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Polyethylene Under Concrete Slabs

Does the poly vapor barrier belong above the rigid foam or below the rigid foam?

Posted on Jul 4 2014 by user-756436

What goes under the concrete in a slab-on-grade home? In the old days, not much — just dirt. Eventually, contractors discovered that it made sense to include a 4-inch-thick layer of crushed stone under the concrete. The crushed stone provides a capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break that reduces the amount of moisture flowing upward from the damp soil to the permeable concrete.

Since the crushed stone layer provides a fairly uniform substrate, it also may also reduce the chance that a concrete slab will be poorly supported by random pockets of soft, easily compressible soil.

Eventually, polyethylene was invented. Concrete contractors learned that a layer of poly helps to keep a slab dry, because it stops upward vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. from the soil.

Finally, some contractors in cold climates began installing a continuous horizontal layer of rigid foam insulation under their concrete slabs. The foam layer isolates the room-temperature slab from the cold soil under the slab.

Getting the sandwich layers in the right order

At this point, we’ve got a sandwich with three or four layers, and the question arises: does the order of the different layers matter? What goes down first, and what goes down last?

According to most building scientists, here’s how the layers should go, from the bottom up: crushed stone; rigid foam; polyethylene; concrete.

Can I still use blotter sand?

Some contractors may ask: Is it a mistake to put the polyethylene lower down in the sandwich? The answer is yes. To understand why, it’s useful to study the history of blotter sand.

Beginning in 1989, the American Concrete Institute (ACI) recommended the installation of a 4-inch layer of granular material between a sub-slab vapor retarder and a concrete slab. ACI standard 302.1 R-96, Guide for Concrete Floor and Slab Construction, included this recommendation in Section 4.1.5: “If a vapor barrier or retarder is required due to local conditions, these products should be placed under a minimum of 4 inches (100 mm) of trimable, compactable, granular fill (not sand).”

Although the ACI specified “granular fill,” most residential builders found it more convenient to use sand. This layer became known as the “blotter sand” layer.

After a rash of flooring failures, the standard was revised

Because a layer of blotter sand between the polyethylene and the concrete allows for faster slab finishing and tends to reduce slab curling, especially if the concrete has a high water/cement ratio, many residential builders readily adopted the ACI recommendation. The use of blotter sand became particularly common in California.

In the late 1990s, however, some builders of slab-on-grade homes began receiving regular reports of flooring failures. Investigators discovered that many of these failures could be blamed on the inclusion of blotter sand above the poly.

Finally bowing to the evidence, the ACI eventually reversed the long-standing but controversial recommendation. In the April 2001 issue of Concrete International magazine, the ACI advised that for slabs with vapor-sensitive coatings (i.e., virtually any kind of flooring), the vapor retarder should be installed directly under the slab, with no intervening layer of granular fill. In November 2001, the ACI included the updated recommendations in a new edition of ACI standard 302.1R-96.

Concrete contractors love blotter sand

Contractors used blotter sand because it provided certain undeniable benefits. Sand helps protect the polyethylene vapor retarder from abuse. With a layer of blotter sand, excess water can drain out of the fresh concrete, making it easier to place concrete with a high water/cement ratio. In contrast, when polyethylene is installed directly under the slab, bleed water dissipates more slowly and concrete finishing is delayed. When excess water leaves the slab from the top side only, the slab may cure unevenly, in some cases resulting in slab curling, shrinkage, or cracking.

When I wrote an article on the topic of blotter sand for the May 2002 issue of Energy Design Update, I interviewed Howard Kanare, who at that time was a senior principal scientist at Construction Technology Laboratories in Skokie, Illinois. “On one side are the folks concerned with construction practices; they don’t want to see a plastic sheet directly under the slab,” Kanare told me. “On the other side are the people who have investigated failures. We almost never see a failure due to moisture infiltrating concrete when the concrete is directly on the plastic sheeting.”

A sub-slab sponge that stays wet

While a sponge under the floor may help contractors during construction, it will probably be a headache for those who have to live in the house. “The problem is that the blotter layer needs some moisture to be compacted,” Kanare said. “It might be at a moisture content of seven percent, and if the layer is four inches thick you have a couple of pounds of water under each square foot. You are essentially forming a bathtub. It’s a long-term source of moisture to recharge the concrete.”

Water added for compaction is just one of many possible sources of moisture keeping blotter sand damp. Another is rain, as I learned when I spoke with Kelley Roberts, a water-intrusion consultant at Construction Forensics in Huntington Beach, California. “At one home I investigated, they put down a bulletproof vapor barrier with two inches of sand on top, and then poured a 6-inch slab,” said Roberts. “During the 30 days between the slab pour and when they got the building dried in, there were 4.5 inches of rain. But they went ahead and installed the flooring and then they cranked the air conditioner up. The VCT blew off in just 45 days.”

Water can also reach the sand layer by capillary action from the edges of the slab, especially if homeowners have lawn sprinklers. In some cases, it may be able to come up from below through rips in the poly. “When you have sand with a moisture content of 18 percent, you are talking about waiting years, not months, for the slab to dry out,” said George Donnelly, a concrete and flooring consultant in Hemet, California.

