Based on these and some other conversations, I think we have a consumer surplus to work with for awhile, meaning the maximum price people are willing to pay is a good bit greater that what we have typically been charging.

The RadonEye RD200? It is quite sensitive and good as a diagnostic tool since you can check flux off of the slab quickly to figure out problem areas. The same company (FTLab) manufactures a couple professional versions as well that are now listed. I have about 5 of the RD200 versions. The problem with clients having them, at least in my experience, is that they can foster obsessive behavior fixating on the minor fluctuations because they are so sensitive. I believe the value is updated every 10 minutes on a one hour rolling average, although there is a one day, one week, and monthly value also, customers tend to watch the 10 min changes

Best to call Rad Elec, but they have always sent a link when they have a software update for Radon Report Manager. My version is 3.8.32 and I have always updated when they send a link.

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

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.

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".

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.

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.

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.