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Radon and a Passive House

Just because your new passive house is very, very tight, doesn’t mean that you don’t need a radon mitigation system

Even though our new Passivhaus had a very tight envelope, we still needed to install an active radon mitigation system. Before the mitigation system was installed, basement radon levels ranged from 3 to 12 picocuries per liter. The EPA recommends mitigation efforts in homes with radon levels that are above the “action level” of 4 picocuries per liter.
Image Credit: Paul Honig

Let me start out by stating that I am neither a radon expert nor a Passive House expert. That said, I do have experience with radon gas in a Passive House that I’d like to share.

According to FAQ page on the Passive House Institute U.S. (PHIUS) web site, Passive Houses are (by design) well protected from radon. PHIUS attributes the radon-preventing attributes of a Passive House to its rigorous airtightness metrics and balanced ventilation. Yet our home is a certified Passive House that had a radon problem until a conventional radon mitigation system was installed. I don’t believe that achieving the Passive House standard did much to reduce the likelihood of our having a radon problem.

According to the EPA’s “A Citizens Guide to Radon,” radon occurs naturally in soil and rocks in which home foundations are placed. Radon can be found all over the country. In our area (northwest Connecticut) there is approximately a 40% chance of radon existing on the site of your home, according to a local expert.

Radon enters a home through foundation cracks

Radon gas enters a home through gaps in the structure (typically a foundation) that separates the below-grade interior of a home from the earth that surrounds it. The local expert we saw explained that even a tiny gap the size of a dime can lead to elevated levels of radon in a home. The atmospheric pressure in the ground tends to be higher than the atmospheric pressure in a house’s interior. As a result, a basement can act as a sort of vacuum cleaner motor that sucks in radon in soil air through any of these small gaps. Radon gas can then accumulate to levels that according to the EPA can lead to increased risks of lung cancer.

In houses without balanced ventilation, this vacuum cleaner effect can be exacerbated by exhaust fans, dryer vents, and mechanical heating and cooling systems. As air is forced out of a house, unconditioned air enters through gaps in the lower portions of the house.

Although a Passive House has a balanced ventilation system, the pressure in the soil around the foundation still tends to have higher pressure than the house’s interior. I conclude this based on the fact that my house has a balanced ventilation system as well as a radon problem isolated in the basement.

Using fresh air to dilute radon concentrations

A Passive House’s balanced ventilation can also reduce the level of radon in a house by exhausting the air with elevated levels of radon from a basement and supplying fresh outdoor air with lower levels of radon. Doubling the rate of ventilation can cut the radon level in half. In our house, I saw short term (7 day) radon levels (measured with a Safety Siren Radon Gas Detector) range over the course of a year from 3 pCi/liter up to 12 pCi/liter. We would have had to quadruple the rate of ventilation in order to reduce the highest detected levels below 4 pCi/liter as recommended by the EPA. This was not possible the way our ventilation system was designed. So, while our continuous exhaust of basement air reduces the amount of radon in our basement, it doesn’t reduce it nearly enough to mitigate our moderate radon problem.

A Passive House’s balanced ventilation has two seemingly positive characteristics with respect to radon infiltration and exhaust which, at least in our case, fell short of preventing elevated indoor radon levels. So, the question now is, why doesn’t the airtightness requirement for a Passive House prevent enough soil air containing high levels of radon from entering the building?

Our house is tight

Our house beat the Passive House airtightness standard by more than 35%. It had an airtightness measure of 0.38 ACH50 (air change per hour at 50 pascals) with a corresponding reading on the blower-door test of about 180 cfm.

This means that under normal unpressurized conditions, air leaks into our house at a rate of about 300 liters/minute. I’ll assume that the air entering our house through the balanced ventilation system has radon levels of 0.5 pCi. Soil air in the U.S. typically has radon levels of 200 to 2,000 pCi. I’ll assume that our soil has a radon level of 750 pCi which is considered in the middle of an average risk area (270-1,350 pCi, according to “A Living Radon Reference Manual”) for radon.

If the source of air in our basement were a mixture of 99.5% 0.5 pCi air from the ventilation system and 0.5% 750 pCi soil air, then we’d see radon levels of 4 pCi in the basement. Our basement contains about 300,000 liters of air, and 0.5% of this is approximately 1,500 liters. If all the air leakage in our house occurred below grade with soil air, it would take about 5 minutes to leak 1,500 liters of 750 pCi soil air. If only 1% of the leakage in our house occurred below grade, it would take 8 hours.

