Membranes, are much cheaper- however there are many benefits to spray foam.
https://www.demilec.ca/documents/Radon-Brochure-English.pdf
Our research council, tested their products in their radon barrier testing facility and it was found to be excellent.

I agree with the spray foam option. A membrane may be cheaper on a material only point of view but add the labour to get 100% gas tight seal and you will see the foam is likley your best bet.. The foam will provide an excellent soil gas barrier with as close to a perfect (air/soil gas) seal as you can get..

Demilec, as well as other foams have recently been demonstrated through lab testing to perform very well as radon barriers; even outperforming polyethylene. Demilec was one of them. You can contact CUFCA for others. There are actual radon diffusive rates available on the foam that you can use to engineering systems.

Below is a link to a paper on interior spray foam insulation of rubble walls. This is written by Joe Lstiburek of Building Science Press. They will have other reports that will fit your needs if the one I have referenced does not.. Joe Lstiburek is a renowned Building Science Engineer so you shouldn't have any problems getting architects, engineers or code officials to accept his designs. I do not believe he has done anything radon specific but if you apply the spray foam sealing and radon barrier characteristics it will get the job done very well. Remember the lions share or radon entry is mass transport of soil gas and not diffusive transport so a great air barrier is more important than a material with a super low radon diffusion coeffient.

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

Keep in mind spray foam contains isocyante and should only be sprayed by qualified contractors with proper ventilation and only when the house is vacant. The isocyantes will be used up during the application and will be vented outside. But if you were to spray it indoors without proper procedures and homeowners at home they could be come chemically scensatized. It can be done very safely just pick the best contractor who can demonstrate they know the hazards and control measures. Maybe get an industrial hygienist CIH involved for oversight.

Hope this helps.

Dick,
My son, Clint, is a CIH (certified industrial hygienist). Colden Corporation in East Syracuse.
Ed Smith

In all my years of mitigating, I have never had to seal a stone foundation to successfully mitigate radon either Dick. I'm thinking it's more science than luck!. A stone wall has no footer, instead relying on the width (18-20" in South Central PA) of the wall to bear the weight of the structure above it. Without the footer to contain your negative pressure field, your often depressurizing the soil on both sides of the wall.

For weatherization, I strongly recommend a spray foam applications with a spray coat of intumescent paint. Hanging a vapor barrier from the sill plate would violate fire code and provide a concealed moist pathway for termites.

Best wishes,
Jay Bauder

One thing that many people don't know about rubble-stone walls is that the mortar that holds them together is not concrete based but is a lime based mortar, these lime based mortars require moisture to keep from sanding out in essence they breathe the moisture in and out. They were not designed to be finished on the inside. Finishing over lime mortar with concrete will result in the mortar sanding out in the interior of wall which will in all likelihood cause a structural failure over time (may take 50 years). In designing a system to weatherize a rubble-stone wall you would need to keep this in mind.

I would suggest something that was done on a historic building rebuild that i was involved with: where the architect had us mechanically fasten dimple-board on interior of the rubble wall and then seal the outside of the dimple-board with a medium density foam 2" thick (like Demilec "Soya" ) to below the concrete floor for a R-12 insulation, air barrier, vapour barrier and radon barrier, and a drainage plane to sub-slab gravel then use open cell spray foam or batt insulation to achieve required insulative requirements. By following this procedure you will allow rubble-stone wall to still breath in moisture and not likely harm the ability of the lime mortar to continue to breath in and pass around moisture.
Don't forget to spray foam and insulate the joist headers, as these i understand can be up to 12% of the energy loss on a modern building.

As Bruce said above do not forget that spray foam must be installed by certified contractors and that Isocyanates are a chemical sensitiser, and as Jay above said spray a coat of fire retardant if they are to be left exposed (not behind drywall)

Bob Wood

Very interesting! Thanks for sharing.

Useful and important information. Thanks

It's interesting to think about the possibility that sub-slab depressurization is reaching under the 18" rubble stone foundation. Would be an interesting experiment to inject tracer gas on the outside of the foundation and see if it shows up in the radon pipe. Even more interesting would be to use a standard wall with a footer as a control for the experiment.

Thanks for responding. I did look at Lstiburek's article. What struck me was the detail that preserved a drainage plain between the rubble stone wall and the spray polyurethane foam. The membrane creating the drainage plain extended below the slab into a french drain water collection system. At no point was spray foam directly applied to the wall. For radon mitigation in an existing house, the foam insulation is not required, but sealing the membrane to create a sufficient radon barrier and preserving the required drainage plain would be a major project. See Bob Wood's comments.

