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I recently purchased a home. During the inspection there was measured radon levels of 6.3 PiC/L.

After having a professionally installed radon mitigation system, an electronic sensor indicates an unchanged level of 6.3. The system was installed 5 days ago.

The contractor noted clay soil underneath the basement slab.

What strategies can I employ to reduce my radon levels further? Should remain optimistic about reducing the levels below the WHO guideline of 2.7?

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  • Blow as much air through the basement as possible (without freezing your pipes) and take another measurement. Make sure you make up the exhaust air with fresh air from outside and "stir" the air in the basement since radon is considerably heavier than air. What was the "professional" solution? Just ventilation? If you are just blowing 10 CFM or so through a 500 sq. ft. basement, the serial dilution rate could be low if the air currents generated by the system don't adequately mix the air. – ScienceGeyser Feb 23 at 2:01
  • I had a sub slab depressurization system installed. They cut a hole in the basement slab and suck air through it into a pipe to the outside. – ugh Feb 23 at 2:19
  • Interesting. They must assume that there is porous material all around your slab. Otherwise, once you pull the air to a static pressure, there is no more movement and the system is diffusion limited. Is it possible to measure an air flow out of the exhaust pipe that they installed? – ScienceGeyser Feb 23 at 3:02
  • I practical terms no. I can tell you there is about 1.75 inWC of vacuum about 2.5 far above the slab penetration. – ugh Feb 23 at 3:05
  • Maybe they found some cracks in the foundation that they want to pull room air into instead of letting radon come through the other way. Pulling a vacuum does not move molecules once you reach a static pressure. If you want to move molecules out, you must have molecules coming in. Otherwise, you just have static pressure and diffusion does what it always did, just at a lower pressure. – ScienceGeyser Feb 23 at 3:11
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A gas or oil burner in the basement could produce a small vacuum if that burner takes the combustion air out of the room where it is installed. If the basement windows and doors are closed, the radon could be sucked into the basement via cracks and openings in the walls/ground slabs. Newer burners do get the combustion air through the exhaust pipe (counterflow in a ring). Also other equipment could provide a tiny vacuum in the basement, f.e. fans blowing outward.

The layer(s) just below the first solid building layer and next to the walls under ground should always be made of material to break the capillary effect, f.e. small pebbles. This will avoid water getting sucked into the basement. The radon can free flow in these pebble layers, and one way to reduce the radon level is to suck the air from this layer and blow it to the Lee side or over the roof of the building.

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