One option would be to install a HRV/ERV to dilute the radon and also pressurize the house too by bringing in more air than exhausting. This can lower the radon and improve overall indoor air quality too.

Thanks Donald, oddly enough the homeowner (who spent some time as a US Navy nuclear engineer) made reference to ventilating the basement, though he didn't bring up ERV.

I'm with Don. There could easily be radon from a source other than the floor, such as emanation from the stone walls.

I appreciate the feedback Henri, sounds like a rational approach. I did a salad bowl test, sealed a monitor inside a bowl by caulking the bowl to the wall; it didn't indicate building material emanation, but I only did it in one location.

You mentioned that the fan pressure for the RP265 was .5". That is really low and indicates a very high flow.Yet you said that the slab was 14" to 22" thick and you measured minimal pressure drop across the slab. This indicates to me that most of the air traveling through the RP265 must be coming from the basement, and the initial reduction in radon could have been due to dilution from negative pressure pulling air into the basement either from the living space above or through the foundation. Did you do any measurements of the pressure in the basement wrt the exterior or first floor with the RP265 on and off? The HRV/ERV will probably work, but you might want to monitor the pressure changes in the basement when configuring the intake and exhaust flows.

Curious what changed between "RP265 running at 0.5" static pressure" and "GX5 which is nearly maxed out at 4.7" static." You sealed off the short-circuiting of air flow present in the original configuration?
Also curious how you reached below the "14 to 22 inches of concrete" to nonchalantly "excavat[e] a couple 5 gallon buckets of soil"

Hi Tom

You could sprayfoam the foundation walls.

I agree that an ERV or HRV would work and could be a cost effective installation. I noticed there are ducts in the basement already.

Emanation from the foundation wall could be a source however I would suspect that radon ingress through the foundation wall to me more likely. Stone foundations can have many pathways in them.

Some sprayfoams have been tested and approved as radon barriers and are much more effective then 6mil poly vapour barrier (or any other membrane for that matter) from both an air leakage and diffusion control perspective. I have attached some supporting documents and details for you.

The spray foam would control radon entry and emanation from the foundation walls by diffusion and soil gas leakage. In addition it would likely improve negative pressure under the slab. As Dick mentioned you seem to be pulling a lot of air which could be drawn from the basement through the foundation walls. The sprayfoam would really seal that up.

Another benefit with the sprayfoam is that when you place the drainage plane over the foundation (done before you spray, see the attached detail) you end up with a tiny gap between the foundation wall and drainage sheeting. If you tie this gap into the SSDS via the sheet metal channel you gain active depressurization along the entire wall face.

The sprayfoam system would certainly be more costly to install but it would use far less energy over it’s life span than an ERV/HRV and would offere other passive control measures that the ERV/HRV wouldn’t. Two things I don’t like about ERV/HRV is they can be shut off by an occupant and they are also maintenance heavy. If the owner is not cleaning filters or intakes regular (every 3 months) then is losses its effectiveness. Don’t get me wrong…I have and HRV and love it. But I am also an obsessive indoor air quality professional who runs and maintains it properly.

Here are 2 links and documents for reference. The first link is the CUFCA and NRC approval of spray foam as a Radon barrier in Canada. The second link is the detail for spray foaming a rubble foundation wall which includes a drainage plane between the foundation and the wall. You will need to confirm local fire code requirements for what the foam needs as a fire barrier. Building Science Press and Joe Lstiburek are renowned for their competence in building science. I can't imagine a building official being able to argue with his designs but check first.

If you sprayfoam you must make sure occupants are out of the building during the application of the spray foam. This doesn't sound like a problem in your case. The isocyanates in the foam are a chemical sensitizer and you simply must control exposure during application.

https://www.demilec.ca/documents/Heatlok-Soya-HFO/CCMC-14073-R_EN_RADON-RCS.PDF

https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations

If I understand this correctly, there are (2) systems? I would look at the RP-265/baseboard system. If you are moving over 280 cfm, some of that is basement air. Any air being pulled from the basement is fighting against you. Try rechecking your pressure field extension with the baseboard system off. If you have a significant increase in the pressure drop, try retesting it with the baseboard system off. You can also consider temporarily replacing the switch with a variable speed control to imitate the performance you would get by installing a smaller fan on the baseboard system.

I have an old RP-265 that we use to check for leaks. We mount it upside down and attach a coffee can with holes drilled in the bottom. Then we drop a smoke bomb in the can and blow the smoke back into the system. Mark any areas that smoke appears in the basement with painters tape. In this case, I would seal any leaks and repeat the process. You may find that once you seal the major leaks there is still significant suction loss thru smaller holes.

