I'm not aware of the history of how the Radon Mitigation Standards for Existing Homes became "Soil Gas Standards" but I can't help but believing this organizational scope creep is causing more problems than it's trying to solve. Radon and Vapor Instrusion systems on the surface may look like the same thing but they are very different animals that need to be kept in their own cages. For starters, VI mitigation addresses human made pollution, culpable entities , environmental agencies, litigation and liability of an offender. Radon on the other hand is a decay product of elements created in neutron stars billions of years outside of any statute of limitations. VI systems need to be designed, engineered and documented to a preponderance of evidence that demonstrates theoretic effectiveness. Radon Systems can be proven effective with a simple inexpensive radon tests. Conflating the two standards of practice needs to be recognized as a mistake by AARST and be undone.

Owen, what percentage of post mitigation test, do you see failing?
Reason I ask, is I’ve only done 4000 systems but never had a failure but I am a Canadian mitigation contractor who knows my PFE with a $1200 manometer.
I couldn’t imagine leaving a home, not knowing if I had full containment.

Robert. Having mitigated over 2000 homes myself, I've had numerous examples of homes where we could not achieve anything close full PFE containment but achieved excellent post mitigation test results and houses with excellent PFE coverage and still needed additional work. Four thousand homes without a single failure. Really?? Here in Minnesota, the majority of suburban homes we work on have drain tile, a code requirement since the 80s. Drilling a bunch of test holes in homes like these with highly finished basements is often impractical and unnecessary. Add in-floor radiant heat to the equation and every additional hole is like Russian roulette, even with an infrared camera.

I have also done over 4000 mitigations and have had a handful of failures including a few that I never solved. We have always done pressure field extension testing using a Minneapolis micromanometer and, in one case, measured -2 pa wrt the basement at the furthest test hole and didn't change the radon level when the fan was running. I totally agree that pressure field extension testing should be done with at least one test hole where feasible, but it is not a guarantee of success. And there are complicating factors including interior and/or exterior drain tile with drain to daylight, highly finished basements, karst geology, etc that make PFE testing problematical.

Agree that one should do PFE but making it a requirement is an overreach.

On behalf of Bob Lewis, manager of state radon program in Pennsylvania, I am posting a link to the final report of our re-entrainment study.

Radon Re-entrainment Study – An Initial Investigation

Could you summarize your findings Ryan?

Your assumption that 50% of the air being pulled from a radon fan in conditioned air from inside the building may be wildly overstated. I've been party to several blower door tests, where every exhaust appliance is turned on and off and measured for volume of air loss. Range hoods, bath fans, ERV etc. In all cases, no measurable air loss was detected from radon fans. One house had a Festa Fury moving more more 200 CFM and still no measurable air loss. That house had a ACH50 score of around 2, so pretty tight building. In all cases, all measurable air from the fans came from outside the building envelope. If the heating and cooling impacts of a radon fan were significant, we'd know about it here in Minnesota by now. The operating cost between a 15W vs 50W fan is really negligible if in fact it little impact on HVAC costs. A larger fan may in some cases reduce the need for basement dehumidification reducing net energy usage and improving overall air quality.

I have been on the MIT committee for nearly 2.5 years. I am writing this not as a committee member but as someone involved in this industry since 1996. I came to the conclusion that much of the complexity in the ANSI-AARST SGM-SF 2017/2020 standard for single family home “soil gas” mitigation is due to the additional requirements for Vapor Intrusion (VI) work that are not needed for radon. The potential harmonization of SF with MF and LB into one standard would also raise the level of the entire standard.

For those interested in how we got here, I have done a bit of research. This was put into motion many years ago –AARST added Vapor Intrusion to its repertoire around 2014, started pursuing ISO 17024 around 2015, added QA requirements in 2014, OM&M in 2015. These things were mentioned in various issues of the Radon Reporter.

There was a committee formed sometime around 2014 to make a new single family home radon mitigation standard as an ANSI-AARST standard. There were 33 people, plus Spruce co-owner Dave Kapturowski as chair, on the committee writing SGM-SF-2017. The breakdown of affiliations has always troubled me. There were 7 HUD people (and no disrespect to them, but I doubt any had hands-on radon mitigation experience). There were 15 government people total, including HUD, EPA and state radon reps. And 10 people specifically as radon reps, but when you take off government people, only 7 of the 33 total were designated to protect radon’s interests.