The entire volume of air in our basement is exhausted by the ventilation system every 16 hours, so 1,500 liters leakage in 8 hours should be enough maintain radon levels at 4 pCi in the basement. I don’t know the actual soil air radon levels (although our well water tested at 2,500 pCi and 5,000 pCi, so 750 for the soil seems reasonable to me) or the percentage of our house’s leakage occurring below grade, but nonetheless, I’m not at all comforted that the Passive House airtightness requirements have any meaningful effect on preventing radon infiltration. In fact, our house exceeded the Passive House standard on the initial blower-door test under conditions that were ideal for radon infiltration, with a gravel basement floor prior to the concrete slab being installed.

All it takes is a dime-sized gap

To prevent radon from infiltrating a house through airtightness requires close to 100% effectiveness below grade. Our house was designed and built with an elaborate air and vapor barrier surrounding the foundation (although based on our initial blower-door test this barrier was not at all necessary to achieve the Passive House airtightness standard). I suspect that others who build Passive Houses have similar below-grade air and vapor barriers. The barrier was designed to be continuous with both penetrations (well and septic) sealed with spray foam.

My expectation was that this air and vapor barrier would keep all soil air with its high radon concentrations out of the house. Unfortunately, it seems to have at least one dime-sized gap as evidenced by the elevated radon levels in the basement.

Here is a link to a photo album with pictures of the foundation and barrier for anyone interested.

A radon mitigation system to the rescue

Fortunately, we were able to prevent radon from entering through the small gaps in our air and vapor barrier through a conventional radon mitigation system.

The conventional system was built by drilling an 8-inch-diameter hole through are basement slab and removing about 5 gallons of the gravel found there. A 4-inch PVC pipe was inserted into the hole and the hole was sealed back up. The pipe travels out of the basement and into the adjoining garage and then out the garage wall.

A 20-watt fan was connected to the pipe and run continuously to create negative pressure underneath the slab. Because the pressure under the slab is now less than the pressure indoors in the basement, there is no longer a vacuum effect and soil air is not sucked into the basement through existing gaps. Fortunately, we were able to achieve the desired result with a small fan drawing only 20 watts to keep electricity usage to a minimum.

So, while a Passive House’s balanced ventilation and airtightness sound like they’d be helpful preventing elevated levels of indoor radon, they don’t do enough to make a difference. In particular, air leakage permitted by the Passive House standard can allow substantial amounts of radon to infiltrate a home.

I don’t believe, unfortunately, that achieving the Passive House standard significantly reduces the risk of having a radon issue in your house. On the positive side, if you have a Passive House with a radon issue, it can be addressed with a conventional radon mitigation system.

Paul Honig lives in a passive house in the northwest corner of Connecticut with his wife Diane and two boys. He enjoys teaching math and playing roller hockey. Paul and Diane are the authors of a blog called Our Connecticut Passive House.


  1. jdom | | #1

    Radon Points of Entry and ERVs
    Hi Paul - Very interesting post. I believe our home is very similar to yours in many key areas. Most particularly for this discussion, the basement was sealed in the same manner and our final blower door test came in at .3 ACH @ 50. We are in the final stages of the PHIUS certification process. In the early stages of construction, I opted out of a radon mitigation system because I was confident that the Passive House detailing would make it unnecessary. In addition, we used a footer form system called Form-a-Drain (Certainteed) which has the (claimed) side benefit of venting the foundation base to daylight. However, your post prompted me to test our basement, which we've now done using the Safety Siren. Our initial (long-term) reading was 8.3 pCi. That has since decreased to 7.2 pCi. I have to admit that I am somewhat shocked. I will be retesting using another method to ensure the Safety Siren reading is accurate. But I have a couple of thoughts. First, an examination of my basement makes me wonder if the lally columns are the source of entry. I say this because they are the only things that pierce the vapor barrier. Although the barrier was carefully taped to the outside of the columns, I am wondering if the interior of the columns may be an avenue for gasses to rise as I don't think it is completely airtight. I will be sealing the top of each column, but I'm wondering if your mitigation contractor had any thoughts about that possibility? The other thought pertains to ERVs. Although I can't put my finger on it, my recollection is that I read an article not long ago that communicated some concerns about ERVs possibly recycling contaminants, such as formaldehyde. Have you learned anything about the possibility of a similar recycling of radon?

