Radon Mitigation Basics for an Older Home
I’ve been telling you the story of radon in my Atlanta home for three years now. In April 2023, I got the results of my first radon test. In July of that year, I installed a continuous radon monitor to see how it varied over time and under different conditions. In early 2024, I began the renovation of our basement, which I hoped might solve the whole problem. But I was wrong. Things did improve, but recently I decided it was time to mitigate. So today, let’s talk radon mitigation basics.
My radon test results
For my 2023 article, I had done only a 7-day activated charcoal test. I placed the test kit on the main floor, where we spend the most time. The result was 7.7 picocuries per liter (pCi/L). Then I did a four month alpha track test over the summer of 2023. For that test, I put the test kit in the basement, closer to the source. The result was 10.3 pCi/L.
In July of that year, I installed an Airthings Space Plus IAQ monitor that also gave me continuous radon readings. Now I have nearly three years of radon data. Let’s look at them in chunks.
As you can see from the graph above, our seven-day charcoal test was pretty accurate. The periods where the radon level dropped into the green zone were times when I had the windows open in the fall.
The graph above shows our radon level for the whole year of 2024. In February of that year, we began our basement renovation with demolition and asbestos abatement. From then until we got the new windows installed in September of that year, we had the basement open and ventilated with an exhaust fan almost continuously. That’s why you see the big drop in radon for most of the year in the graph above.
2025 was a normal year. Phase 1 of the basement renovation was finished. I had sealed up a bunch of the leaks in the slab, and it made a difference in our radon level. We were down about 2 pCi/L from before the renovation.
Here are the data for 2026 year to date. Again, we’re in the mid-5s for picocuries per liter of radon.
House open or closed?
One thing you may have noticed in the 2023 and 2025 data are the dips into the green zone for radon, which means the level was lower than 3.0 pCi/L. You may also have noticed that those dips were in the fall and spring. Turns out that opening the windows in the basement and main floor is a good way to keep your radon levels down. Unfortunately, that’s not realistic to do throughout the year here in Atlanta.
The graph above shows the December, January, and February data for this past winter. As you can see, our average radon level is back to the old number when the house is closed up. So maybe the slab sealing I did during the basement renovation didn’t help that much? Maybe it’s just because I’m averaging in the low radon periods when the house is open in spring and fall.
Radon mitigation basics
When your average radon level is above 2 pCi/L, the US EPA recommends doing something to mitigate it. And the thing to do is called sub-slab depressurization. If you’re lucky, you can do it without a fan. You install a pipe beneath the slab, and the soil gases rise passively in the pipe because of the stack effect. The passive system can work well, but it’s hard to do in existing homes…especially mine, as you’ll see below.
If that doesn’t take your radon down to an average of less than 2 pCi/L, then you add a fan. With active depressurization beneath the concrete slab or crawl space membrane, you get rid of radon and other soil gases before they enter the living space.
CAUTION: If you decide to take this on yourself, make sure you understand not just the radon mitigation basics but all the details, too. One thing you definitely want to do is make sure the radon fan, if you install one, is outside the living space. You want all the pipe running through the conditioned space to be under negative pressure. Otherwise, any leak on the positive pressure side of the pipe will pump soil gases—including radon—into your home.
The EPA has a detailed guide on this topic called Building Radon Out (pdf). Click that link to download it for a full explanation.
Our radon mitigation
On Monday, Smart Space Atlanta came out and installed a radon mitigation system in our home. Our house has a couple of complications that make it a bit more difficult. First, our foundation is part basement and part encapsulated crawl space. Ideally, you’d want to depressurize the sub-slab and sub-membrane areas separately, but Smart Space has had good luck tying them together. They do have a valve that allows them to adjust how much air comes from beneath the crawl space membrane and how much comes from beneath the slab.
Second, our 1961 house has no gravel beneath the concrete slab. No plastic liner either. And we’re sitting on the red clay of Georgia, so that makes depressurizing the sub-slab area more difficult.
They used a concrete hole saw to drill through the slab (photo above). Then they reached in and pulled out enough dirt to fill about four 5-gallon buckets. The 6 inch PVC pipe connects to the hole, which is sealed with caulk. The crawl space has a flexible, perforated pipe beneath the membrane. As mentioned above, the two are connected with a valve that allows for adjustments of the air flow.
The radon fan (RadonAway RP265) is mounted just outside the house where the pipe comes through the wall. Yes, it’s on the front of the house because that’s the best place to put it here. But it’s sort of hidden by the azalea you see and the Japanese maple you mostly don’t see.