But also, this committee had 16 VI representatives (or 14 – 2 EPA members were not specified VI or Rn but were among the VI people). Anyone would expect a higher level than minimum requirements, based on the number of both government and VI people. This is why the standard has so many new and cumbersome requirements – desirable and needed for VI but not for radon.

I do acknowledge the time and efforts made by people who are AARST volunteers, on the board and on the committees as well as for NRPP. This work requires a significant amount of time and most people are highly invested in their work. Many have been involved with AARST in various positions for decades, and it is understandable that they would be offended to have their work product criticized.

But public comments on standards is by its nature a form of criticism of the standards. When SGM-SF-2017 was being written and issued, it was required to be used nowhere, and got little attention. But the ANSI-AARST standards now have the force of law and are enforced with fines and threats of losing one's license in some states, and the same is planned for all states. So everyone should be reviewing each word carefully and not be inhibited from making comments/objections/suggestions, and AARST should be receptive towards making changes to requirements that people who are using the standard in the real world have found to be problematic.

Maybe this is a great example of why there should be term limits/time limits on people's service, and limits on the number of employees or volunteers from each company being involved in AARST. It is unfair that only very few companies bear the burden of AARST work, and on the other hand, unfair that only a few are making the decisions and setting the direction for the entire radon industry. And we, the radon community, have no idea how many people are involved on all the various committees, since only the chairs are listed, but I think some companies are sacrificing significant manhours to AARST while others are left out. I think making this transparent, with all committee meeting minutes published, would help us truly appreciate the work of AARST.

Hopefully Dallas can commit to making these administrative changes, and that the AARST, NRPP and the Standards Consortium will be a partner with radon professionals instead of an adversary that uses policies and procedures to ignore valid complaints and concerns.

Hi Stephen,

You are right.... the 50% figure that I used in my previous email is simply an assumption.... what we have been taught over the years to assume.... maybe those numbers and assumptions will change in the near future with more research. With the building techniques that I have personally witnessed here in Canada where the contractors put styrofoam panels on the basement foundation walls and sit those panels on top of the foundation wall footings and then pour their concrete floor to come butt up against those styrofoam panels, we often see cold joints forming that are as large as 1/4" to 3/8" along the perimeter walls because of the shrinkage that occurs in the concrete floor slab when it dries. I have also personally witnessed backdrafting of wood stoves and fireplaces when the radon fans were running even when the 20 watt fans were installed.... Up here in Canada, a very tight home with that size cold joint and good clean crushed stone under the concrete floor slab has to be closely monitored. Even a small amount of air drawn from the basement of a small home in those situations is too much. That is where we need a good micromanometer to adjust our pressures under the slab to a point where we have just a bit more suction pressure under the slab versus the stack effect in the home. Here I am talking about a differential of 1 Pa (.004 " WC) or less. In my area, we often see temperatures go down to -40 to -50 deg F in the coldest months of the winter. (Last winter was a good example)... Something we have to deal with here in Canada...I agree that the building techniques in different provinces or states over the years have differed and we need to consider those facts during our installations.... My main concern is not so much the loss of conditioned air (although I do consider that important) but the backdrafting of combustion appliances.which I have witnessed personally over the years...

Marcel, I live in Minnesota and share the same climate zones and 90% of the Canadian population. Look at page 7 of Ryan Fox's post https://files.dep.state.pa.us/RadiationProtection/RadonDivision/RDPortalFiles/2021/Re-entrainment_Final_Report_Aug-12-2021.pdf . The most egregious example was a radon fan pulling 14.1 CFM with most being much lower. Small data set sure, but if like the half dozen blower door tests I've been involved with depressurization from radon fans is very small at worst and non existent at best. If a radon fan causes backdrafting imagine what a 100 CFM bath fan will do.

Ryan: Thank you for the research. It appears to validate Bill Brodhead's research and that of Canadian researchers.

All of this research compels us to update our standards so that our mitigation practices are science-based and ultimately benefit the consumer.

I would like to add a few observations from the viewpoint of a state regulator. In VA, our trained professional radon testers outnumber our resident mitigators by roughly 5:1 and the disparity has grown worse over the last few years. Most of our mitigators are located around the major metro areas which has caused several mitigator "deserts" in many high risk areas of the state that have smaller populations. The recent hyper-competitive real estate market has also proved devastating for the businesses of many radon professionals. It seems to me that if our main goal is to protect the public from radon exposure, then we should be striving to make things easier on the mitigators, rather than more difficult. We should also do what we can to make the idea of installing a radon mitigation system more attractive to the typical home owner. To these ends, I think we already have enough scientifically valid information for the US to strongly consider adopting the Canadian model for both ground level discharge and internal fan installations. Of course we should also consider adopting additional and reasonable protective standards/guidance for both practices that would be more appropriate for the US market.
If it is AARST's view that we have not yet achieved scientific consensus on these issues, then they should commission their own scientific studies as soon as possible to settle these matters once and for all. In my opinion, the present regulatory disparity between the US and CAN has persisted for far too long and no longer adequately reflects accumulated knowledge and conditions in the industry.