Building scientist Joe Lstiburek agrees with Donnelly. Lstiburek wrote, “The sand layer is wetted by liquid phase wetting in a time frame measured in minutes. Whereas the sand layer can only dry upwards by vapor phase drying in a time frame measured in years.”

Slow down and be patient

Flooring failures over damp slabs have particularly plagued builders in the South. “In Southern California and Florida there is a tendency to do things quick and cheap, and concrete tends to be made with a higher water/cement ratio,” Kanare told me. Such a mix results in concrete that is less dense and more permeable than a mix with a lower water/cement ratio. “Even in a relatively dry environment, the moisture in the earth under a slab will eventually come up to 95 percent relative humidity. When the air conditioning is cranked up, the relative humidity of the indoor air is low, and so you have a terrific pump going to pull moisture up from the slab.”

In most cases, damp blotter sand eventually dries, but it may not dry as fast as the flooring contractor would like. “Everyone wants to get the flooring down in a hurry, and no one wants to take the time for a calcium box test,” said Roberts.

The flooring just popped

Until recently, homeowners with slab problems were mostly upset about their ruined flooring. But some homeowners have begun to notice that their failed flooring has mold on the underside, adding a whole new dimension to flooring litigation.

“Flooring failures over damp slabs are a huge issue in these class-action suits against builders,” said Donnelly. When flooring pops off a slab, repairs aren’t cheap. Once the flooring is demolished, the slab is usually shot-blasted in preparation for topside sealing.

After the sealer has cured, new flooring can be installed. “We just finished investigating a failure in a 1,200-square foot room,” Roberts told me in 2002. “It cost $10,000 to seal the slab. That’s not including $30,000 it cost to remove and replace the flooring.”

All kinds of flooring have been known to fail over damp slabs, including vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate)., carpet, and wood flooring. “The sheet vinyl flooring discolors, and a wood floor will swell,” said Donnelly. “Sometimes you lift an object like a child’s toy box, and underneath the carpet is damp.” The common denominator in all of these failures is a damp slab. “With VCT, the first thing you see is the white glue oozing out of the joints,” says Roberts. “With a wood floor, you can get cupping and buckling. If they find mold on the underside of the floor, now it turns into tens of thousands of dollars.”

Concrete specifications matter

Many residential builders pay little attention to concrete slab specifications, at least until they have flooring problems. But “anyone who has been successfully sued starts to pay attention,” said Donnelly. With the recommendations of the ACI 302 committee now in line with most building science experts, there seems to be a firm consensus that the vapor retarder belongs immediately under a concrete slab.

Lstiburek tells contractors, “Repeat after me: don’t ever, ever, put a layer of sand between a plastic vapor barrier and a concrete slab — don’t even think about it.”

Still, some concrete contractors may be reluctant to abandon blotter sand, fearing problems with excess bleed water, slab curling, and cracking. But by switching to higher-strength concrete, reducing the water/cement ratio, and using adequate steel to prevent curling, these problems can be controlled. “The water/cement ratio should not exceed 0.45,” said Kanare. “You should specify a 5-sack mix, and the largest possible sized aggregate. For a 4-inch slab, you should be using 1-inch aggregate.”

In an article on blotter sand, Lstiburek imagined the objections of a concrete contractor from California: “‘Yeah, but if I don’t put the sand layer in there it will take too long to finish the floor.’ Yes, that’s true if you use crappy concrete with too much water in it. The easy answer is: don’t use crappy concrete.”

Some builders who have long used blotter sand may be tempted to substitute a layer of crushed stone for the blotter sand layer. According to Lstiburek, however, this won’t work. “Large quantities of water are still held in the pea gravel — think surface area of the gravel and the fines in the pea gravel. Pea gravel does drain, but it retains huge amounts of water even though it drains.”

Test the concrete

If you are worried about flooring failures, it makes a lot of sense to perform a calcium chloride test before you install the flooring. This test will verify that the slab’s moisture level has dropped to within the specifications of the flooring manufacturer.

After all, the cost of the test is a lot cheaper than litigation.

Can I put the polyethylene under the rigid foam?

Some builders like to install polyethylene under a layer of rigid foam, so that the sandwich looks like this (from the bottom up): crushed stone, polyethylene, rigid foam, concrete. Is that OK?

Well, it’s not as bad a sandwich as one that includes a layer of blotter sand above the poly. But it’s still not optimal. If it rains after the rigid foam is installed but before the concrete is placed, the polyethylene can hold hidden puddles.

It’s always best to install the polyethylene on top of the rigid foam, so that it ends up directly under the concrete.

Is it important to make sure there are no holes in the polyethylene?

Some energy-efficient builders misunderstand the purpose of the polyethylene under a slab. They evidently think that the polyethylene is an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., so they go to great lengths to seal every seam with expensive tape and to tape all pipe penetrations. These builders stay up at night worrying that the concrete contractors will put a little hole in their precious polyethylene.

