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Q&A Spotlight

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

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 VOCs 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 GBA senior editor Martin Holladay points out, concrete itself is a good air barrier, 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 LEED Platinum project and was pleased with the result.

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  1. Interested Onlooker | | #1

    HRV for the basement

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

  2. GBA Editor
    Peter Yost | | #2

    Does the radon mitigation HRV service the upstairs?
    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).

  3. Ed Voytovich | | #3

    Arlene's Dilemma
    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.

  4. Ron Flax | | #4

    HRV as Radon Solution
    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

  5. Carl Mezoff | | #5

    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).

  6. Steve Rossello | | #6

    VOC detection and mitigation
    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.

  7. Danny Kelly | | #7

    Is dilution really a solution?
    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?

  8. Arlene DiMarino | | #8

    Thank to all for the
    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.

  9. Peter Marsh | | #9

    Potential basement VOC problems
    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.

  10. Jim Craft | | #10

    VOC detection and mitigation
    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.

  11. Tony Kiburis, NH Radon Mitigator | | #11

    Problems with vapor barriers
    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.

  12. Venkat Y | | #12

    Radon in basement
    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.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Venkat Y
    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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