@Michelle: The main takeaway from the PA re-entrainment study is that we didn't find any real difference between ground level and roofline discharge.

There are a few things to consider. "Ground level" in this study was more like three to four feet above ground based on the system design. Still comfortably below roofline, but not actually right at ground level. The pre-mitigation levels weren't all that high (for Pennsylvania basements anyway), half had initial concentration levels at 10.0 pCi/L or below. The others were above 10.0 pCi/L. The exhaust gas concentrations, collected using Lucas cells and air pump with tubing going down the "throat" of the ground level discharge, were surprisingly low.

So does that mean you would expect re-entrainment with higher pre-mitigation levels/higher exhaust concentrations? Maybe. But maybe not. What if you have pre-mitigation levels above 100.0? Above 1000.0? Maybe ground level doesn't make sense in the latter case, but that would be a pretty rare situation. It wouldn't hurt to have more research.

Here are some of my personal opinions/thoughts after reading through this thread. I don't think there's going to be an end all be all study that answers our radon re-entrainment questions from now unto eternity. I also don't think the scientific method cares one whit about "consensus." What is a consensus today could change tomorrow. What won't change is the underlying reality, though our understanding (or lack thereof) is going to change over time unless we remain dogmatic.

So I wonder, is there enough data and compelling research to challenge the conventional wisdom of above-roof discharge and above-roof discharge only? Seems like things are pointing in that direction. I'm not claiming to know the answer, but I'd be happy to see more discussion/debate/questioning around the issue.

Ryan: I appreciate your respect for the Scientific Method (a la Karl Popper): we must continue to challenge our assumptions about radon re-entrainment and all other facets of our field. But I also believe that we should not hesitate to make informed choices and take action when the existing research and current practice (as in Canada) directs us to a logical course of action.

Are we going to act, or are we going to do more studies?

I say we should do both.

The last part of this post is the conclusion of my ground based exhaust study.

What I learned from reviewing all the original research done in early 1990's was the basis for exhausting above the roof occurred because Arthur Scott who I truly respected screwed up on his 1st EPA project of mitigating 40 homes in Pennsylvania. He aimed almost all the 40 system exhaust directly to the side of the building and half of the buildings failed the post mitigation radon test. The extremely high house he did direct away from the house passed the post test. Arthur corrected all those badly exhausted systems by mostly venting above the roof and EPA made the decision all systems needed to be vented above the roof. I got ahold of Bruce Henschel, the lead scientist who published the original EPA mitigation guides, and he told me his studies including a final wind tunnel test came to the conclusion that ground based system exhaust would work but by the time he finished his work the EPA had already set above roof venting as a requirement.

Although my study was on a single home the multiple CRM measurements for over two months confirmed that as long as the exhaust is straight out from the house as Bruce Henschel advised 30 years ago it is highly unlikely to raise the radon levels indoor. This is because of three factors.
1) There is no significant radon increase around the exhaust port, only in the air stream in front of the exhaust.
2) The area around the exhaust port is a very small fraction of the entire perimeter of a home.
3) If the wind blows the radon back towards the house, the dilution of the wind dramatically reduces the radon concentration.

Obviously some guidance needs to be included of how and where systems can be vented towards which the Canadian guidance goes not include. And further research is as always a good idea.

Here is the conclusion of my study. The entire study is on my website and due to be included in AARST papers this year after final review.