They should stop worrying. The polyethylene is a vapor barrier, not an air barrier. It's perfectly OK for the polyethylene to have a few holes.

The concrete slab is your air barrier. If you want an airtight floor — and there are several reasons you should, including radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. worries — then you need to seal cracks at the perimeter of the slab with high-quality caulk, and you need to seal penetrations through the slab. This work is performed after the concrete has cured.

A vapor barrier can have a few holes and still perform perfectly well as a vapor barrier. If 5% of the surface area of a vapor barrier is missing entirely, the vapor barrier performs 95% as well as a vapor barrier that is intact.

Here's what Lstiburek has to say about about polyethylene under a slab: "You can poke holes in it, you can puncture it, you can tear it, you can leave gaps in it, and pretty much have your way with it as long as it is in direct contact with the concrete. ... Air barriers need to be continuous and free from holes, but vapor barriers do not need to be. Lots of vapor moves by air movement, not a heck of a lot of vapor moves by vapor diffusion. The concrete slab is the air barrier, and the ripped and torn and punctured polyethylene sheet is the vapor barrier. ... I could wear golf shoes and march around the plastic vapor barrier and not do much damage. "

[Author's note: Portions of this article appeared in an article (“Blotter Sand Woes”) that I wrote for the May 2002 issue of Energy Design Update.]

Martin Holladay’s previous blog: “Banish These Details From Your Plans.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. R. Ian Parlin / Oyster River House

Jul 4, 2014 8:20 AM ET

Edited Jul 4, 2014 8:21 AM ET.

Perfect timing
by Oberon

Thanks Martin, your timing was perfect.

I will be pouring a basement slab for a small room addition within the next couple weeks and I was just starting to research how to stack the layers - assuming it quits raining for more than two days straight any time soon.

Have a great 4th of July weekend!

Jul 4, 2014 8:58 AM ET

Edited Jul 4, 2014 9:01 AM ET.

Which poly?
by user-940291

This question is secondary to the main issue addressed in the article but has anyone else come across the discussion of polyethylene degradation below slabs? A supplier pointed out "research" that said polyethylene deteriorates below concrete slabs due or the conditions of the subslab environment and lime in the concrete; they recommended polyolefin as a superior specification for subslab vapor barrier. However, looking into the issue the "science" I found was mostly from manufacturers and struck me as similar to debates over which rigid foam is superior below grade; mostly it is competitors sniping at one another.

Jul 4, 2014 10:19 AM ET

Good subject but..
by ahamel

I alway wondering why it was so important for architect and engineers to put poly on top and this subject always initiate coarse discussions because it depend on a lot of factors IE: you have to install a radiant floor on one of these preformed insulation panels that hold pipe in, how do you think it have a chance to work. In this case, poly goes under,end of the discussion.
the only thing you should matter is to put a roof before doing slab preparation and this solution is also valid with standard foam when you have to fix some pipes on wire mesh, usually the poly end up with a lot of holes in it.
Honestly, i think, especially in case of radiant floor installation, that it is easier and safer to put it under the foam, and with new rules about radon and other soil gas barrier, you have to protect this poly from concrete contractors hooks ( the one they use to lift the mesh). So, to answer the question, yes, using the blotter sand technique was dangerous and illogic but putting the poly up or down is a matter of job site condition, that's it.
You have to adapt continuously to new rules, techniques and materials but the bottom line is, it's always important to use common sense on each job because they are all unique...

Jul 4, 2014 1:47 PM ET

Alain - Poly on top is best
by JackofAllTrades777

The other potential issue with polyethylene at the bottom of the sandwich below the rigid foam is that the rigid foam can trap water between the poly and rigid foam. This would mean that the concrete slab might not be cured for over a year. Best to put the poly on top like Martin recommended. It will cure faster than with poly on the bottom underneath the foam.

Jul 4, 2014 7:58 PM ET

Edited Jul 5, 2014 10:15 AM ET.

Peter- sometimes
by ahamel

That's the only issue Peter, There is only a problem when you put low permeance floor covering on fresh concrete, this is as stupid as putting a poly on foundation wall. Just do the roof and windows before preparing the slab and you are safe. I do not wonder about slow curing because as we all know, slow is better than fast for apparent floor like polished concrete, it is,as far as i know, the next best thing for a basement after no basement at all...

Jul 4, 2014 9:58 PM ET

Utilities details
by user-347767

Interesting article. I understand the reasoning behind this stack but what about plumbing and electrical connections that go through such as in a slab on grade house. I assume that the poly vapor barrier needs to be carefully sealed around each pipe and conduit. What about remodeling - how does the poly barrier get repaired after the slab is cut open to change plumbing? Or are these leaks to small to matter?

This whole slab curing time issue and sealing around utilities seems to make unvented, conditioned crawlspaces attractive. Just put in a 2-3 inch rat slab, build the main floor, run a dehumidifier in the crawlspace for a year, and no drama. Buy an auto mechanics scooter to make working in the crawlspace easy. All of the wiring and plumbing is relatively easy to access.

