for the life of the slab.

How do you test a slab for moisture before flooring?

You test the slab's moisture before any flooring goes down, with one of two ASTM methods, and you do it because the barrier protects the slab from the ground but the slab still has to dry out its own mix water before it can take a floor. A new slab is wet from the day it is poured. The test tells you when it is ready.

The in-situ relative humidity test, ASTM F2170, is the one most flooring manufacturers now lean on. You drill a hole into the slab, set a sleeve, let it equilibrate, and read a calibrated RH probe down in the slab. For a slab drying from one side, the hole goes to 40 percent of the thickness; for two-sided drying, 20 percent. It reads the moisture deep in the slab, which is what predicts how the floor will behave over time. A common acceptance band is around 75 to 80 percent RH, but the flooring manufacturer's number is the one that governs.

The calcium chloride test, ASTM F1869, measures the moisture vapor emission rate, the MVER, by sealing a dish of salt under a dome and weighing the moisture it pulls in over about three days. It reports pounds per 1000 square feet per 24 hours, and a common limit is 3 pounds. It reads the surface, not the depth, which is why the RH method is preferred for the floors that fail expensively. Skip the test and you own the flooring failure. We cover coating and self-leveling underlayment moisture limits by topic in the related material.

Do you need a vapor barrier under a slab?

You need an under-slab vapor barrier any time the slab will be inside a conditioned space or will carry moisture-sensitive flooring, a coating, or a finish that traps moisture. That covers most interior slabs in occupied buildings: offices, retail, warehouses that will get sealed floors, anything with vinyl, wood, tile, carpet over adhesive, or epoxy. For those, the residential code now requires a minimum 10-mil ASTM E1745 Class A sheet, laps at least 6 inches, between the slab and the base or the prepared subgrade, and commercial specs follow the same logic.

Where you do not strictly need one is the slab that stays open to the weather or unheated, where there is no moisture-sensitive floor and no comfort concern. The code carries explicit exceptions for that: garages, unheated accessory buildings, small unheated storage rooms, carports, and exterior flatwork like driveways, walks, and patios, plus cases the building official approves for local site conditions.

The trap is the slab that gets enclosed later. A garage or a storage building that is unheated today and finished into a conditioned room in five years now has a moisture-sensitive floor over a slab with no barrier, and there is no way back. If there is any chance the space gets enclosed and heated down the road, put the barrier in now. It is cheap during construction and impossible after.

The vapor barrier and radon

The same sheet that stops water vapor also slows soil gas, so under-slab vapor barriers do double duty in radon and methane control. Radon comes up out of the soil and into the building through the slab, the same path the moisture takes, and a continuous low-perm sheet is part of the resistance to that flow.

On its own the barrier slows soil gas but does not move it. In a radon-prone area the barrier works as part of an active system: sub-slab depressurization, sometimes called active soil depressurization. A network of perforated pipe in the gravel under the slab connects to a fan that pulls a slight vacuum beneath the slab, and the sheet helps hold that low-pressure field so the fan draws the radon out from under the building instead of letting it seep up through the floor.

If the project has a radon or soil-gas requirement, that requirement can drive the sheet selection, often toward a heavier 20-mil or a purpose-made gas barrier, and it adds the pipe and the riser to the under-slab work. Coordinate the gas membrane and the moisture barrier as one system rather than two, because they share the same sheet and the same sealed penetrations.

Slabs that carry sensitive equipment

Some slabs carry more than flooring. Data halls, labs, clean spaces, and any room full of electronics live on a slab whose moisture and the alkali it brings up can foul access-floor systems, corrode hardware, and feed the kind of humidity swings sensitive gear hates. On those jobs the under-slab barrier is not a flooring afterthought. It is part of keeping the room dry, and it usually gets specified at the high end, a heavy Class A sheet, sealed hard, with the moisture testing held to a tight number before anything goes on the slab.

The below-grade case is its own animal. A slab against the water table, a basement floor, or a pit, is fighting liquid water under hydrostatic pressure, not just vapor, and that is a waterproofing problem, not a vapor-barrier problem. We cover below-grade slabs and walls by topic in the below-grade waterproofing guide. Do not reach for a vapor barrier to solve a water-table problem; the two are different systems for different forces.

The pre-pour inspection

The barrier gets inspected before the pour, because after the pour there is nothing left to inspect. This is a hold point, and the slab on top of a bad barrier looks exactly like the slab on top of a good one. The only chance to catch it is while the sheet is still bare.

Walk the whole sheet. Check that it is the specified product and class, not the 6-mil roll someone grabbed. Check the laps are at least 6 inches and taped continuously, not just weighted down with dirt. Check the perimeter is sealed to the wall, grade beam, or slab edge. Then work every penetration, every pipe, every conduit, every column and footing, and confirm each one is booted and sealed. Last, hunt for punctures across the field, especially under and around the rebar where the chairs sit, and confirm the repairs are patched and lapped 6 inches past the damage.

