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Vapor Barriers, Radon, Basement Slabs, and VOCs — How to Stop the Poison?

When radon or volatile organic compounds are a threat from below, the barrier needs to stop more than moisture, and it needs to be continuous and tight

Posted on Nov 1 2010 by Scott Gibson

How's this for a dicey scenario: Arlene DiMarino is a homeowner with chemical sensitivities who lives a couple of blocks away from an EPA Superfund site.

"I am aware of a toxic plume of underground water that is close by," she writes in a Q&A post. "I am concerned that these VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production. can permeate the cement floor and foundation."

She plans to pour another layer of concrete over the existing basement slab. This will give her an opportunity to insert a vapor barrier to block any toxins from migrating into the house. But what's the best product to use?

Alternatives to 6-mil poly
As senior editor Martin Holladay points out, concrete itself is a good 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., as long as any cracks that develop are sealed with caulk.

But, he adds, it's important to install a vapor barrier — typically 6-mil polyethylene with taped seams — under any concrete slab.

Other points of view are quick to emerge, like this flat statement from Robert Riversong: "I would never use standard 6-mil polyethylene under a concrete slab."

Instead, Riversong recommends a product called Tu-Tuf, a robust, cross-laminated, 4-mm-thick plastic that's advertised as free of pinholes. (It's made by Sto-Cote Products and sold widely over the Internet.)

David Meiland suggests a product called Stego Wrap, which is 15 mils thick and said by the manufacturer to resist tears, splits, and punctures.

There's also something called SlabShield, writes Andy Ault. SlabShield combines two layers of polyethylene and one of aluminum. The polyethylene sandwich protects the aluminum, a highly effective vapor barrier, from lime in the concrete. The manufacturer also claims that SlabShield provides a thermal break.

Ault says he used SlabShield on a Habitat for Humanity LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Platinum project and was pleased with the result.

"It is much stronger than typical 6-mil, so it is designed for that type of rugged application that Robert alluded to," Ault writes. "We had volunteers walking on it, and it never missed a beat. The ship-lap edge design also gave us a high comfort level that the taped seams will perform well."

Yet another possibility, writes Craig, is Red Guard, a waterproofing and crack-prevention membrane made by Custom Building Products.

"Typically it is used under tile, and it is being used in some applications as a shower pan liner," Craig says. "It seems perfect for a situation between slabs because it acts as a crack isolation membrane and vapor barrier, so it should seal any cracks in the original slab and prevent any new cracks from leaking vapor through. "

A building scientist disagrees
As posters recommend tougher and tougher vapor barriers, Garth Sproule points to a conversation with Joseph Lstiburek, who couldn't care less if a sub-slab layer of plastic has a few holes in it.

Even if Arlene were to don a pair of golf spikes and walk all over her plastic vapor barrier, it would still do a pretty good job, Lstiburek says in a Green Building Advisor podcast. "So what’s the total surface area of the punctures compared to the total surface area of the plastic?" he says. "If I’m there for about two hours, maybe it’s 10 percent. So I basically have reduced the vapor control layer effectiveness of that plastic sheet by 10 percent."

When the barrier is topped by four inches of concrete, she'll have a very effective air barrier and "a darn good vapor retarder.

"So I haven’t increased, even from a measurable perspective, the amount of water vapor transmission from the ground into the floor with the ripped and torn plastic sheet," Lstiburek says. "That’s why I always laugh at the people that say, 'Well, you gotta tape the joints and you gotta be careful not to puncture it.' Give me a break!"

Replies Riversong: "I disagree with Dr. Joe. With a good sub-slab radon-mitigation system that effectively depressurizes the soil, an intact concrete slab might be good enough. But, if there is any positive soil gas pressure, then no concrete slab will be tight enough to prevent radon intrusion, which can pass through a crack too small for the eye to see."

What about a real radon-mitigation system?
In new construction, the area beneath the basement slab is often vented to the outside, giving radon, a naturally occurring radioactive gas, a way to escape. The vent can be passive or powered by a fan.

This is the retrofit measure recommended by Riversong, who suggests removing the existing concrete slab, and adding a layer of crushed stone and a perforated pipe connected to a roof vent.

Others agree. "Pulling everything existing and starting from scratch is never a bad strategy," writes Ault, "but that will at least double your cost (if not more), so you need to be sure that the cost/benefit ratio makes sense for you."

As Ault points out, ceiling height also is a concern. If a new slab lowers the basement ceiling height to less than 7 feet, it could be a code problem should Arlene or a future owner want to make the basement a living space.

Our expert's opinion
Here's GBA Technical Director Peter Yost's opinion:

Looking at Arlene's basement problem from the radon perspective is a good one, in my book

In my own home, with a leaky basement floor and no spare head height at all, and a radon problem exacerbated by the installation of spray foam insulation on the basement walls, I tried a variety of ways to air seal the radon out, to no avail.