I once helped to replumb a slab on grade house in Los Angeles where some of the pipes (copper - it was a long time ago) under the slab were damaged due to ground movement so I'm a bit negative on that style.

Jul 4, 2014 11:31 PM ET

by user-1132906

If one of the desirable attributes of a green building is its ability to be easily modified or repaired over time then it's hard to beat a single story structure on a crawlspace with a trussed roof.

Jul 5, 2014 5:31 AM ET

Response to Alain Hamel and Karl Overn
by user-756436

Alain and Karl,
Both of you seem to worry about sealing holes in the polyethylene.

Alain wrote, "You have to protect this poly from concrete contractors' hooks."

Karl wrote, "I assume that the poly vapor barrier needs to be carefully sealed around each pipe and conduit."

You are both wrong, fortunately -- because this is an unnecessary worry. I decided to address your concerns by adding a new section at the end of the article, with the heading "It is important to make sure there are no holes in the polyethylene?"

Isn't it great when building science knowledge allows builders to relax, and makes our lives easier instead of harder?

Jul 5, 2014 5:45 AM ET

Response to Matthew Nolette (Comment #2)
by user-756436

Q. "Has anyone else come across the discussion of polyethylene degradation below slabs?"

A. The issue has come up a few times on our Q&A forum. According to my understanding, the main reason for the degradation of polyethylene is exposure to UV light (sunlight). As long as the polyethylene is protected from sunlight -- as it obviously is when it is installed under a concrete slab -- it should last indefinitely.

Whenever I have seen holes drilled through concrete slabs, I have never noticed any degradation in the polyethylene layer.

My own advice is to install 6-mil poly and stop worrying.

Jul 5, 2014 6:27 AM ET

More information on PEX for Alain Hamel
by user-756436

There are many ways to handle PEX tubing during a concrete pour. For a good discussion of the options, I suggest that you read How to properly insulate a poured slab with PEX?

Some contractors like to use reinforcement mesh; some contractors like to use rebar. Some contractors like to use chairs to raise the mesh; some think chairs are useless and awkward.

There are many ways to do this, but here are a few principles to keep in mind:

1. It's good to have a method that prevents the mesh or rebar from being pushed to the bottom of the slab. Choose a method that works for you.

2. It doesn't really matter very much whether the PEX is near the bottom of the slab, in the middle of the slab, or at the top of the slab, as long as there is plenty of insulation under the slab.

3. Don't worry if the concrete contractors put a few holes in the poly.

Jul 5, 2014 7:06 AM ET

alternative to gravel?
by jhuang2014

hello all, just joined GBA, excited to pick the brains of so many experts. on the topic here of insulated slabs, would it be possible to replace the 4" of gravel with a drain mat in order to save on headroom in a low basement area? or at least reduce the amount required?

Jul 5, 2014 9:50 AM ET

No brainer
by user-417066

Hello All,
For me this is a no brainer. I will always place the poly on top of the rigid insulation as spoke of in the superinsulated slab article. Placing poly under the rigid foam runs the risk of the concrete while being placed will find its way below the rigid foam thru cracks and joints. Once this happens the foam becomes buoyant and pokes up thru the concrete like icebergs. It doesn't happen all the time, but the times I have seen it happen it was nothing less than a horror show. I just won't take that risk. Although the rigid foam with its semi-impermeance slows the migration of moisture thru the slab assembly by itself, I consistently spec the poly on top to simply hold the rigid down while the slab is being placed.
As for degredation, I have taken apart numerous homes with existing poly, and have never seen any severe degredation. Besides that, for years many, many homes were built without poly. My personal home built in the 50's is a sandy base with a 4" slab cast atop it, and no problems exist.

Jul 5, 2014 10:50 AM ET

Finish Flooring
by user-417066

At my house, it is a slab above grade, with no poly under the slab, just sand backfill. When I remodeled, and added 3/4" xps, sleepers, and a plywood subfloor, and then wanted to install a finished oak wood floor I was worried about the floor cupping as I have heard stories. I ended up putting a clear poly coat on the underside of the wood flooring prior to installation. Many contractor friends said it was a waste of time. Well, it didn't take long, put the hardwood on some saw horses, roll out a quick coat, it can be sloppy because the floor will get sanded and finished on the top side. After 10 years, the floor is still perfectly flat, and the joints have NO seasonal swing, stability is maintained. I know some will talk about cost, but before you do, pitch the cost of prevention vs. the risk/cost of repair.

Jul 5, 2014 10:54 AM ET

Edited Jul 5, 2014 1:52 PM ET.

to Martin about Pex, eps and poly
by ahamel

Hi Martin
I've used a lot of different methods to built radiant slab, poly on top with mesh or rebars, poly under with Isorad or Hydrofoam, but the thing is i'm not comfortable with Lstiburek position on this one.

It's Now part of the 2010 NBC (and valid in most of the provinces except Quebec) to put a 6mm poly or even thicker under the slab and to seal each penetration correctly and perimeter to the air barrier, the called it radon protection, so nobody have the choice, it's mandatory. If you didn't do it right and they found radon in the basement, you could easily be sued by the owner.