  2. Paul Honig | | #2

    Response to Jerome Lisuzzo
    Hi Jerome, The radon expert who visited the house never mentioned anything about lally columns as a potential source of entry. I was under the impression that they were filled with concrete, but I guess if there were a gap in the concrete that could be a possibility. He did mention the electrical conduit to the well as a potential source. After sealing the well's electrical conduit our short term reading went from something like 7 to 3.5 very quickly. I took this to mean that the electrical conduit was one source of radon in our basement, but not the only one.
    I haven't heard of ERVs retaining radon, so I don't have any comment on that topic.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Jerome Lisuzzo
    I don't think that you should pursue the idea that you can reduce your indoor radon levels by sealing cracks in your foundation.

    GBA has been reporting for years that tight homes can have high radon levels. For example, Peter Yost reported in 2010: "When you make a home more airtight, the good news is that you could be reducing the entry pathways and reducing the pressure differential between the soil and home. The bad news is that you are also significantly reducing the air exchange rate in the home. So, what really happens? The short answer is: it depends."

    In my 2013 article on radon, I wrote, "High radon levels are possible in new homes as well as old homes, and in tightly sealed homes as well as leaky homes."

    Your best bet is to call up a radon mitigation contractor.

    An ERV is a ventilation device; it introduces fresh outdoor air into a home. While it is theoretically possible for some of the radon in the exhaust stream of an ERV system to be transferred to the incoming fresh air stream, the ERV is not the source of the radon, and in any case the ERV is helping (slightly) to dilute the concentration of radon in the home by introducing a measured amount of fresh air.

  4. jdom | | #4

    Your responses
    Thanks guys! Always appreciated.

  5. Paul Honig | | #5

    Response to Martin Holladay
    Hi Martin, I have a slightly different take on trying to reduce the amount of indoor radon by sealing the openings through which the radon passes. If you're able to find the gaps and seal them why not? The chances of actually finding the perpetrating gap may be pretty small, but the cost of trying may also be small. In my case, if sealing the well's electrical conduit (5 minutes and $5 worth of spray foam) reduced the indoor radon level sufficiently I wouldn't have put in the conventional mitigation system. No harm in buying the powerball ticket if you're already at the convenience store counter and have a dollar to spare.
    And whatever mitigation path you choose, it makes sense to periodically retest radon levels to make sure the mitigation is still effective.

  6. GBA Editor
    Martin Holladay | | #6

    Response to Paul Honig
    Your advice makes sense. I agree. Thanks.

  7. Hannah_H | | #7

    Hi Paul, great article! I have a double envelope passive solar house constructed in the 80's and I think I may have a problem with radon. I can't find a radon mitigator familiar with this type of home in my area. I'm wondering - would it be better to have the radon mitigation system installed in the envelope (the chamber surrounding the house proper - where we also have a geothermal cooling tube) or in the house proper? I assume the house proper...right?

  8. charlie_sullivan | | #8

    What type of radon mitigation system are you considering? A typical design depressurizes under the slab. Does your double-envelope design have a shared foundation for both? Do you have any drawings of the foundation design?

  9. Hannah_H | | #9

    We're looking at the typical depressurizating system, is there another kind? It is a shared poured foundation, unfortunately I can't find much on the foundation. Here are a couple old diagrams from the 80s! We have a geothermal cooling tube that we shut from October - May but typically keep open for fresh, cooled air in the summer.

  10. charlie_sullivan | | #10

    This discussion isn't getting a lot of attention--you might try making it a new topic on the Q&A. but from the drawing, it looks like there's one footing aligned with the outer wall, which means that the hole through the slab could be anywhere and theoretically that whole sub-slab space is connected and it would work. I'm not sure I fully understand the drawing--there a black line going through the slab where the inner wall rests, and I can't read the label on that, so I'm not fully sure I understand the setup. But that's what I think from what I see and understand.

  11. yzerham19 | | #11

    Hi everyone I have a radon mitigation system and my Airthings radon monitor shows my levels range from 0.3 to about 1.5 pci/L which is decent but I want to further decrease radon levels and was thinking of putting down a vapor barrier on my concrete floor in the basement before installing water proof flooring on top. Can anyone recommend the best barrier on the market and also if there is a better way? I live in NY. Thank you!

  12. CamSustainable | | #12

    Can I mitigate with the infusion of passive air to offset the pressure? with exchanger to heat pump /AC combo only?
    That way the air comes form outside, passes trough the air filter at the right temperature and then fills the house naturally as the exhausts remove the air. I have 2 (400 feet of linear underground pipes each) - 4 inch pipes (1 per floor 2000 sq feet each)

    1. charlie_sullivan | | #13

      It's not clear to me what overall scenario you are talking about. It might be better to ask a new question in the Q&A section, and give a little background. New build or an existing buildling? If so at what point in the project? What is the construction, particularly the foundation? What mechanical equipment is installed now?

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