That’s it. Pretty simple setup, and they were done in one day.
Early results
We’ve had the radon fan running for about 18 hours so far. I marked the beginning of it on the graph below.
As you can see, the radon level is mostly in the green. There’s some yellow (between 3 and 4 pCI/L) and we hit 4.1 pCi/L for a couple of hours before it turned down again. That’s an encouraging start because I’ve had the house completely closed up since they started the fan. Now look above for my three months of winter radon. We averaged 7.4 pCi/L with a closed-up house then. When I’ve closed up the house in the past, the radon has gone up and hit the red zone with hours…and then mostly stayed there.
If for some reason our longer-term radon level averages a higher number than I’d like, we have some options. The first thing probably would be to adjust the valve to change the ratio of air coming from crawl space versus basement slab. If that doesn’t work, we could separate the depressurization beneath the crawl space membrane and the basement slab.
More info
After I first published this article, Ross Trethewey sent me a link to this nice video about radon mitigation they did in a hundred year old home. One thing they show in the video is drilling multiple test holes to measure how well one area beneath the slab communicates with other areas. And when you find out what happened to the dog, you may be motivated to get your house tested if you haven’t done it yet.
So, there are some radon mitigation basics for you. If you haven’t had your home tested, that’s the place to start and I recommend it. Both of my parents died of lung cancer, and I can tell you it’s not a pretty way to go.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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Do you have and ERV installed or would it be beneficial to have an intake communicating with your cold air return?
I failed to mention that this home has no air conditioning or forced air at all (unless you consider a swamp cooler to be “forced air”). I plan to install mini-splits (1:1) as much as possible, and optimized for heating. Installing an ERV is a good idea and it can be used to add a bit of positive pressure to the living-area of the home, provided that the windows are not open. There is no reasonable provision for traditional duct-work but the smaller ducts used by an ERV can likely be retrofitted.
David: Yes, we have a Zehnder ERV. You can read about it here:
https://www.energyvanguard.com/blog/ventilating-with-a-super-efficient-erv/
I’m not sure what you mean by the second part of your question. Are you referring to a central fan integrated supply (CFIS) type of whole-house ventilation?
Somewhere in your system you have a leak. When I was doing energy audits using a blower door, I was also checking any movement in the water Column on systems I came across. I sometimes would make a home to airtight and one primitive way was to introduce a fresh air intake, Y-8150 Honeywell controlled damper, directly into the cold air return.
I also used the REDCALC software to set up systems in home for a balanced system. However being a government agency, I was never able to connect House exhaust, Bath or Kitchen fan to this system.
I would hope with the passive system that your numbers would subside. I wonder if you had a blower door with a thermal camera to locate these leaks in your system.
I think I remember you installed a insulated sub floor. Would it beneficial to identity another location and install a second location?
Would it be crazy to do multiple mini holes and monitor them to isolate of find the highest concentration?
Over 9 years ago we bought a large 2-story home (built in 1982) in Pine, Arizona, which is in the mountains just under the Mogollon Rim (200 mile edge of the Colorado Plateau). It is still a second home for us, but we plan to move there. Over the years I have read about radon, and I decided to do some testing. Virtually no radon data was available for Gila County / zip code 85544, but I had a sneaking suspicion that there could be high radon because there are uranium mines in the county and the area is geologically amazing.
I installed Airthings radon testing devices. First only one (in the living space) was installed, but after observing high radon (a peak of 9), I also installed a device in our garage/half-basement. The Airthings device in the garage has peaked as high as 31! Our long-term radon averages are 2.9 pCi/L in the living area, and 8.7 in the garage.
I have spent much time sealing the concrete in the half-basement/garage, which is perhaps only 15% under below-grade, but divided by a cinder-block wall from a dirt crawlspace of similar area. The dirt crawlspace (a steep slope) had no venting at all. I had been doing a lot of air-sealing in the crawlspace and this resulted in a hike in radon and to dangerous levels of humidity. Recently I installed a 200 CFM exhaust fan in the crawlspace (still with no inlet vents) and it seems to have made a difference for the living area in that the radon readings are vastly better (average of 0.8 pCi/L). The adjacent garage was not improved at all. I plan to put 13 mil plastic down in the crawlspace, sealed to the walls. Perforated 4″ plastic pipe will be laid along the crawlspace walls and connected such that there will be a place where a radon fan can draw from if necessary.
