We purchased a home that had one installed during construction. I'm just wondering if people have had a positive experience with this type of system and can explain how it works. We just had a radon test completed and it came back above 4 and would require mitigation to sell. The monitor on the tube shows that it is "working" as the pressure is not equal. Anyone have this type of system and any problems with it? Thanks!

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    The system can be working, but not working goof enough to remove all the radon gas. Here, we'll install a gravity system (without a fan) and if that doesn't work, we'll add an exhaust fan. I'm not sure what a "reverse pressure system" is... Basically you're trying to remove the gases in the ground so they don't enter the house. Does "reverse" mean you're making the house have positive pressure so the radon can't enter the house? (Btw, I don't think that would work.)
    – Lee Sam
    Aug 9, 2017 at 6:57
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    Thank you for your feedback, Lee! That's what I'm wondering...if by "reverse" they mean they're trying to make positive pressure in the house. I have a call out to the company who installed it. When we purchased the house 2 years ago the test came back at 2.8. I've been trying to search for more information about a reverse pressure radon mitigation system and can't seem to find anything online.
    – Amy E
    Aug 9, 2017 at 12:48
  • Radon is much heavier than air, and just creating positive pressure inside the box of the house is not enough to expel it. You are going to need an exhaust duct with a fan. The duct must start very near the floor of the basement. Quite often the water heater and furnace act as such and are sufficient, but every case is different.
    – ajeh
    Aug 9, 2017 at 14:17
  • A typical radon mitigation system depressurizes under the slab by pulling air out, often that's called a negative pressure system, I'd imagine that's what they mean. If you have a tube with an indicator, you most likely have an exhaust fan. Does the pressure gauge read around 1? Do you have a full basement or partial with a crawl space?
    – Hart CO
    Aug 9, 2017 at 14:38
  • We have full, finished basement with a crawl space. The new buyers do not want the system to run on the outside, but not sure how that can be accomplished since the house was not constructed with piping roughed in for a radon system. The current system does have a fan at the top and it is sucking air down through the pvc pipe.
    – Amy E
    Aug 14, 2017 at 18:50

1 Answer 1



Radon is a byproduct of radioactive decay. You don't have to live on top of a uranium deposit; there are trace amounts of radioactive minerals in common rock, like granite. So virtually anywhere you live, the soil slowly releases minute amounts of radon.

Outdoors, the wind disperses it. What seeps into your basement tends to accumulate. Your HVAC system will circulate some, and the exchange of outside air will get rid of a little. But radon is heavier than air and basements usually aren't well-ventilated, so the basement is usually where you see the highest concentrations.

Because radon is heavy, it also tends to remain in your lungs when you breathe it, and being radioactive, it's a source of lung cancer.

The 4 pCi/L threshold isn't a magic number below which you're safe and above which you die; it's somewhat arbitrary. Any amount carries some risk, and the government had to weigh risks and costs in order to pick a number at which they recommend doing something about it. They may just have picked 10x the average outdoor level of 0.4, perhaps because inexpensive technology can typically keep a basement below that level, and that represents a minimal health risk compared to other things people are exposed to every day.

So if you do a radon test and it comes back 4.2, the risk to your health isn't substantively different than if it came back 3.9. If you don't spend time in your basement, the risk to your health is relatively low.

However, you would likely need to mitigate it when you go to sell, so you might as well do what you can do cheaply now, and benefit from it yourself.

Negative pressure systems

The negative pressure systems are a pretty simple mechanism. There is a layer of gravel under your basement slab that has air filling the voids between the stones. The radon collects there. A pipe is placed in that space. The pipe contains a blower that sucks air from the gravel and vents it at roof level. This removes a lot of the radon before it can migrate into the basement. The pressure indicator shows the relative pressure. As long as the pressure in the pipe going into the gravel is lower, it will theoretically be exhausting radon from under your house.

What can go wrong?

The system relies on a few things to work:

  • A "normal" level of radon. The level of radioactivity in the soil varies. If your house is located over veins of uranium, this kind of system may not be enough.

  • A working blower. Like any mechanical device, the blowers don't last forever. If it completely dies, the pressure indicator will show no pressure difference. But before that point, it may become less efficient, and less effective at removing radon. If the system is old, check how much air is being exhausted.

  • An unobstructed pipe. Anything that reduces air flow in the pipe will reduce its effectiveness. Check for anything that shouldn't be there at the exhaust end. Obstruction at the intake end will leave the pressure indicator showing a pressure difference but little air being removed from the gravel. Water in the gravel would be an obstruction. The gravel can also accumulate silt if there is a water problem. The amount of air being exhausted will be a clue.

  • Air in the gravel. If the foundation has a water problem (high water table, rain water collecting, broken sewer pipe, etc.), the gravel voids can fill with water. This won't reduce the release of radon, but it will prevent the abatement system from removing it (and the pressure difference will still look like the system is working). Dampness or water in your basement will be a sign of this. If you have a sump pump, the basement may be dry even with water in the gravel, but that will provide a path of easy access for radon. If there is water under the slab, it will, at a minimum, reduce the effectiveness of the abatement system.

So those are a bunch of things to check. Beyond that, there is a general process of the abatement system degrading with age while the basement becomes more permeable to radon. The foundation settles, concrete and mortar develop more cracks, etc. If the basement has a large footprint, a radon abatement system at one end may be challenged in exhausting radon from the entire area.

If there is no obvious cause for the system not achieving levels below the threshold, one solution may be to install a second system at the other end of the basement; they are relatively inexpensive.

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