If you can't keep it out, flush it out

And when I called a local radon technician to see about a sub-slab depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. system solution, he let me know that it was unlikely that he could pull hard enough with even two fans to depressurize such a leaky slab. So our solution was a heat-recovery ventilator servicing just the basement.

But exhaust fans use energy

The HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. drove the radon concentration well below the EPA threshold of 4 pico-curies per liter. It was a practical, easy solution to a soil gas problem, albeit with a steady energy penalty (our FanTech HRV pulls about 1 watt per cfm on any of the three 100/150/200 cfm settings).

If you can't give up the height, and you don't have the stomach to completely redo your basement slab, I would go after any potential or actual soil gas problem with some sort of exhaust strategy, purchasing the highest-efficiency fan your money can buy (there are HRVs in the GBA Product Guide with efficiencies up to more than 5 cfm per watt).

One way to offset the extra energy use

One last note: I reduce the energy penalty of our HRV in the late fall/winter/early spring by hanging our wash in the basement. The HRV easily handles (by exhaust exchange) the 4 pounds of water left in a load after our horizontal-axis washer is done with it. And I've checked the interior relative humidity in the basement to verify that this approach is not a problem; it barely bumps the interior RH.

Nov 3, 2010 12:37 PM ET

HRV for the basement
by Interested Onlooker


Does the whole house exhaust via the basement with the incoming air warmed by the HRV being distributed upstairs?

Nov 3, 2010 3:48 PM ET

Does the radon mitigation HRV service the upstairs?
by Peter Yost

No, the HRV is dedicated to radon mitigation in the basement. We use our 2nd floor bath exhaust fan for whatever whole house ventilation we feel the home needs (overall airtightness of about 4 ACH50).

Nov 3, 2010 5:30 PM ET

Arlene's Dilemma
by Ed Voytovich

If Arlene has chemical sensitivities and she lives within a couple of blocks of an EPA Superfund site, I strongly suggest that she move to an uncontaminated place even if it means a significant financial loss. Her health need to be the first and highest priority.

Six mil poly or any of the other products recommended here may minimize gas transmission from under the slab, but they are extremely unlikely to protect her adequately. Contaminants will unavoidably find their way into the house via other pathways.

Just for instance, there is potential (and likely) infiltration in all the other usual places from stack effect. If the pollutants outgas from under the existing slab it's hard to imagine that they are not present in the air outside the house, and especially near the ground, where stack effect draws outside air in. Never mind the pollutants blown from her neighbor's yard against the side of her house by the wind, the soil and dust from the outside that comes in on her shoes, and so on.

Admittedly, radon gas dissipates rapidly in the outside air, but we have to wonder if the gases affecting Arlene do so at the same rate or in the same way.

Her case is special enough to merit a change more radical (and probably costly) and any subslab vapor barrier: a change of address.

Nov 3, 2010 6:09 PM ET

HRV as Radon Solution
by Ron Flax

Your HRV solution is very interesting. I am wondering about some of the details. How big of a basement, and how much air are you moving? I wonder how the energy penalty of the HRV stacks up when compared to the energy penalty of a continuous radon fan. My estimates are that a radon fan seems to use around 600kWh per year just to power the fan. Would you share some of the details of what you did in your basement? Thank you very much, - Ron

Nov 4, 2010 10:52 AM ET

by Carl Mezoff

An alternative to suffering the electrical penalty of operating an HRV for taking radon (daughters) out of the basement would be to install a generously-sized bathroom exhaust fan in the basement ceiling instead of using an expensive HRV. Connect the exhaust fan to an occupancy sensor.

Assuming that that the basement is occupied only a few hours per week, there is no benefit to constantly evacuate the basement air if nobody is down there. The effects of elevated radon levels are barely measurable even for very long term exposures (measured in years), so the hazard from a momentary exposure (while the fan evacuates the basement air) would be extremely low. Remember that we are all exposed to Radon gas every day because it is present in outdoor air (albeit at very low levels).

Nov 4, 2010 3:22 PM ET

VOC detection and mitigation
by Steve Rossello

I'm an environmental scientist and from my viewpoint, the measures recommend above are premature and may not even be necessary. Records for the Superfund site near Arlene's home are available online ( and the paper reports should be on file in her local library. If the VOC plume passes near or under her house, then she can demand that an air vapor survey of her basement be conducted. If the results show the presence of VOCs then it is the responsibility of the owner of the Superfund site to mitigate the problem and Arlene should not have to bear any cost. If there is no known owner, then it is the responsibility of EPA or the State (if the State has primacy) to mitigate the problem. However, is only radon is present, then the homeowner bears the cost of mitigation because radon is naturally occurring.

Nov 4, 2010 11:09 PM ET

Is dilution really a solution?
by Danny Kelly

Good timing on your article Scott - I just asked a similar question the other day:
From my responses and from reviewing Peter's article on his deep energy makeover:
it seems to me that dilution is not a very good solution (although I wish it was to solve a problem I have). In Peter's article, he mentions that the radon level remains the same in the basement after the HRV was installed but went down inside the house. I would think this would prove that dilution is not a very good method and the reason it went down inside the house was crawl space air was no longer entering the house due to the exhaust/negative pressure in the crawl.
Will a dilution method work? If so - is there a proper calculation to determine the required CFM to dilute the air but not provide an energy wasting excessive air exchange?