Something which became very obvious to us is that opening the downstairs windows short-circuited the stack-effect and causes the radon to go to almost zero. Luckily, opening the windows is usually very pleasant there.
Bob: It sounds like you’ve done a great job investigating and mitigating. Is there some reason you can’t just leave the garage open? Another option would be to install an exhaust fan there. Unless you spend a lot of time in the garage, though, you probably don’t need to worry about it as long as the radon level is low in the house.
Allison, I wish I could say that I have made a lot of progress with investigating and mitigating, but I am still only just beginning. I did read the “A House Needs to Breathe…” book (cover to cover), as well as Michael Church’s “Crawl Space Repair Myths Busted”. I have also learned a lot from some YouTube channels. While I have now spent a lot of time in the crawlspace, I have not yet entered the attic. Your book details many things to look for in the attic, and elsewhere. I have already found many things to fix which are not yet fixed!
Regarding leaving the garage “open”, that would be a bad idea due to possible theft, bears, rodents, and freezing weather. Adding forced ventilation (with a switch) there is a good idea, regardless of radon. The temperatures drop to 6 degrees during winter and during the summer monsoon, it is temporarily humid like Georgia.
I did leave the garage door and windows open for substantial time. The radon readings did not drop much and I found that to be surprising. I read that the Airthings reading is only updated once per hour, it takes time for air to enter the chamber, and they add to a rolling average so it could take time to change. I mounted the Airthings device to a constructed plywood panel which I am suspecting may be hiding an added foundation support for the upper floors, which goes through the concrete (maybe open to dirt on the perimeter). If so, I may have attached the radon monitor to a major radon source!
Allison, i believe in radon mitigation but think the gravel.base emits more than existing soils. I build with a perforated pipe around the perimeter set in eps peanuts in the trench using vapor barrier as a cap and bearing foam above slab on top
Dave: I hadn’t thought about that. Yes, granite gravel can contain uranium, which is where the radon decay chain begins. And with lots of unfinished surface area, that gravel could be adding radon to a house that might not otherwise have it…or more to a house that does. But if you have a good amount of perforated pipe in the gravel and a vertical pipe to vent it above the roof, it seems like that would solve the problem.
Sounds like you’ve got a good method that should work. Have you measured radon levels with it?
When we built our house, the contractor only wanted to place a t-fitting under the basement slab. I had him add perforated pipe there and under the conditioned crawlspace membrane the length of the house. Tests have shown levels below 1.5 pC/L using a continuous monitor without the need for a fan. Very glad I extended the pipe.
Norman: Good call! I made your builder’s mistake when I built a house in 2001. I just put a tee under the slab with extending it. It probably was OK because we had a good layer of gravel beneath the slab.
I was lucky that UGA came out with an 82 page Radon Prevention set up just before I had my slab poured. The plumbers installed the perforated pipe. No right angles were to be in the stack and I had to ask that one be removed! Trust but Verify!
Hi Allison!
Your post are always so timely. We finally got around to testing our house for radon (10 years in) and got our test results back earlier this week. Our results came back at 2.1 pCi/L, which seemed acceptable till I saw the level of variance in your tests. It makes me wonder if our radon is high enough at other times we should pursue mitigation. Should we keep testing just to get a more accurate sense of our levels over time? Install the IAQ monitor you linked? I loaned your book to a friend who is undergoing an extensive home renovation and don’t have my trusty guide to consult.
Alyce
When I was a grad student in the early 1980’s, I did a literature survey and wrote a report on radon risks since that was when this topic was first investigated. There was solid evidence relating cancer risks to radon from uranium minors who were exposed to much higher levels. The acceptable limit of 4 pCi/L was based on an extrapolation of the cancer risk from data that were two orders of magnitude higher in terms of radon levels and cancer rates. There were no direct data at that time relating radon exposure to cancer risks at around 4 pCi/L and I don’t think there has been any since. So is this really the right “limit”? Beyond that, do you really think that there is a significant risk difference between the levels of 2 and 10?
I have a medical opinion that Radon should be less than 2.0 especially for those under 55 yo
Up in Canada our federal guideline also allows two additional options – depressurisation through the sump pump lid and through the wall exhaust with the fan indoors. The latter is to prevent condensation, frost, and fan damage if the fan is outside or in an unconditioned space. I’ve seen many houses retrofitted with this approach.
https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/radiation/radon-reduction-guide-canadians-health-canada.html