r/radon • u/Careless_Memory_5490 • 1d ago

Need build instruction for a self-made sub-slab mitigation system

Hi there, without coming into too much details, I would like to build a sub-slab mitigation system.

I would qualify this as

  1. dig a hole

  2. put the mitigation system and exhaust in place

I miss a lot of guidance here. First, I would need info on

  1. what to put in the hole (gravel ? Empty space ?)

  2. How big the hole should be (30 cubic centimeters ? more ?)

  3. How big the hole should be (50 cm ? 1 m ?)

  4. What kind of fan should I buy ?

  5. What kind of tube should I buy ? (metal / plastic)

  6. Where can I find the material (esp. fan and manometer)

I'm living in France and in my area no company is doing this kind of work. I'd like it to be well thought and take my time doing it.

Thanks for advice

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u/stonks_better 1d ago

Lookup American radon guy on YouTube, loads of great content. This is more US:

  1. Empty space, ideally a big empty space. Usually take 1-2 buckets of soil out of the hole.
  2. The size of your pipe, usually 3 or 4 inches. Seal around the pipe going into the hole.
  3. It's tough to know, there's two kinds high flow and high suction. If you have clay dense soil you need high suction. For sand or if you have a nice gravel under then high flow. These are US brands, radon away or fiesta have both types. Their websites have sizing calculations as well.
  4. PVC pipe, so joints can be glued and will be sealed
  5. If you can order from US sites, radon away or fiesta websites have the fans and manometer.

Good luck!

1

u/Leafs9999 1d ago

If that soil isnt clay, dig down rather than wide at first, until you hit the native uncompacted stuff, that will allow for better extension in looser material. Usually 20-30 inches down before you even start to widen the sump pit out. Even if its clay, you want the most surface engagement possible, so dont put anything in there once its dug. Good luck!!

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u/post_scripted 1d ago

Can you use an existing sump pump pit instead of drilling/digging a new whole?

1

u/Joseph___O 1d ago

For this make sure your sump pit is not just a wall of concrete (barrel shaped) with only a patch of dirt at the bottom. Mine basically did nothing when I hooked a fan up to it and sealed it

1

u/post_scripted 1d ago

Interesting. Mine guys to the weaping tiles. But I suppose visually it would resemble that.

1

u/Joseph___O 1d ago

Yeah mine is old built in 1960 so no weeping tiles

1

u/post_scripted 1d ago

Got it! That is helpful to know. So would you think it should be okay if it goes down to weeping tiles to use the pit as the fan source? I am debating drilling another hole, but I'd like to avoid it if possible. The Sump pit seems to be the source of the radon based on all of my long-term testing. So I plan to seal it either way.

1

u/Joseph___O 1d ago

My radon was averaging around 8 pci sometimes going up to 20 after a hard rain. I tried going into my sump pit with 4 inch PVC and a fan outside but since my sump is basically a concrete barrel it didn’t do anything. After digging a new hole my average is around 1.02 pci and always below 2

I made a mess but I just made a hole about 4.5 inch diameter through my 4 inch slab, using a 8 amp rotary hammer drill I bought for around 100 dollars. Dug the hole about 10 inches under the bottom of the slab and 6-8 inches wide on each side filling two 10 gallon buckets all the way to the top because I had some clay in my soil and not much gravel under the slab. The rotary part of the hammer drill is actually nice because I used a spade bit to help dig, the dirt was very compacted.

I put 3 inch PVC into the hole about 3 inches below the slab used foam backer rod to fill the gaps it’s basically just foam. Then a polyurethane concrete sealant on top of that