Sub-slab radon mitigation system

Should a radon mitigation system be roughed-in for a high performance build or should it be assumed that following best practices for sealing the slab and foundation will prevent radon from entering the basement in the first place?
We have low-moderate level of radon, above where it should be but we have 15 mil poly VB under the slab and everything is well detailed so we don’t know how the radon is even getting inside. Our builder skipped the interior weeping tile system and radon boot (we have exterior weeping tile draining to daylight already for water management).
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I think you're missing some details here like your slab assembly, your weeping system or specifically what that purple squiggly line is supposed to be.
Hopefully there is a vapour control layer on the foundation walls and sub-slab as well.
To to my understanding of your description your contractor installed an interior weeping system, and of that system there a pipe sticking up out of the slab? And that's what you're to use to add a randon, if needed?
I don't see much wrong with this, outside of a contract breach, (not sure if the squiggly like was part of the contract)
My suggestion is to start running the ERV and retest the radon and see if it's a problem or not.
If it still is problem, start sucking air from the weeping system and blowing it to the outside.
Hope this helps,
Jamie
No, I’ll update the description. The slab is properly sealed, poly VB and 4” EPS… The squiggly line I’ve just drawn to highlight the very faint dotted line shown in the architecture plan here which is labelled “interior weeping tile to sump basin”. One end goes out the basement walkout (gravity drainage downhill) and the other on the top left corner the dotted line ends at “typ. Radon rough-in”.
Our GC did NOT install the interior weeping system shown because they did weeping tile around the exterior perimeter already which is the weeping tile and no sump pump because this is built into a slope so it can drain downhill.
There is NO stub for the radon mitigation. Meaning as far as I know: the GC decided to skip both the interior weeping system and the radon stub rough-in. So if we need to mitigate (which we might not) then I think we need to drill into the slab if access below the slab is required.
Since you have gravel under the slab, it will allow any gases to move. If you have radon, you can drill into the slab and seal to the poly bellow. As long as the gravel is continuous under the slab (ie no steps in the slab) any radon can be drawn up through the gravel over the whole slab area.
Thanks Akos. So the mystery is where the radon is coming from, how is it getting inside? GC is confident in their detailing so theoretically no radon should be getting inside with 15 mil poly under slab, ICF also sealed inside with taped poly, etc.
Interestingly, it seems like it is usually assumed that following best practices is enough, so nobody actually tests/monitors. But with my sensor, it is clear there is radon so something was either missed or is failing. The house is very tall, I wonder if stack effect is contributing here?
Radon particles are extremely small and can easily get through most poly vapor retarders. The retarders are useful in allowing the sub-slab area to be depressurized, so the radon particles can be removed before they make it into the house.
As Akos wrote, as long as there is a gas-permeable layer below your vapor retarder, you can still create a functional mitigation system. There is a decent chance that it could be a passive design instead of requiring a fan.
They can be combined. The pipes need to be in a gas-permeable layer, usually crushed stone where I am, which would communicate between interior footing drains and radon loops anyway.
You might be interested in this quick article I wrote on this topic: https://www.finehomebuilding.com/2016/07/08/radon-big-deal.
Thanks Michael! Interesting topic for me because people totally overlook radon as a health risk and I suspect someone close to me died of lung cancer from radon in her home. I got some “looks” from a few trades while I was setting up the radon monitor in the mechanical room… they said radon is the last thing I should be worried about but given that they were smoking cigarettes all day, I think I’ll do my thing to ensure the level is consistently below 2 pCi/L or ideally under 1 since we are planning to stay in the house for decades and we work from home/plan to have kids (who are more susceptible to radon). At our condo now the level is usually about 0.5 and I’m comfortable with that.
Do you think it is worth getting the water tested? I know that can be a source of radon.
Thanks for sharing your article. If I’m understanding - the way our architect drew the plans was actually good aside from draining the interior weeping tile to daylight because it reduces the ability to depressurize below slab? It’s a bit blurry in my photo (reattached above) but basically they showed the exterior and interior weeping tile connecting to one line to daylight. However my builder only did the exterior perimeter for water drainage because they thought adding interior was redundant. Generally as a design principle I like redundancy… I guess I was just bothered because the whole idea was to build it *ideal* to depressurize below the slab by including what you call the “radon superhighway” and passive or active ventilation if needed. Our new PM is great and he said putting the stub is more for standard homes without the level of detailing that we have, but admittedly he said they don’t usually have radon monitoring like I have so it’s usually just assumed to be fine after sealing everything.
So if we do need to mitigate after the fact here they will basically just drill one hole somewhere in the slab all the way down to the gravel below the poly and EPS? And then put the 4” pipe there? What are the options for either passive or active mitigation at that point? If levels are low, just vent that pipe out a wall? Alternatively if levels are high enough, add an inline fan? Again really small sample size so far but I am guessing our levels after ERV is running will either be fine or low enough that passive mitigation could be enough.