about a year ago I had my crawlspace conditioned. Among other things, the company put in a concrete slab for the floor and added a sump pump in the corner.

The strange thing is, there is a 1-4 inch gap on the surrounding slab (see attached picture).

NOTE: Where the crawlspace connects to the basement the gap is not there i.e: where there is "no wall"

Does anyone know why this is? I am assuming this is related to the drain for the sump pump?

Another concern is my radon levels in the basement are extremely high and I am wondering if this big gap is contributing to the spiked levels.

Thanks in advance


  • How high is extremely high? You might want to consider a lunos hrv instead of a radon mitigation system and get heat exchange and fresh air instead of just dumping your heat to the outside. Probably the air being swapped will mitigate the radon issue. Jan 6, 2020 at 17:21
  • @FreshCodemonger it was around 10.8 - Ill bring it up to the contractor and start looking into the HRV. Are the mitigation systems effective? I have my concerns on if it will truly help
    – Andrew
    Jan 7, 2020 at 18:33
  • Certainly a radon mitigation system can be effective. In your current situation I am not sure what they would do. Typically these are put in before the slab is poured and depressurize the underslab area by sucking air out. Radon is heavier than air so it lingers in the low areas - so really all you need to do is ensure the air at the lowest point of your house is swapped out. What kind of radon detector do you have? 10.8 is quite high. 2.4 is where they recommend remediation. Jan 7, 2020 at 19:12

1 Answer 1


Almost certainly for drainage. Carrying any water leaking in from the walls to the sump.

My understanding of radon mitigation would be that this gap and the sump pit need to be sealed from the top and power vented to the exterior. Any underslab drainage on the "basement" side should be connected to the vent system as well.

If you already had a radon mitigation system in place and the company did work this way they are a bit clueless/irresponsible. If you did not have such a system, you evidently need one now.

  • Makes perfect sense, had a buddy come over who puts radon mitigation systems in place and said exactly what you did :) getting a system put in this week. Appreciate the help
    – Andrew
    Jan 6, 2020 at 15:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.