Dick. I know Bob very well and we talk about this stuff all the time. I am a certified Building Scientist and Certified Engineering Technologist in the Civil discipline. I don't know the full scope of your task or budget expectations which put me at a disadvantage. The foam is simply a tool to provide a very effective air barrier/radon barrier. It is an option to use or disregard as you see appropriate to your project parameters. You are correct that the insulation is not "needed" but in your case the foam is the radon barrier/membrane and the insulative value is just a side benefit. In Bob's example (and likely Joe's for that mater) I am sure the drainage plane (dimple board) was not sealed and they relied on the foam as the seal. You could try and manually seal a rubble/stone foundation wall manually but I would expect labour cost to be very high and efficacy of the seal to be very low. A low seal efficacy would not be good particularly if you are looking for passive controls as you indicate is the goal. But perhaps there is a migrator out there who can do an adequate manual seal at reasonable cost. Building Science is a very, very, very complex thing. Managing moisture and it's movement in any building envelope needs to be considered carefully as do budget implications. In Joe's French drain configuration he is managing moisture in the wall and providing a free and clear drainage path to the subsurface. It is important to not that you would still have to provide a membrane barrier on a rubble wall a place to drain any accumulated water as you can't simply let it pool at the bottom of the wall with no where to go. Lstiburek and his firm is available to consult on projects.

You captured the problems I'm dealing with exactly:
"You could try and manually seal a rubble/stone foundation wall manually but I would expect labour cost to be very high and efficacy of the seal to be very low" and any barrier of any kind I install would have to have the capacity to "drain any accumulated water as you can't simply let it pool at the bottom of the wall with no where to go", and I can't figure out a way to do this inexpensively and effectively. As for the budget issue, I have been asked by low-income weatherization agencies in two states to write passive radon mitigation procedures to include:
1. Floor crack sealing
2. Floor and wall penetration sealing
3. Sump sealing
4. Crawlspace sealing
5. Rubble stone wall sealing
And budget issues are huge because it is in the context of low-income weatherization work.

I don't envy your task. Items 1 to 3 seem straight forward enough but items 4 and 5; wow! Good luck. As we all know Items 4 and 5 can certainly be done but trying to find a low cost solution and provide accurate cost estimates will be difficult. I wonder, would you have some luck discussing the pros and cons with the client? If you explain that passive Rn controls have low effect and the challenge and cost with sealing rubble walls they may be more inclined to go to an active system. As Jay eluded active slab depressurization seems to work in controlling radon intrusion through rubble walls for him.

If the walls prove to be a significant source of Rn intrusion, even after sub-slab mitigation, you could consider an active wall system ( e.g. depressurize or pressurize a cavity created by dimple board placed on the interior of the rubble wall). The active system would manage any imperfect seals in the dimple board. You could have the cavity drain to the soils below by drilling holes through the slab at the base of the wall and ensure the dimple board covers the holes. Make sure you drill enough holes to manage the anticipated water and get the low spots in the floor. You would need to make sure that the negative pressure under-slab is greater than the negative pressure on the wall cavity so that water pooled at the bottom of the wall is not slurped up into the wall because of back-draft up the drainage hole. Again there is lots of buildings science to consider with pulling or pushing air/soil gas through the foundation wall. If you pull too much cold air in from outside you could cause some wall freezing. If you pump too much warm air into the cavity you could warm the exterior soils in the winter (no so bad but lots of energy loss).

I have a question.

Are they asking you to write passive radon mitigation procedures, not active?
Maybe that's the key disconnect..
The radon systems should be active to avoid having to mess with the other variables hence what Jay stated earlier.
Let the weatherization experts handle making the home green and let us fix the radon in the best way possible
Just my 2 cents

I agree with everything you are saying. Here's the fundamental problem. This is a mandate from the Department of Energy to Weatherization agencies. Active mitigation measures are not approved, even when a simple SSD would cost less in labor and materials than sealing a rubble stone wall. It's government bureaucracy at its worst. But the whole issue has raised a larger and more interesting question. For years, in the weatherization industry there has been a concern to establish best practices, which has culminated in a project to create Standard Work Specifications. The SWS's are publicly available (https://sws.nrel.gov/). They were created mostly with input from the industry and technical experts. Nothing like this exists for the radon mitigation industry. It's a project that might be worth thinking about for the limited number of procedures we use in our work.

Sump sealing is not straightforward. Sumps can be standard crocks with pedestal or submersible pumps. The can have two sump pumps installed or elaborate water backup systems. They can be ceramic crocks with raised, rounded tops. They can be precast concrete 11" square pits. They can be shallow holes in a dirt floor. They can be plastic buckets set into the concrete slab projecting 2" above the slab. and on and on. The principle for sealing is simple. The practice can get complicated.