So when Joe is talking about taking a walk on my poly with is golf shoes, i'm imagining me shooting his butt with a 12 gauge coarse salt shot, the result is funny but it's only the kind of reaction my brain can have when someone goes against basic common sense... It's simply the f---ing law, for now. If Joe want to invest some of its time to change the code, he is welcome.

The other thing always forgot by some experts is that concrete always crack even with control joints, it's really difficult to predict accurately how concrete is going to react, it depend on a lot of factors. Code ask for a sealed radon, soil gas, air or vapor barrier because concrete crack and shrink and it is not considered as an air barrier by itself because of this flaw, that's it .

And a note to Steve, you better change your foam installator because in 30 years of building insulated concrete slabs, i've never seen any foam pops like an iceberg. It is not a matter of poly it's installation related.

For the concrete pouring thing, i use chairs on stairs, balconies or structural slabs never on basement slab, its useless, rebars or reinforcement mesh are only there as an attachment for the pipes, i ask concrete finishing team to slightly shake the pipes support to be sure that they are well coated with concrete.

So, as i can see, everybody have his own way of doing things, in some cases they are right and in other cases it could be food for thought, but the most important thing is to be here and share our knowledge and experiments so everyone can learn from each other.

This is the link to the isorad,
and the hydrofoam

Jul 5, 2014 11:47 AM ET

Wood Flooring on slabs
by user-1132906

It's a small point but coating wood on all six sides doesn't stop it from taking up moisture over long periods of time, it just stops it from taking it up differentially so that it doesn't cup. I would bet the moisture level in your flooring closely matches the surrounding ambient level at the slab in all seasons, much as it would if it were uncoated. If you want to keep the moisture out of the wood you need to stop it migrating through the slab using a sealer or crystalline coating.

Jul 5, 2014 12:11 PM ET

Edited Jul 5, 2014 12:16 PM ET.

Malcom Taylor,
by JackofAllTrades777

Crawlspaces are area based applications. Crawlspaces in Arizona are almost never used. Termites, spiders, scorpions, black widows, rattlesnakes, and the list goes on. That is what would be living in your crawlspace and coming into and around your home if you did a crawlspace. Slab-on-grade is best for this area. Other cold zone areas require deep footings. Crawl spaces would not work there either. Properly insulated and DRY crawl spaces are super rare. Most are poorly insulated, leaky and moisture ridden.

Plumbing is buried in slabs a million times a year and if it's done right, will outlast the life of the building. Besides, it's called a "crawl space" but 90% of the time you can't crawl under it properly and do any work properly under there due to the limited space.

I pity the fool who would dare crawl into a crawl space on a home out here. Might was well have some vile's of anti-venom on stand-by because you are going to need it...

Jul 5, 2014 5:34 PM ET

by user-1132906

Your description has convinced me. I'll never recommend anyone use a crawlspace in Arizona. After reading that list I might be dissuaded from visiting too!

Jul 6, 2014 4:49 AM ET

Response to James Huang (Comment #11)
by user-756436

Q. "Would it be possible to replace the 4 inches of gravel with a drain mat in order to save on headroom in a low basement area?"

A. Anything is possible, as long as your plan meets local code requirements. But I'd like to make a few comments.

First, the standard way to address low headroom in a basement is to excavate soil from the basement before pouring your slab. In other words, lower your floor. This is usually done the old-fashioned way -- with a pick and shovel. Put the soil in drywall mud buckets and haul it outdoors. Make sure that the contractor who does this work knows how to do it in a way that doesn't undermine the footings.

Second, the guideline that calls for 4 inches of crushed stone is a guideline. If you installed 3 inches, it wouldn't be the end of the world.

Third, the crushed stone layer is one element of a passive (or active) radon mitigation system. For more information on radon, see Radon mitigation systems.

Jul 6, 2014 4:59 AM ET

Edited Jul 6, 2014 5:00 AM ET.

Response to Alain Hamel (Comment #14)
by user-756436

You wrote, "It's now part of the 2010 NBC (and valid in most of the [Canadian] provinces except Quebec) to put a 6 mm poly or even thicker under the slab and to seal each penetration correctly and perimeter to the air barrier -- they called it radon protection -- so nobody has a choice, it's mandatory."

If this approach is legally required in Canada, you should, of course, follow the code. Taping the seams in the polyethylene layer, and sealing penetrations, causes no harm, and probably makes air sealing easier once the slab has been placed. This sounds like a rare phenomenon -- a code provision that encourages builders to pay attention to airtightness.

Codes differ from country to country, from province to province, and from state to state. Unless the code is promoting a method that will perform poorly or lead to problems, it's always important to follow your local code.

For the record, I think that your reaction to Joe Lstiburek's writings on this issue ("So when Joe is talking about taking a walk on my poly with his golf shoes, I'm imagining me shooting his butt with a 12 gauge coarse salt shot") is unnecessarily extreme.