The results of the study indicate that radon system exhaust of about 234 pCi/l ( 8658 Bq/m3) vented at-grade directly away from the house produced on average radon levels around the exhaust location that never averaged greater than 1.5 pCi/l (74 Bq/m3) above the ambient radon levels. The average radon level beside the exhaust for the study was 0.5 pCi/l (18 Bq/m3) above ambient. When you consider that this portion of the exterior rim joist (about 1 meter) has slightly elevated radon as compared to the entire perimeter it appears from this study it is extremely unlikely that at-grade radon system exhaust could raise the indoor radon levels even if the exhaust concentration was four times or more higher. During the study no elevated indoor radon was measured that could be attributed to system exhaust except when a window directly above the exhaust was left open for four days with the system airflow of 40 CFM. During the open window test the indoor radon levels at the window was 0.4 pCi/l ( 15 Bq/m3) higher than another CRM twenty feet away. Lower exhaust rates did produce a higher though insignificant radon level to the side of the exhaust. Exhaust flow rates less than 20 CFM were not tested. Further studies should be considered with a building with significantly higher radon exhaust in combination with low flow rates and significant openings into the building during periods of strong stack effect.

Although the radon levels to the side of the exhaust were very low there was significantly elevated radon levels at one meter in direct line with the exhaust of about 7% to 16% of the exhaust concentration. The average at 2 meters dropped to less than 5% and at 3 meters to less than 2% and at four meters less than 1% of the exhaust concentration. One meter away at a 45 degree angle the concentration was less than 2.5% of the exhaust concentration. At 2 meters at a 45 degree angle it was less than 2% and at 3 meters at a 45 degree angle it was less than 1% of the exhaust concentration. This study indicates the location of grade level exhaust should not be direct towards any occupied location within 3 to 4 meters of the exhaust outlet in a area that is less than 45 degrees angled from the exhaust direction.

Ryan I would agree with your comments. I am a mitigator in Va. and have 30 years of experience and have done thousands of mitigations. I feel that we indeed need good sound regulations that provide consistency in mitigation systems. I also feel that how regulations effect the cost and availability of mitigation for the average consumer is very important. Our goal should be to create regulations that lead to the installation of safe, effective, and durable mitigation systems. Not put undue burden on mitigation contractors that may reduce the amount of mitigation systems they are able to perform. I would agree that our area probably has a shortage of quality mitigation contractors. The ultimate goal should be to provide as many safe, efficient, and durable mitigation systems as possible. There has to be enough regulation to create and maintain the industry without hampering the number of systems the industry is able to install.

After reading these posts I can’t help but bringing up the current beliefs of the current mitigation committee. And Bill Brodhead you are on this committee as you are well aware. Curious what your vote was on the 20 ft above grade requirement was.
Recently they (the committee) updated the standard to allowing discharges 20 ft above grade along with testing in the room on the other side of the discharge.
Note - that I do not believe in the science of “at grade discharge” like Canada. But do you really think they (the mitigation committee) will ever allow a discharge like this.
Based on the retesting requirement in the room near the discharge, the committee actually believes the radon gas might actually move from the dilution of the outside air after discharge and travel through the siding, insulation, and sheetrock of the house and back into the structure at elevated amounts from 20 ft above grade.
Please think about that for a second.
With what’s in the current standard (that by the way Minnesota is the only state so far to legally follow word by word) there is no way at grade discharge will ever be allowed. Which is strange because a 1 story rambler can be discharged at the roofline which ends up around 10ft above grade - which meets protocol. But you end up with a 2 story home with no windows or openings on one side and it needs to be 20 ft above grade to meet protocol. Because??? Just venting my thoughts.

Correction to the post above. I “do” believe in the science of at grade discharge like Canada.

