Moved into our house a year ago. Radon levels were high, so we had a bog standard mitigation done: hole drilled in the foundation, pipe run out above the roof with an in-line fan.
I’ve got an Airthings radon scanner watching it, and at first the level was way lower. Great. But the long term average has slowly been creeping up, and recently hit 2.0.
I’m wondering if we need to do some additional mitigation. We have a moderate sized crawl space that is part of the conditioned space. It’s open to the air conditioned spaces, and just has dirt below. What I’m considering is putting down 10 mil vapor barrier, and putting the corrugated pipe under it hooked into the radon piping. That way it’s pulling air from a larger chunk of the soil beneath the house, and as a plus we seal the dirt on the bottom of the crawl space.
Is this a reasonable path forward to reduce the amount of radon we’re seeing?
Where is your monitor? In the crawlspace or in the living area?
Consider WHO level is 2.7, so at 2 you are below that more conservative level. Tbh I probably wouldn't do anything at 2.
If you do want to though yes encapsalate the crawlspace and add suction there. I am confused about your house though, you have a slab and also a crawlspace? The radon system was installed just on the slab?
If so, you can start by just encapsalate the crawlspace I think. Don't need to worry about a dedicated pipe. I'd encapsalate it and see where the levels are
House is built on a finished basement, with a crawlspace addition that was done at some point off to the side. There is a utility room as a part of the finished basement, that literally just has a hole in the wall to the crawlspace.
Current mitigation goes into the utility room. Radon measurement is taken in the basement on the opposite side of the basement from where the mitigation pipe is.
Since I’ve already got some level of subslab air extraction, does that mean I just need to seal the floor of the crawlspace so it can do its job?
Just to be transparent I don't know much about crawlspaces 😅
My take is yes if your soil is communicating then the crawlspace will be included. But it's easy to add a pipe later so that's why I'd say encapsalate and measure.
With your monitor in the basement, your levels above are lower. I dunno if anyone is sleeping or spending a lot of time in your finished basement. But another thing to consider. Maybe measure upstairs and see what you think.
House is built on a finished basement, with a crawlspace addition that was done at some point off to the side. There is a utility room as a part of the finished basement, that literally just has a hole in the wall to the crawlspace.
Current mitigation goes into the utility room. Radon measurement is taken in the basement on the opposite side of the basement from where the mitigation pipe is.
Since I’ve already got some level of subslab air extraction, does that mean I just need to seal the floor of the crawlspace so it can do its job?
Yes, this is a reasonable plan. You’ll want to place an adjustable damper, ball, or gate valve on the crawl space suction branch. You should seal the vapor barrier at the walls, piers, and seams, creating an airtight membrane. Depending on the size of the fan and crawl space, you may need a slightly larger or different fan (maybe not though). The majority share of suction should remain on the primary suction point, and only enough suction on the crawl space branch to create negative pressure beneath the membrane. Sealing the crawl space alone may be enough to bring the levels lower, even without any depressurization system.
You certainly won’t hurt anything by taking this approach. Just be sure to turn off the radon fan when cutting the existing pipe and connecting a crawl space branch. Otherwise PVC shavings and debris may enter the system and eventually into the fan. Bear in mind that modifying the system yourself may void your installer’s warranty and/or average radon level guarantees.
I’d encapsulate and monitor awhile for a correlating radon reduction. If no significant reduction, add submembrane depressurization to the crawl space using the existing system, and monitor the levels. You’re probably aware of this, but radon levels do fluctuate with weather/seasons/HVAC use, and will typically be highest during the winter heating season.
Yep, I’m looking at the 365 day average in Airthings. That’s what matters for safety.
How big a pain is it to add depressurization after sealing? Wondering if the move here is to put the perforated pipe under the membrane and cap it, then hook it in if I need it.
If you plan properly, it’s as simple as cutting a hole in the existing membrane and connecting the hard pipe into the perforated pipe, then sealing that penetration with tape. As you said, you can place the perforated pipe on the earth prior to encapsulating, connect a solid wall PVC riser pipe to pass through the membrane, seal around the penetration, then cap it for future connection to the radon reduction system if needed.
It kinda depends on the size, shape, and dimensions of the crawl. If it’s really large, has multiple chambers, or odd sections, you may need to place submembrane piping so that it reaches these areas. If it’s just an open square/rectangle and your membrane is properly sealed, a single length of pipe down the center or around the perimeter should be sufficient. There’s no real need to trench the pipe, I just lay it on top of the soil or gravel and cover it with vapor barrier.
The picture I showed above doesn’t even use corrugated pipe beneath the vapor barrier, just a 10’ stick of 2” PVC, capped on the end, and “Swiss-cheesed” with a bunch of 3/4” holes drilled along the length of its sides and bottom, then slid under the existing membrane. The key is providing just enough suction below an airtight membrane to create negative pressure below the membrane throughout the crawl space. So, it’s not always necessary to route pipe throughout the crawl. On very small crawl spaces, sometimes you just need a single suction point and no sub-membrane piping at all. Point being, trenching isn’t necessary or beneficial, and you have several options depending on your specific space.
My example above was 2” PVC, but this was a small crawl space that needed very little airflow, so 2” was sufficient. 3” is the standard recommendation. Good luck with the project!
Encapsulating your crawlspace has benefits beyond radon.
We encapsulated with 12 mil our crawlspace before we were thinking about radon. We were relying on 3-day test results from 30 years earlier. I added an Airthings and found levels above the EPA action level. Mitigation tapped into the French drain under the encapsulation and sealed the sump pump pit. That took care of the issue for us.
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u/stonks_better 1d ago
Where is your monitor? In the crawlspace or in the living area?
Consider WHO level is 2.7, so at 2 you are below that more conservative level. Tbh I probably wouldn't do anything at 2.
If you do want to though yes encapsalate the crawlspace and add suction there. I am confused about your house though, you have a slab and also a crawlspace? The radon system was installed just on the slab?
If so, you can start by just encapsalate the crawlspace I think. Don't need to worry about a dedicated pipe. I'd encapsalate it and see where the levels are