Shortly after my wife and I bought our 1961 home in 2019, I discovered a serious problem in the basement. Where I could see it, the slab was cracked all along the edge of the exterior walls (photo above). I figured it might be causing elevated radon in my house, so I did what any normal person would do. I avoided testing my radon level. Until last spring, that is, when I found it was above where the US EPA says you should fix it. Now I’ve finally started doing something to get some reduction in our radon level.

What was wrong with my basement?

After doing a seven-day radon test in early April last year (2023), I dug into those cracks in my basement slab. You can see below what I found. Beneath the cracked slab was a piece of plastic sheeting. Beneath that was a layer of granite chunks. And beneath that was a corrugated plastic pipe.

Yes, this was an interior perimeter drain system installed decades ago, but after the house was built. In addition to the cracks in the slab allowing soil gases to leak into the house, that piece of metal wasn’t sealed either. In some places, there were large gaps at the top edge or between it and the slab.

And then there was the mystery hole I saw shortly after we bought the house. That’s red Georgia clay you see in the middle of hole (photo below). Because it’s in a closet near the middle of the basement, it couldn’t be the same problem I had at the perimeter.

After getting the drywall removed, I discovered the reason for that mystery hole: a plumbing drain pipe that was added during a renovation after the house was built.

That hole is still open, but I’ll fix it after I remove that interior wall and get the bottom plate out of the way.

Soil gas leak sealing, so far

The first part of the basement renovation exposed the basement to more soil gases because we opened up the slab in multiple areas. But we also separated the basement from the house with plastic sheeting and put in continuous ventilation to exhaust air from the basement. (You can see the fan in the window below.)

Since then, we’ve been closing up the slab and the concrete masonry unit (CMU) foundation walls. The photo above shows the new concrete over the new interior perimeter drain. And the plastic on the wall is sealed with tape and polyurethane sealant (mostly).

Then, the concrete contractor came and covered up the big hole where the plumbers replaced cast iron drain pipes (photo above). They also dug a hole for a footing to hold a new column and then filled the hole with new concrete.

So, there’s a lot less leakage area for soil gases to get into the house.

Radon level timeline

Now, how has the radon responded during all that’s happened? You’ll see the graphs in the next section, but let me provide a general timeline here of my radon measurements and basement status.

Early April 2023. Did a 7-day radon test and radon level was 7.7 pCi/L. Tested main floor, not basement.

20 May through 23 Sep. 2023. Alpha track test resulted in 10.3 pCi/L of radon. Tested basement.

13 July 2023. Installed Airthings Space Plus continuous radon monitor with sensor on main floor.

27 May 2023. Opened old perimeter drain in one room of basement.

9 Feb. 2024. Basement renovation started with day of demolition.

27 Feb. 2024. Basement slab closed up after replacement of perimeter drains and new concrete over other work.

That will provide the context for the radon level graphs below.

Early radon reduction results

The first graph here shows our daily average radon level since I put in the Airthings monitor on 13 July. Before the renovation started, we were averaging pretty much the same radon level I got in my seven-day test last April: 7.7 pCi/L. With the recent reduction in radon level, that average is now down to 7.4 pCi/L, as you see below.

Looking at the radon data over a shorter time period, the past month, we can see that the radon level dropped about ten days into the basement renovation (~19 Feb.). After the slab holes were filled last week, it dropped even more.

The 48-hour graph below shows several periods of the radon level staying at 0.0 or 0.1 pCi/L for hours at a time. Wow!

So, we have some significant radon reduction going on here. But…

Is it too soon to declare victory?

Yes, absolutely! It’s definitely too soon to declare victory. First of all, the house isn’t in its normal state because of the basement exhaust ventilation and the plastic sheeting separating the work zone. Also, things change seasonally and with weather patterns. Notice in the first graph that we had a couple periods of low radon levels last September and October.

But, I definitely have reason to be hopeful that there’s some significance in the radon reduction I’ve seen already. The lows we had last fall bottomed out at 2.0 pCi/L, whereas the recent lows have been 0.0 pCi/L. Also, I still have some sealing to do. I had to remove some of the wall plastic and haven’t resealed those edges yet. And there’s the closet hole and a few other little places still to seal, too.

I didn’t expect to see such a dramatic and quick reduction in my radon levels at this stage, but it makes me happy to see where we’re at now. And it provides (with the caveat of needing more data) some justification for all the money we’ve already spent on this project.

Radon update, May 2024

After I published this article in March, I made an accidental discovery. In the second section above, “Soil gas leak sealing, so far,” I mentioned the exhaust fan I’ve had running in my basement since I began this project in February. Well, I turned it off one day and then forgot to turn it back on until the next day.

The graph above shows what happened. The radon level spiked up to 12 pCi/L. When I turned the fan back, the radon level dropped back to the safe level. So it seems that the slab sealing I’ve done so far may not be the reason for low radon levels for the past few months.

Stay tuned. I’ll let you know what happens when the basement is all done and the temporary fan removed.

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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 subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn. Images courtesy of author.