Radon with Slab on Grade – How much pipe is enough?
Hey All,
We are working on our slab-on-grade, single level, 2100 sf house in Idaho. Just in case we find radon seeping in, I want to have some mitigation measures in place under the slab (cheaper/better design possible during construction). I have Stego 10 mil for above the 6 inches of clean/washed gravel. I plan on securing that at all penetrations and at the top of the concrete stem walls with Stego TakTape.
I have been reading, for what seems like days, about the piping. I have found all types on info. about whether or not to use the drain as a duel purpose radon mitigator, what diameter of pipe to use, etc. But nothing on the recommended linear feet of pipe to install. I have decided to use 3″ PVC.
I’ve seen retrofits where they just put a hole in the slab and seal the pipe that exits through the slab then routing out of the house; implying that there isn’t a need to extend the pipe everywhere – more is perhaps better, but not ‘required’.
But I am still left wondering…
1. How far does the pipe need to go from the exit point?
My Propose route is shown in the atch’d image. The route is somewhat limited by plumbing pipes.
2. What type of perforate pipe should I use?
Option 1 – HDPE triple-wall
Option 2 – DWV PVC
Option 3 – Pressure PVC
Option 4 – Something Else
Any thoughts/guidance/suggestions appreciated.
Thanks.
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Replies
Pre-positioning piping seems to be a regional preference sort of thing more than a science-based one. With 6" of washed gravel and a well-sealed slab otherwise, it doesn't really matter all that much what pipe you use and how much of it. Gases can move through the spaces between the gravel freely, so once you create a suction point, radon will be drawn to it and won't be able to vent into the house. My local radon guy actually told me not to use any pipe. If we have radon, he'll drill my slab and seal his own pipe to the vapor barrier and slab. He says he doesn't need any pipe underneath. If you want to be extra-sure, go ahead and put in some pipe. After all, the pipe is pretty cheap. I would do 2 ten-foot pipes with a "T" extending up through the slab and capped. DWV pipe is fine, though the others would work too. If your slab and house are well sealed and your ERV is balanced, there's not much pressure to drive radon into the house anyhow, so definitely test before spending any more money on mitigation.
idahobuild,
https://foundationhandbook.ornl.gov/handbook/section4-1-radon.shtml
Okay, I just bought 130 feet of Cell-core PVC and misc. fittings. It seams as though it may be overkill, but in only cost a few hundred dollars. Since I am installing and drilling holes myself, that should be the total cost of my "for good measure" approach to radon.
idahobuild,
Certainly does no harm at all.
Howdy! I’m in NW Montana and breaking ground on a new build. Montana Basement Solutions suggested to me that if you have 6-8” of clean stone below the slab, you don’t need any pipe. Just put passageways through the interior footers. Makes sense to me so that’s what I’m doing. They also said if you’re concerned with the long term efficacy of that strategy to put down landscape fabric between the native soil and washed stone. For what it’s worth, they are well recognized as the experts in basement waterproofing, mold, radon, etc in the region.
Think of the gas-permeable layer of crushed stone as the main way that radon will find the riser pipe, and the perimeter pipe as a superhighway in case levels are extra high or the crushed stone becomes clogged for some reason. I'm in a high radon area and always spec 4" perforated pipe within 12" of the exterior walls; I have never needed to add active ventilation after the house was built. (Knock on wood...)