You could also consider installing a suction point closer to the center of the slab on the GX5 system. If you are applying suction 4 to 5 feet from the foundation, you may be losing some suction under the foundation. if none of that produces satisfactory results, try repeating your "salad bowl" test in any areas that have an upper slab like a patio, garage or addition butting against the foundation.

Good luck!

I'm in agreement with Bruce. I've had good success in eliminating reason intrusion through rubble walls with foam. I was surprised at the reasonable cost and very pleased with the radon reduction.

An associate of mine, in a similar situation installed a 6mil soil gas barrier over the rubble walls. He sealed it top and bottom with 2part foam after installing a perforated 2" PVC pipe along the bast of the walls (similar effect of your baseboard channel) and tied it onto an exterior vent pipe with a RP145.

My method (foam) blocked entry, his captured the radon and discharged it. Foam was less expensive and not as successful in reducing in home radon levels, but in that home there was no SSD system installed. The system with the soil gas barrier was actually 2 systems - the walls with the RP145 and a separate SSD with a 10gal extraction pit in the clay sub-slab and a 501 fan.

The insulation contractor that did the foam was great and the foam looks much nicer.

The ducuments Bruce notes are excellent references for this scenario.

Best of luck

I should have clarified that there are two systems, one is the SSDS with the GX5 sucking on two 5" suctions pits; the other is the one I described in detail pulling air from the foundation wall along the floor using the RP265. I would like to check the pressure difference with the 265 system on and off.

So the RP265 system has less resistance pulling from the baseboard configuration. I imagine the membrane (caulked to the baseboard and stone wall) allows the RP265 system to pull some air from the conditioned space, and I should foam that boundary to ensure it's not depressurizing the basement. I believe the two suction pits connected to the GX5 are affecting a fairly small area of influence based on the minimal pressure field, but they are contributing to lowering the radon concentration.

I cored through 14" of concrete to the subslab soil using a wet core rig turning a 5 inch core barrel. I was able to turn a 3" soil auger at a pretty good angle to loosen up the soil and lift it out to get a rather large plenum. I avoided the area with greater than 14" of concrete, the deepest measurement of which was 22".

That's a good bit to digest, but I am up for it. With this homeowner, I wouldn't be surprised if he went for both solutions, the HRV/ERV and the spray foam. I'll try some experiments playing with the configuration and testing the pressure field before working out a plan.

I am leaning toward having the foam as at least part of the solution, now to find a good contractor.

My client had already had the walls of his crawlspace foamed so we invited his insulator in to discuss it with him. This was the first time he had done it for radon, but had lots of experience foaming walls of old crawl spaces and attics in new home construction.

Check with your builders association and see who does foam insulation. If they are using the right kind it is likely that about 2" of foam will create a vapor barrier. My client went with 3" and in some areas that put in more to even out deteriorated areas.

Tom, shoot me an e-mail and I'll send you a couple of before and after photos.
davidmprintsatyahoodotcom

Hi Tom

Some of the guys who have commented my have some contacts for you for good sprayfoam contractors. If not I found a few US groups that may be able to help. I have also listed the Canadian Association (CUFCA), which won't have members in the US but they likely know their US counterparts and could help.

SPFA
3927 Old Lee Hwy. #101B
Fairfax, VA 22030
Tel: (800) 523-6154
Fax: (703) 222-5816
http://www.sprayfoam.org/contact-us

or

https://polyurethane.americanchemistry.com/polyurethanes/Spray-Foam-Coalition/

In Canada

Andrew Cole at CUFCA may be able to direct you to someone
3200 Wharton Way
Mississauga, Ontario, Canada L4X 2C1
Toll Free: (866) 467-7729 (1-866-GO-SPRAY)
Toll Free Fax: (877) 416-3626 (1-877-416-FOAM)
E-mail: cufcaatcufcadotca

As a radon mitigator i went out and bought a spray rig to be able to spray foam as a radon barrier for new construction. Spraying rubble walls over a geotext sheet is easily the best way to fix a rubblestone wall that may be emitting radon and if it is a lime mortar it is good for walls too, (lime mortar needs to keep moisture in it or it will sand out over time). Get the spray foam contractor to do joist pockets as well as this is usually a tough place to seal and can have huge energy savings, up to 15% of energy loss in modern construction occurs at joist headers.
2" thick Demilec soya 2lb spray foam is the minimum for radon that has been certified in Canada, I understand that BASF product line is in process of getting that certification as well but do not have it yet.
i would strongly support (if it were my client) that they consider a 3 or 4" thickness just from an energy savings perspective 2lb foams are about R-6 per inch