Mitigating with French Drain and Weep System
I've had a few customers with elevated radon that have a sub-slab waterproofing system installed. The systems are configured with a "weep system" that leaves a quarter inch gap at the foundation wall base. This allows any water entering the basement through the wall to drip into the drainage system. I'm looking for advice on how to mitigate the radon without 1) drawing air from above the basement slab and 2) compromising the drainage system. (Tapping into the drainage system would void a lifetime warranty.) I've been referring customers back to their waterproofing companies, but giving away work is aggravating.
Both Bob Wood and myself, have encountered a product called Leak Bye, basically core plast running the entire perimeter of slab most of them, we spray foam over, only one, have I managed to use a central suction point and an RN1 fan and managed to mitigate the radon, without any pressure change, within the structure and no smoke loss, through the leak bye.
30 years ago I said that it wouldn’t break my heart to never do another one of them, but we still do 2 or 3 a month. Most are a breeze, some are a major pain. To understand what you’re dealing with, go to
and scroll down to “Basement waterproofing”. You’ll see how Minnesota aftermarket drain tile is done- with a 2 inch layer of rock on the footing and the re-poured slab only 1-1/2 inches thick. Our homes with aftermarket drain tile almost always have block foundations, not poured.
The 2 inch 'air gap' between the slab and the footing is the major problem. Air is freely pulled from hollow cavities of the blocks through this gap. The gap between the slab and the wall is the lesser problem. We only seal them as a last resort and the improvement in sub slab communication is usually minimal.
With normal drain tile, we’ll cut a football size hole in the top of the drain tile at the suction point. With aftermarket drain tile, we’ll use a hole saw to make a tight fitting hole in the drain tile, cut the end of our suction pipe to the radius of the drain tile and then insert the suction pipe ½ inch into the drain tile for a tight fit. If the sump is where we want the suction, we’ll go through the floor next to the sump, turn 90 and enter the sump through the side wall through the proper sized hole saw hole. My theory is that we lose less vacuum through the air gaps with an “airtight” connection to the drain tile system.
95% of the time, one suction with a RP145 does the job. If not, we start doing communication testing. Avoid the temptation to just put on a bigger fan- you’ll exhaust a lot of house air, and could cause back-drafting.
We’ve found that unless we’re creating some vacuum in the drain tile, it usually doesn’t work.
Sometimes you’ll need more suction points on the drain tile. If your suction is tight to the drain tile, the end of the suction pipe is inches from the bottom of the drain tile. This creates turbulence and you lose a lot of air flow requiring more suction points.
Sometimes you’ll abandon the drain tile suction points and do suction away from the drain tile.
Sometimes you’ll do suction away from the drain tile with dampened suction points in the drain tile.
Sometimes you’ll wish you never took on the job.
In our region the waterproofing Installers cut the concrete slab about 18 inches from the wall (in the section to be addressed) the concrete is broken up and removed leaving a trench wide enough to shovel the sub-slab material out to the desired depth. Before the perf-pipe and aggregate are placed in the trench, the Installers use a star chisel to punch holes into each hollow core of all the Concrete block. After the concrete over vapor barrier pour-back is complete, the walls are wide open to the entire wallcore structure. ASD now pulls from any wall penetrations, step cracks etc. and top blocks that are uncapped. They also includes “egg crate” (a high density polyethylene internal drainage membrane) placed against the wall from the top of the footing to about 4 inches above the slab pour-back. The egg crate opens the entire sub-slab soil air and the block core air to the basement air. Touch any of it and the Waterproofers will void the Warranty.
We address this by explaining this to the client and asking them if they have a Waterproofing Warranty. If they have a Warranty we call the Waterproofer and explain that we need to seal the top of the “egg crate”, breech the pipe, and seal the sump pump bucket assembly. If they refuse we suggest they call the client to propose their sealing solution. If the Water-proofing Company becomes difficult, we ask if they want to be responsible for a radioactive gas in the client’s house. We suggest that the Radon may actually have been caused by their system. If they refuse, we don’t take the job.
After 30+ years in business, we know most of the Waterproofers and have working relationships with them. I’ve explained a very small portion of this issue. I could write a book on the subject.
Kevin M Stewart
Reflecting my ignorance here:
Wondering if Randy's solution of "Draintile technique # 3" in the document to which he refers readers (
) is (or might become)
(or among the) water control technique, recognized as consistent with possible future radon mitigation techniques, recommended (or even code-specified) for installation in
construction where it might reasonably be anticipated that water incursion has a fair potential to be a problem? (I live in such a house where I wish the builders
done more to so anticipate.
My points in asking this are the following:
- By encouraging home-builders to preemptively follow technique #3 from the outset, such a thing could, in time, make mitigators' work, where it proves to be needed, so much less troublesome to carry out.
- Emphasis on this technique for water incursion control from the outset could also perhaps help establish it as an SOP for the water-control industry to follow should they be called to retrofit houses not so equipped.
- I know John could write a book, but maybe if there could be more steps made to get out in front of this issue in a cooperative way among the builders, the mitigators, and the water-proofers, maybe there wouldn't be the need for second volume 25 years from now. Is there any advantage in holding, say, a joint webinar to help those stakeholders hear one another and address this one issue? Let me know.
Kevin M Stewart
The better job we do at promoting this, the more effective our mitigation systems will be. An example of the problem: In a previous life, I built a home for Joe. It had technique 3 drain tile with no capillary break- the slab was poured tight to the concrete block foundation. He had water on the basement floor, and wanted me to replace the 'defective' drain tile. I told him that the drain tile was fine, but water was wicking up the face of the concrete blocks, and then running down and wetting the floor. He didn't believe me. We agreed on an experiment. I punched a 2" hole in the bottom of one of the blocks, inserted a garden hose and turned it on full force. After about 30 minutes, the rock bed was saturated and water began trickling into the sump. If I'd built the home with Ice & Water Shield starting at the top of the weep tubes and extending 24" up the wall- the floor would have been forever dry. (I've often said that by the time a builder knows how to do everything right, he's dead or retired.)
After market drain tile contractors give life time guarantees installing drain tile the way Dad taught them. My suggestions to them to use technique 3 with Ice & Water Shield on the wall have gone over like a turd in a punch bowl. I hope you, and others can be more effective than me!
Bruce Decker BGIS
I think Rob is suggesting something like the solution in Fig 3 of the link below or attached picture.
Both are from Building Science Corporation and Joe Lstiburek. Cruise his web site as there are lots of articles and designs free that you can take to building officials for approval. Joe is a well known and extremely qualified building scientist so using his design may get you traction/acceptance that you would not otherwise get.
Of course these assume you have access to the foundation wall. You may need to modify the drainage sheet depending on how it is installed. Just be careful that the drainage path drips freely below the slab, otherwise the concrete will absorb the water and the slab move moisture (just like if your radon pipe doesn't protrude below the slab to make a clean drip edge).
Bruce Decker BGIS
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