Don't think this is right

This is a mostly unfinished basement with the exception of this bathroom (on the other side of this wall). The builder finished the bathroom. If you look under the tub, there is no concrete floor, it’s gravel. Which would be fine if it were a crawl space but it’s not. Even if it were a crawl space it would have a vapor barrier.

So is this an issue, having a big hole in your basement floor? It will be walled in soon but I feel like there should be some kind of barrier there.

What do you all think?

are you saying the tub is sitting on gravel? If so, I would call it out as needing solid support

If sitting on gravel and no support, probably all those moisture will cause some movement and soon or late, the buyer will sea crack and leaks, etc… Thank for sharing.

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Good catch. In my area, moisture coming up from the floor will cause bigger issues than one may think. You don’t see the hole in the slab in the photo, but this growth is adjacent to an unseal plumbing cut-out in the basement floor.

It looks like it may just be the trench they cut out for the new plumbing, but should have been filled back in. Just lazy contractors

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You are correct. If the hole in the slab is not sealed both moisture and radon can migrate into the basement.

no contractor will pour concrete around the TUB P-trap. its a small square hole filled with gravel to still have access to the trap if it was to ever clog. it will look exactly the same if the concrete was a wood subfloor. Look up tub install instructions.

They could have sealed it with some plastic, but its typical and correct.

and by the way Radon can travel through concrete just as easy as gravel.

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Do you believe this to be true?


Yes, concrete is porous. If water can get through it so can radon atoms.

Porosity of Concrete

Concrete cures by reacting with water – hydration. But almost half of the water added to the concrete mix for workability is surplus and has to evaporate. As the surplus water in newly poured concrete pushes to the surface, it leaves behind a [network of capillaries (pores).

The pores constitute 12 to 18% of the concrete by volume. Their diameter is much smaller than a single human hair but the pores are much larger than radon atoms or water molecules. They let in soil gas carrying radon and water vapor.

The dominant transport mechanism by which radon gas moves through an intact concrete slab is concentration-driven diffusion. Studies have shown that indoor radon entry rates can be greatly affected by the type of concrete mix employed.

Radon can travel through concrete just as easy as gravel?


Depending on where the aggregate in concrete was mined, it also could contain radon.

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Question of the Week


Where did you get this information???


No, this is not true. Radon will not travel through concrete as easily as gravel/aggregate.

During a typical sub-slab radon mitigation, we love to see aggregate underneath the slab as the radon (gas) flows very easily through it…and the slab (concrete) provides a great barrier for the gas to be funneled toward our vacuum field.

Notice I am NOT stating radon will not go through concrete… merely pointing out the fact if you have two materials with large porosity differences… gas most def choose the easiest route.


I simply asked a question based on another member’s statement.
What’s up Michael

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I am not a licensed mitigator but I would think if this house had a test that resulted in high Radon readings that that area under the tub would need to be sealed, if not to help mitigate the high reading, but also to help create a vacumm under the floor.


Yes sir. I was answering that question you asked. Hope it helps.

Hello Michael Bird, welcome to the forum.
As soon as one of the members stated that radon can travel through concrete just as easy as gravel, I posed the question “do you believe this to be true?” not because I don’t know the answer, but because I wanted the person making that statement to re-evaluate his statement and realize the absurdity of it.

Marc and a few other members also chimed in with similarly INDIRECT quips and questions as a way to encourage further thought and hopefully research by the person making this claim.

Welcome to the Forum.


Thank you for the warm welcome Bert.

I am but a roving visitor to this forum but figured since I mitigate radon I could help offset the lack of truth in William’s post. The same reason I have misread the room is the also the same reason I posted. It APPEARED as if one person was using “science” to back his incorrect claim and everyone after were merely asking questions. Glad to see there is more knowledge than apparent… may be worth thinking about how things can appear to lurkers whom may be seeking information. Indirect can be hard to differentiate from just text…though I have no doubt it helps keep the peace. :+1:

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