The inspector and the flooring installer care about different things here, and both are right. The inspector wants the code-required class, thickness, laps, and sealed penetrations. The flooring installer wants a barrier that will actually let the slab hit the moisture number later. Pass the walk before the concrete trucks are on site, not while they are backing in.

Maintenance and the next floor

A vapor barrier is buried for the life of the building and needs no maintenance, which is exactly why getting it right at the pour is the whole game. There is no service interval, no inspection cycle, no fixing it later. The maintenance happens once, before the concrete, and then never again.

The decision that comes back around is the next floor. When a building gets a new floor in ten years, somebody is putting a moisture-sensitive product on a slab whose history they do not know. Re-test the slab with ASTM F2170 before the new floor goes down, treat the old slab as an unknown, and check whether the original barrier was ever there at all. An old slab that has no barrier under it will keep failing floors no matter how good the new install is, and the only real fix is a topical moisture-mitigation system applied to the slab surface before flooring. That is a known product category for exactly this case, but it is a repair, not a substitute for getting the barrier in at the pour.

What to document

Write down what went under the slab, because the barrier is invisible the moment concrete covers it and the questions come years later. The record is what answers them: which sheet, how it was sealed, and whether the slab tested dry before the floor went on.

For each slab area, capture the barrier product and ASTM E1745 class, the thickness in mils, the lap width and the seam tape used, how the penetrations and the perimeter were sealed, and the result of the pre-pour walk. Then capture the moisture test before flooring: the method, the readings, the acceptance number, and the date. If a topical mitigation system was added, record it too. The person laying the next floor reads this record, or they fly blind.

Common mistakes

• Calling 6-mil construction poly a vapor barrier when the slab takes moisture-sensitive flooring; it does not meet ASTM E1745 Class A or the current code minimum.
• Putting a sand or granular blotter over the barrier under a moisture-sensitive floor, which traps water under the slab and feeds it up for years.
• Lapping the seams but not sealing them, or weighting them with dirt instead of taping them, so the sheet is not continuous.
• Leaving pipes, conduit, columns, and footings unsealed, so moisture funnels through the penetrations the field sheet was supposed to stop.
• Puncturing the sheet with sharp rebar chairs and foot traffic and not repairing the holes before the pour.
• Installing no barrier under a slab that will be enclosed and heated later, leaving no way to fix it once the floor is on.
• Laying flooring without an ASTM F2170 or F1869 moisture test, then owning the adhesive failure and the cupping months later.

Field checklist

Standards and references

ASTM E1745 is the standard specification for the plastic vapor barrier sheet used under a slab, and it sets the perm rating at or below 0.1 plus the Class A, B, and C grades for tensile strength and puncture resistance. ASTM E1643 is the companion installation practice that covers the laps, the seaming, the penetration sealing, and the repairs. Cite both: one for the sheet, one for the install.

ACI 302, the guide for concrete floor and slab construction, is where the placement guidance lives, including the recommendation to put the barrier directly under the slab for moisture-sensitive floor coverings and to drop the old blotter layer. ASTM F2170 and ASTM F1869 are the slab moisture tests, the in-situ RH probe and the calcium chloride MVER, that decide whether the slab is dry enough for the floor.

On the code side, the residential code, in its slab-on-grade floor provisions, requires a minimum 10-mil ASTM E1745 Class A sheet with 6-inch laps under conditioned and moisture-sensitive slabs, and carries the exceptions for garages, unheated structures, and exterior flatwork. The exact section numbers and the thickness floor move between code cycles, so confirm them against the edition the jurisdiction has actually adopted and any local amendments before you cite them. Above all of it, the flooring manufacturer's installation requirements and the project specification control, and where they are stricter than the code or the ACI guide, they win.

Units, terms, and conversions

The vapor barrier shows up under a few names and a couple of unit systems, so the same sheet reads differently across a drawing set, a manufacturer cut sheet, and a spec.

It gets called a vapor barrier, a vapor retarder, an under-slab membrane, or a sub-slab membrane, and the names are used loosely, which is why the perm number matters more than the label. Permeance is measured in perms, US perms in the ASTM test, where 0.1 perms or less is the barrier line. Thickness is given in mils, where 1 mil is one thousandth of an inch, so a 15-mil sheet is 0.015 inch. Puncture resistance comes in grams and tensile strength in pounds-force per inch. Moisture results come two ways: relative humidity in percent from the F2170 probe, and MVER in pounds per 1000 square feet per 24 hours from the F1869 calcium chloride test.

Perm

Permeance unit, grains of water vapor per hour per square foot per inch of mercury; 0.1 perms or less marks a vapor barrier

Vapor barrier vs vapor retarder

Barrier is 0.1 perms or less; retarder is 0.1 to 1.0 perms and only slows vapor

Mil

One thousandth of an inch; common under-slab sheets are 10, 15, and 20 mil

ASTM E1745 Class

Grade A, B, or C by tensile strength and puncture resistance; all classes are 0.1 perms or less

MVER

Moisture vapor emission rate, pounds per 1000 square feet per 24 hours, from the ASTM F1869 calcium chloride test

In-situ RH

Relative humidity measured by an ASTM F2170 probe set into the slab, reported in percent

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