Our house is a 1993 build, two story detached, block foundation and finished basement.
Last year I bought an Airthings Corentium radon tester, because I was curious about radon. I monitored the main living area of our basement for several months to see what levels there were, and how much fluctuation there was seasonally.
The levels did fluctuate, and were close to but not above, the Canadian recommended level of 200 Bq/m³.
Out of curiosity, I placed the monitor in the cold cellar. The cellar has an exterior door leading into it from the main basement, and also contains a sump pit with a rudimentary plastic lid. After a couple days, I checked the reading and was shocked to find a level of 1100 Bq/m³. Yikes.
We keep the cat litter box in the cold cellar (for odour reasons), and I had installed a cat door a while ago. Since radon gas is heavier than air, my guess is that it was seeping into the main area through the cat door.
Initially I was going to just seal the sump pit and continue monitoring, but with readings that high in one section only, I figured I should just go directly to mitigation since there was most likely seepage elsewhere.
After reading that attempting to seal the entire foundation is basically a fool's errand, I ordered a radon exhaust fan and set about with creating sub-slab depressurization.
Fortunately my furnace room is directly beside the cold cellar, which allowed me to cut a hole in the foundation wall and the floor slab on each side, join the two, and vent both the main slab and the cold cellar slab. (The footings would have caused a barrier since the gravel under the slab would have been separate.)
I routed the exhaust pipe out through an existing air vent in the cold cellar wall.
One of the issues I ran into was capping the sump pit. The pit tank had a diameter of 20 inches. A company called Radon Detect sells a 24 inch radon sump cover that would fit, but it's $179. I couldn't justify paying that much for a chunk of plastic. I was going to just cover the sump with some 6 mil poly and tape, but that was a bit too ghetto for me. A chunk of Lexan would have worked, but it's pretty pricey as well, and while it's strong stuff, I was worried that it may get stepped on.
I went to Lowes, and wandered from department to department looking for something suitable. Finally, I found just the thing. A Bigfoot deck/pier form, 24 inches in diameter. The top had steps that could be cut off to accommodate different sizes of sonotubes, which was perfect. I did have a small piece of clear acrylic hanging around that i could cut to size as the "lid", and would allow me to see into the pit to inspect the sump pump. The "step" that I picked was large enough to allow the sump pump to be removed without having to pull the Bigfoot away from the floor.
This is what the final product looks like.
Here's the fan
Fan in place
72 hours in, the radon levels in the main basement area have dropped to 2 Bq/m³. I'd say this battle has been won.