Jul 6, 2014 6:51 AM ET

thanks Martin for reply
by jhuang2014

Martin, thanks for your response to my query. yes, for headroom, i would also be excavating the existing dirt floor down 12-16" as long as the rubble foundation allows for this safely. the basement doesn't seem to have active leak problems but is damp due to lack of vapor barrier and exposed walls. radon testing came back negative so that is not a pressing issue. besides using the drain mat to augment the capillary function of some portion of the traditional gravel, i'd like to run the ends up the rubble wall in preparation for a future interior insulation project (closed cell spray foam). i saw this detail in one of Joe L's post on another site.

Jul 6, 2014 7:56 AM ET

Martin, about Joe
by ahamel

Hi Martin

Excuse me for virtually imaging my reaction to Joe article but when you know the personnage, it's the kind of answer he sometimes shoot at us without any apparent smile, but everybody's know that he is simply joking, most of the time... So excuse me for my rudeness this time, it was an ironic and stupid joke.
By the way, it's only coarse salt, i think is virtual butt will survive this.

And for the code, i think most of the state have radon regulation and the sealed membrane is always a part of the mitigation solution.

Jul 6, 2014 8:18 AM ET

Response to Alain Hamel
by user-756436

Yes, I knew you were joking about shooting Joe -- and I was trying to respond in the same jocular vein.

You're right: he is perfectly capable of surviving your virtual threat.

Jul 6, 2014 10:28 AM ET

by user-1132906

Hmmm. That much salt in his diet, however administered, might raise Joe's blood pressure to a dangerous level.

Jul 8, 2014 11:42 AM ET

Any instances where no vapor barrier is applicable?
by jmskennedy

Thanks for the article. Its nice to have something clear and concise to point my subcontractors to when they are pleading their case for a different arrangement. My question is are there instances where it makes sense to omit the vapor barrier under the slab? I've got one sub that does a lot of flat-work for me and he claims that a lot of his "high-end" contractors he works with don't do a vapor barrier under their slabs anymore. I noticed a couple of assemblies noted in Joe Lstiburek's Cold Climate book where he notes slabs with rigid insulation underneath but no vapor barrier that are allowed to dry back to the ground and to the interior. Is this really only applicable where you wouldn't be putting another finish over the slab or is this really not a recommended practice at all?


Jul 8, 2014 1:52 PM ET

Response to JMS Build
by user-756436

It is impossible for a damp slab to "dry back to the ground." The ground is always damp.

6-mil poly is cheap, and I see no reason whatsoever to omit it.

Jul 9, 2014 5:06 PM ET

The best place for the poly is between 2 layers of EPS
by user-466045

There is no reason that the poly cannot be an effective air,vapour and radon barrier if it is well sealed and protected. It is easy to do.
We put the ploy between 2 layers of EPS. With the first layer down, it is easy to tape the seams with a high quality tape. The second layer of EPS protects the poly during the pour and keeps if from getting punctured from either side. The poly is brought up around the edges of the slab to be connected with the rest of the air barrier. This strategy has solved our low grade radon problem and given us a bump in air tightness.

Jul 9, 2014 5:33 PM ET

Edited Jul 9, 2014 6:03 PM ET.

Soil Up Order In Our PassivHaus Slab
by user-942869

In the recent construction of our pre-certified PassivHaus in NJ we did exactly what Peter (above) described. The order upwards for the slab is: undisturbed soil, crushed stone, 4-inches of Type I EPS, 6 mil polyethelene sheet, another 4-inches of Type I EPS and then the 4-inch concrete slab for a total insulation value of R30. The footings are completely encased in EPS too, with Type IX under them and Type I around them. The poly sheeting is run up the walls and joined to the first floor air barrier.

Jul 10, 2014 5:57 AM ET

Rigid insulation as vapor barrier
by user-466192

Is not the rigid insulation a vapor barrier? Would 2 layers of 2" rigid layered perpendicularly to each other work? Or spraying 2" of closed cell?

Jul 10, 2014 7:08 AM ET

Edited Jul 10, 2014 7:11 AM ET.

Response to Sandra Heiser
by user-756436

Q. "Is not the rigid insulation a vapor barrier?"

A. It depends on the type of insulation and its thickness. One inch of extruded polystyrene (XPS) has a vapor permeance that ranges from 0.4 perm (that would be a vapor barrier) to 1.6 perm (a vapor retarder, not a vapor barrier), depending on the brand. Thicker samples of XPS would have lower perm ratings.

One inch of expanded polystyrene (EPS) has a vapor permeance that ranges from 2 perms to 5.8 perms, depending on the brand. So one inch of EPS is not a vapor retarder. Thicker samples of EPS would have lower perm ratings.

Q. "Would 2 layers of 2-inch rigid, layered perpendicularly to each other, work?"

A. Yes, 4 inches of most types of rigid foam would act as a vapor barrier.

Q. "Would spraying 2 inch of closed cell work?"

A. An application of 2 inches of closed-cell spray polyurethane foam would have a vapor permeance of about 1 perm, which meets the definition of a vapor barrier.