Nov 8, 2010 11:02 AM ET

Thank to all for the
by Arlene DiMarino

Thank to all for the responses to my question. I see that this topic has generated much interest. Thanks for the link for the EPA Steve. I had been trying to find current information on the site and got exactly what I needed. I actually got to speak to the project manager from the EPA. The site has been remediated but there is still some metal contamination mainly hexavalent chromium and nickel at a depth of > 20 feet which is being treated at the site. During the remediation the soil was removed down to 15-20 feet depending on the area but there is some residual at a deeper lever. I was told that the metals are a danger to the drinking water and have not migrated off the premises. There was also some problem with voc's from solvent chemicals but those also are no longer a problem. I live about 4 blocks west of the site. So as far as he is concerned there is no danger from the site to my home. That being said, I would like to take precautions in the event that there is something else that we don't know about. We had dry cleaners in the area there are also gas stations nearby too. I am always concerned that underground plumes may carry voc's .So while I am doing this work I would like to do the best I can to make the indoor environment as healthy as possible.

Nov 8, 2010 11:25 AM ET

Potential basement VOC problems
by Peter Marsh

The suggestion to vent the basement with a bathroom fan is counter-productive as venting air out of the basement without supplying make-up air will cause a pressure differential that will increase migration of any gases through the slab. I have found a ten-fold increase in basement radon levels when a fan was running in a basement window. The HRV is a better solution. Also weatherizing the upper levels of the house and, in particular, any penetrations into the attic will help reduce the stack effect which exacerbates gas migration through the slab.

Nov 8, 2010 12:15 PM ET

VOC detection and mitigation
by Jim Craft

I agree with Mr. Rosello's comments and with Ms. DiMarino's concerns so why not simply test the subslab vapors and indoor air with Summa canisters for VOCs and Radon. Cost may run several hundreds $ but why fix a problem that may not exist? In our work in NY, we find that Radon (a known carcinogen) is widespread and unpredictable and often a greater concern to the general public than scattered VOC plumes. Moreover, mitigation if often cheaper than testing; I installed a Radon fan (Fantech HP2190; 90W) and piping for < $200 in my existing slab to mitigate a Radon problem. But sealing almost never works to seal out toxic vapors; if a problem exists, go with depressurization; cheap and effective. And why the simple mitigation measures are not code-required during construction (vapor barrier, gravel with PVC pipe; maybe $100-200?), I'll never know. If an issue arises later, just hook up a wind turbine or inline fan to the stack.

Dec 2, 2010 3:34 AM ET

Problems with vapor barriers
by Tony Kiburis, NH Radon Mitigator

Sub slab vapor barriers by themselves are not effective barrier to radon entry. There are too many other pathways around the vapor barrier for the radon gas to travel. I believe the vapor barrier's prime benefit is to block moisture wicking into the concrete.

It seems to be common practice around here for concrete finishers to slash with a knife the vapor barrier prior to the pour in order to drain as much water from the wet concrete. This allows them to use their power equipment to finish the concrete the same day of the pour rather than using hand tools or coming back the next day.

I suggest searching the WEB for "Radon Ready New Construction" (RRNC) for builder specs. for passive radon mitigation system that can be designed into new homes in high radon areas. If necessary, they can be made active by adding a fan later. The added benefit is the fan will be located in the attic of the house or attached garage not outside the house.

Regarding the comment about removing the concrete floor - Don't do it!
In a single day a certified radon mitigator can install an active "Sub Slab Depressurization System which is the most effective way to remove radon from a home. Total cost should be about $1,000 to $1,500.

Check in with your State Dept of Public Health; they will have a radon office which can help you with radon issues & locate a certified radon mitigator in your area.

Jan 23, 2014 7:41 AM ET

Radon in basement
by Venkat Y

Peter Yost said: "The HRV drove the radon concentration well below the EPA threshold of 4 pico-curies per liter. "

Is this reduction in radon because of the positive pressure created in the basement by the HRV? Is the generation of this positive pressure based on some delta P measurement the the HRV makes? Or "balancing an HRV" about skewing it for positive pressure? Thanks.

Jan 23, 2014 8:10 AM ET

Edited Jan 23, 2014 8:12 AM ET.

Response to Venkat Y
by Martin Holladay

Q. "Is this reduction in radon because of the positive pressure created in the basement by the HRV?"

A. No. The HRV is balanced. The reduction in radon is due to the fact that the fresh air introduced by the HRV dilutes the concentration of radon.

Q. "Is the generation of this positive pressure based on some delta P measurement the the HRV makes?"

A. No. There is no positive pressure. It's balanced.

Q. "Is balancing an HRV about skewing it for positive pressure?"

A. No.

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