Radon Mitigation using a French Drain

by Doug
(Ridgway, CO)

Radon Vent Discharge  10' above Grade

Radon Vent Discharge 10' above Grade

Three or four years ago, I measured the radon levels in my newly constructed house with a lab test kit and it measured 19pCi/L. I stuck my head in the sand then and ignored it because I thought it was a big deal to fix. Recently, I read that one can use the French drain that is placed around my foundation to suck out the radon, so I thought a system designed around that would be much easier. Otherwise, I would have had to drill a hole in my slab and then another hole in my concrete basement wall to have an exit for the pipe(I have no attic and the house is open floor plan so there was no real easy way to go up inside the house). I also didn't want an unsightly fan on the outside of the house, so the French drain design seemed to be ideal.

The concept of using the drain didn't seemed to be sound to me though since the drain is on the outside of the stem wall - the fan would have to suck air from under the footer, up the inside of the stem wall and then under the slab. I was leery of that being able to happen. But I set up a temporary installation by just putting an elbow at the day light end of the drain and put the fan on top (with a long extension cord powering it!). To my amazement, the readings started at about 14 using a Pro 3 radon monitor and went to 2.0 in 6-7 days. That told me it would work, so I installed it permanently. I thought I would have to move the fan closer to the house than the 150' I tested it at but since it tested good there, I just installed it near the end among the trees. I was also concerned about the unsightly look and the potential for hearing the fan noise at night, so that was perfect.

As you can see from the picture, my pipe design was a bit different as well. I was hoping that the 4" PVC pipe would provide enough structure to hold the system vertically without the need for guy wires so I decided to modify the condensation system a bit to make the structure more rigid than the rubber fan couplers would provide. Now, condensation falls through the vertical pipe, but then hits a rubber disk I placed inside the top Wye fitting (I made an annular ring out of plastic that I put in first to support the disk). The disk has a 3/32" hole in the center where the water is sucked through and falls to the bottom. I left the top stack loose(not glued) to allow for inspection of that disk. The condensation collection seems to work OK.

At the base of the system, I installed a PVC "T" fitting in the corrugated drain pipe that went horizontally to another "T" fitting. One end of that fitting went into the ground and the other end pointed upward where the fan would go. I concreted that fitting in the ground to provide structure.

I also needed a backwater flapper valve at the end of the drain pipe in order to allow drainage water to exit the system. You can see the grey valve in the background in the picture. I think that works OK but may have a tiny vacuum leak.

The black box in the pictures house the manometer and a remote sensor system to monitor the vacuum from the house. I got a vacuum valve used on furnaces that senses .47"WC and connected the normally open contacts to the battery wires inside an outdoor wireless temperature sensor. So now, I know that when an outdoor temperature is displayed in the house, the vacuum is at least .47"WC and the fan is working. Otherwise, the temperature is not displayed and I know something is wrong.

Val was a great help to me - I'm glad I found him!

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May 19, 2021
Using my french drain gets me 0.5-2.0 values
by: Anonymous

$500 all materials, fan, etc. outside 24" vertical culvert access to french drain bottom of foundation. have sump pump in bottom with 1" pvc up and out thru new hole in 24" cover. Then put in new hole to take the 4" or whatever I used radon exhaust and extended it down 2-3', ran pipe up outside of house to roof. Caulked heavy all around 24" cover and culvert and pipe holes.

works perfect. radon was originally>8. Winters are worse at maybe 2.0 but spring summer fall I'm 0.5-1.0. Any klutz can do this.

Feb 20, 2020
Still working
by: Doug

After some 8 years, the system is still working great. As measured in real time, my values are 4-6 consistently

Feb 20, 2020
any further testing
by: Anonymous

Im thinking of installing the same system. did you do testing later or a more long term test? If so what were the results? Mine just tested at 18 and I have a baby on the way.

Jan 06, 2020
Great idea
by: Curtis

I had the same idea - can I use my french drain for Radon mitigation and when I googled it discovered your site. Thanks so much for posting this.
Black Forest, Colorado

Feb 26, 2019
Your design
by: Monte

I like your idea...how do I do what you did except I want to keep it against the exterior of the house.

my real confusion is how do you tap into the French drain inside and where at do you do that? My French drain was installed by Basement Systems and cement was poured over it by them. what it leaves is a 1/8"
space from the foundation wall to the drain and the slab floor all around the basement with 2 clean out spots. Where do you put the PVC into the drain and how far down do you go. I already have several spots that are 8' high from the slab that you could run the PVC from the slab out to the exterior and add the pump outside. Can you copy my email also merickson01@msn.com

Monte(Coal Creek Canyon)

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