Jul 10, 2014 1:37 PM ET

Southern Arizona slab on grade with radiant tubing
by user-875633

To insulate or not; to steel reinforce slab or not; wire fabric in slab or not; radiant floor (heating) pex tubing attached to suspended #4 at 2'oc each way or not; what about potential (some say inevitable and the only thing one can do is mitigate) cracking slab; sandwich the vapor barrier or not; on compacted base or under........

How about the following proven concept (DOE Build America testing, monitoring and 3 year lived-in experiencing) compact sub-base, compact AB or similar, 40 mil plastic liner (vapor and radon restrictive; perhaps too much but one time expense), 2" R10 Creatherm (pex ready snap-in plasticized topped eps foam), pex tubing at bottom of 4.5" slab with no steel reinforcing, finished concrete floor, no cracking, year round radiant floor thermal comfort

It's called the KIS (keep it simple) principled way of building.

77 wheel barrel 2.jpg

Jul 11, 2014 1:19 AM ET

Edited Jul 11, 2014 8:14 AM ET.

2 Reasons Why We Place Poly Under the Insulation
by StollerB

Living in a wet, coastal environment in Canada where a large portion of our houses are slab on grade, there are two reasons why we continually choose to place 10mil poly underneath our rigid foam, instead of on top of it.

Reason 1: As foam absorbs water it's r-value drops substantially. Most people use EPS instead of XPS below slabs, and EPS absorbs much less water, so it's r-value remains much more stable. However, we choose to put the poly below it regardless, because we don't like our insulation's r-value being degraded at all. Sub-slab foam serves no other point than to insulate, so why risk defeating that purpose at all?

Reason 2: Bugs (namely carpenter ants) love rigid foam. LOVE it. And what else do ants love? Moisture and wet environments. If we have a bunch of EPS sitting on potentially continually damp ground with a poly layer on top, that EPS is going to be a magnet for insects looking for a cozy wet place to hang out. I don't know what it is about the foam that they love so much, but we have seen some insanely extensive ant tunnels through rigid foam around here. (Maybe ants are Passive House enthusiasts themselves?) Put 10 mil poly under your foam, and any moisture in your ground will not get into your foam layer and will thus not attract ants. And beyond preventing a moist environment from existing in your foam to attract the insects, 10 mil poly is actually a pretty effective barrier to them itself.

As a third, but less influential reason, we actually prefer to place our foam on about a 2" layer of compacted sand (which is in turn over compacted rock) because we find it easier to get a sand surface very flat and smooth, which helps our foam lay much flatter during installation. This leads to less cracking when it gets trampled by trades, and it largely negates the risk of the floating icebergs that Steve Baczek mentioned. Because our sand in this environment generally is quite wet all year round, we do want to keep whatever moisture is in that sand out of our EPS, hence the poly between sand and EPS. The rigidity of the 10 mil poly also does a pretty good job of keeping the sand from getting "dug up" by boots while we are walking around on it placing our foam.

We really see no downsides to placing the poly below the EPS. Because concrete is an extremely effective vapour barrier, we aren't concerned about vapour from the house condensing on the "cold side" of our insulation against the poly and building up there below the foam (which is why one would normally place poly on the warm side of their insulation a-la walls and roof). And because foam goes down fast, we just wait for a window when the weather is good for a few days so we can get the foam down and get the slab poured immediately after. Not a big deal. Anyway, those are our reasons for putting the poly under the foam.

Jul 11, 2014 12:27 PM ET

40 mil correction
by user-875633

Sorry readers, I misread the old invoice from 2010. The poly vapor barrier is 10 mil, not 40. It's called Perminator a product from WR Meadows. Comes in different widths and lengths so it's easy to economize and minimize seams. Great stuff.

Burke- it's great to hear your prepour installation method in wet, coastal environment in Canada. Your sequence is identical to what we did in hot/dry climate of southern Arizona.

It seems like the only difference is in the particular foam product choice. We used 2" R10 Creatherm product because it had the plasticized 'reversed muffin tin contour' (my description for visual perception) in which PEX just pops in place perfectly uniform in position.

Do you do any hydronic radiant heating?

Jul 11, 2014 12:44 PM ET

WR MEADOWS white paper on vapor retarders/barriers
by user-875633

definitely worth reading:

Jul 11, 2014 11:55 PM ET

Response to Michael Ginsburg Comment #32
by StollerB

Michael- we do some hydronic heating, but generally the mechanical contractors we use just tie the tubing (with zips ties) to the rebar matt or the welded wire mesh grid to keep it from floating up in the concrete. Pretty basic.

Jul 12, 2014 2:16 AM ET

Burke Shouldnt the EPS/XPS be
by BobHr


Shouldnt the EPS/XPS be elevated above the bulk water layer by the stone. The bulk water should be controlled by drain tile. Now if the poly is under and has holes if there was a high water incidence than water could be trapped between the insulation and poly. Although I would assume this would be minor.

So below ground the insulation should really only be subject to RH and not bulk water, We dont worry about rigid insulation being subjected to high RH behind a rain screen wall. Should we worry about the RH below grade.

Lastly the insulation is being exposed to the moisture of the concrete when it is wet. Like the sand it can only dry via the vapor phase. It would seem the stone would allow greater drying via the vapor phase particularly is there was a radon mitigation system,

Jul 13, 2014 12:52 AM ET

Response to Robert Hronek, Comment #35.
by StollerB

These are valid points. There shouldn't be any issues with bulk water coming up to the level of the EPS, unless there is major flooding and the water table is high enough that the storm sewers are flooded and not draining the perimeter drain. Or perhaps a less dramatic example; tree roots eventually knotting up and clogging the perimeter drain, an errant excavator shredding the landscape fabric over the drain rock and spilling dirt into the drain, curious kids dumping mud down all the perimeter drain clean outs, etc. As a long suffering pessimist, I believe that if something can go wrong, eventually it does.

Because it is pretty easy to seam seal the poly when it goes down, and because you've really got to give 10 mil poly the full beans to tear and puncture it, we're pretty confident that even though the rock below should prevent any bulk water from reaching the underside in 95% of all cases, for the rare times 10 or 15 years down the road, it really doesn't hurt to have a well-sealed, intact layer of poly below the EPS. (And as Peter Amerongen suggested, connecting the poly to your air barrier at the slab to foundation wall seam can be an important part of the puzzle).

I am not concerned at all about the water content in the concrete. We order the concrete ourselves for our finishers and specify slump, so it is never poured soupy. Most of the water is used up in hydration or bleed water at the surface. After 28 days, the moisture coming out of the concrete is small enough that it's not much of a concern. Our EPS is generally a fairly tight fit, and I am confident that the very small amount of moisture that does absorb into the EPS (I can't see it absorbing much at all during the few hours it takes the concrete to set up) isn't going to cause any major concerns working it's way back out during the course of construction.

While I don't think there is a huge difference whether the poly goes directly under the concrete, between the foam, or under the foam, I still see a couple of slight advantages in our climate, our location and with our practices to placing it under the EPS. Context could sway the decision from one place to the next. This doesn't strike me as one of those building science issues with warning signs written all over it. Poly under a bunch of wet sand, yes. Well-sealed 10 mil poly under or over your foam, no.

Jul 15, 2014 5:12 PM ET

With EPS being greener than
by Bill_L

With EPS being greener than XPS, it seems a sure winner for below slab applications. However, it does present a problem that putting poly above the insulation can help with - small beads of EPS making their way into the top surface of the concrete. As I think most people who have used EPS will attest, it is impossible to prevent a swarm of EPS beads, and when they get inadvertently blown around and troweled into the top layer of the slab, they lie in wait to open up like a canker sore on an otherwise unblemished finish. With the poly on top of the EPS, there are less beads getting blown around into the top layer of the concrete.

Jul 18, 2014 12:03 PM ET

poly placement and radiant heating
by user-875633

Excellent points on the poly placement location

Regarding tubing placement for radiant heating, I have always been concerned about potential problems with the pex tubing being tied to rebar mat: (1) uniformity of placement both horizontally and more importantly, vertically (2) potential known and UNknown damage to pex occurring during concrete pouring, (3) just so many potential what if's with so many laborers trying to maneuver around during the pour.

With the use of Creatherm product almost all the above mentioned concerns are just taken out of the equation. Additionally, I used poured stem walls and a 'floating' slab (thermal and structural isolation) and NO rebar in the slab. The only add to the 4.5" pour was fibermesh. I had NO cracking at all. I personally think there were four reasons for this easy, non-cracked floor: fibermesh, pouring in shade (SIP walls and roof were up) and most significantly the physical embedded reverse muffin tin shape of the Creatherm product: 4" oc, 2" diameter, 1" tall knurled protrusions into the bottom of the concrete and finally NO uneven water wetting the subbase just prior to pour thereby NO uneven drying or curing.

Jul 18, 2014 6:26 PM ET

I fully agree with Martin's blog and not some recent posts
by wjrobinson


Jul 21, 2014 7:27 PM ET

I am with Alain
by badgerboilerMN

Alain has it exactly right. Icebergs, now that is a thin mix!

We install a lot of basement slabs, both new and in our extensive renovation work. We are called in to design and install radiant floors, but end up address ground water, radon, plumbing and insulation of floor, wall and most especially rim joists.

If you have high ground water and/or radon here in Minnesota you get drain tile and a passive vent. If it warrants a receiver tank and pump (mandatory in new construction) we address that as well.

As for vapor retarders, 2" of 25psi XPS (the only thing you see here in the Midwest) you start with a very tough rigid platform and with a vapor permeance of 0.7. This is and effective vapor retarder in and of itself. Where a redundant vapor barrier is placed is of little concern except to those whom making a living splitting the proverbial hairs.

We have installed miles of radiant tubing with and without a vapor barrier with no appreciable difference in the quality of the slab or the measurable radon. We have not placed the poly barrier over the XPS for reasons accurately and astutely described by others whom actually take responsibility for systems they design, install and warranty.

As a side note, we in the mid-west have managed to ignore the former "accepted" science and did not use "blotter sand", though once radiated I defy any slab to go soggy!

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