In my previous posts I was subtlety hinting that the design and documentation process we use in Canada could be adopted in some way in the US and that it might help lower your costs - smaller fan, decreased noise complaints, reduced call-backs, more referrals, added value etc. Adding the design and documentation components could be included starting tomorrow in the next mitigations that you perform and in a short while you might be able to see this added value pay you back real quick.
Frankly, discharging at the rim joist is not all that advantageous and is the less interesting aspect of the Canadian approach and is something that probably should not be considered without also taking the design and documentation and sealing approaches first.
Why? Well for one thing, having your fan outside and exhausted above the roof helps to hide the big noise footprint of using more powerful fans. When you take the "super-sucker 4000" fan and mount it inside the house you are asking for trouble. When the house is real quiet at 2am and that 100watt fan that seems not too loud during the day suddenly becomes intolerable at night - you will hear back from the customer - then what do you do? Install a smaller fan and retest? Isn't that backwards? Also, when that exhaust noise that you have been pointing up and into the stars suddenly gets pointed at the neighbor's house or in the backyard, it can also be too noisy. Mufflers help to take the edge off but they are not magic.
I'm not sure which came first the chicken or the egg - in this case but I feel that the design process that predicts the correct amount of negative pressure for each particular house goes hand in hand with the ability to install a fan indoors and the exhaust at ground level because the process helps to ensure that the smallest fan that can be shown to work is used from the start.
Along with this is also our guidance which says that we must seal "all accessible leaks". This is close to pure magic. I have fixed many homes that would require the "super sucker 400" only to allow me to use a 20 watt fan after a thorough sealing effort (which can take 30% of the time onsite). Sucking from a perfectly sealed container takes almost no fan power while sucking on a leaky container sometimes isn't even achievable with large fans.
Finally, exhausting at ground level has you competing for precious wall space. We have clearances to other openings, mechanical air intakes, corners, gas regulators, windows, and everything else that poking through the mechanical room wall. Often, we would have to first go into a garage or adjoining room to get the clearances we need and this can get real tricky. Or we're moving other ductwork that's in our way. Just poking through the wall anywhere and pointing up to the roof sounds like the easier approach in many instances.
My advice is to start with the design and documentation. Add this value which will get your profits up and reduce costs. Get used to spending a lot of effort sealing. Reduce callbacks. Start using small fans when appropriate (we are 96% 20 watt fans regardless of building size or soil material) and get the noise footprint down. When all that is second nature - go ahead and drop the exhaust to the ground - re-entrainment is almost a unicorn. But I think you will find the unintended (mainly noise) consequences of starting with ground level discharge to be not worth it if you don't do the other first.

Colin: Thank you for your perspective. However, allowing US radon contractors the option to vent at ground level would not prevent them from using current guidelines for venting above roof lines. I can think of many cases where ground level venting would not be appropriate, especially in city centers with tight alley spaces between buildings. Also, as you mention. if you are concerned about noise, venting at ground level might not be appropriate.

I can also think of many cases where I was forced to run exterior pipe 30 feet of outside wall for no good reason. There were no exterior doors, windows, wall penetrations nearby. The fan was well away from other homes. I don't believe in these situations that adding extra length of pipe above the fan helps a smaller fan run more efficiently.

I appreciate your regard for properly sealing where you can and choosing a properly-sized fan for the home, but I see the issues of ground venting and fan diagnostics as being separate issues.

I have been on the MIT committee for nearly 2.5 years. I am writing this not as a committee member but as someone involved in this industry since 1996. I came to the conclusion that much of the complexity in the ANSI-AARST SGM-SF 2017/2020 standard for single family home “soil gas” mitigation is due to the additional requirements for Vapor Intrusion (VI) work that are not needed for radon. The potential harmonization of SF with MF and LB into one standard would also raise the level of the entire standard.
For those interested in how we got here, I have done a bit of research. This was put into motion many years ago –AARST added Vapor Intrusion to its repertoire around 2014, started pursuing ISO 17024 around 2015, added QA requirements in 2014, OM&M in 2015. These things were mentioned in various issues of the Radon Reporter.

There was a committee formed sometime around 2014 to make a new single family home radon mitigation standard as an ANSI-AARST standard. There were 33 people, plus Spruce co-owner Dave Kapturowski as chair, on the committee writing SGM-SF-2017. The breakdown of affiliations has always troubled me. There were 7 HUD people (and no disrespect to them, but I doubt any had hands-on radon mitigation experience). There were 15 government people total, including HUD, EPA and state radon reps. And 10 people specifically as radon reps, but when you take off government people, only 7 of the 33 total were designated to protect radon’s interests.

But also, this committee had 16 VI representatives (or 14 – 2 EPA members were not specified VI or Rn but were among the VI people). Anyone would expect a higher level than minimum requirements, based on the number of both government and VI people. This is why the standard has so many new and cumbersome requirements – desirable and needed for VI but not for radon.

I do acknowledge the time and efforts made by people who are AARST volunteers, on the board and on the committees as well as for NRPP. This work requires a significant amount of time and most people are highly invested in their work. Many have been involved with AARST in various positions for decades, and it is understandable that they would be offended to have their work product criticized.

But public comments on standards is by its nature a form of criticism of the standards. When SGM-SF-2017 was being written and issued, it was required to be used nowhere, and got little attention. But the ANSI-AARST standards now have the force of law and are enforced with fines and threats of losing one's license in some states, and the same is planned for all states. So everyone should be reviewing each word carefully and not be inhibited from making comments/objections/suggestions, and AARST should be receptive towards making changes to requirements that people who are using the standard in the real world have found to be problematic.

Maybe this is a great example of why there should be term limits/time limits on people's service, and limits on the number of employees or volunteers from each company being involved in AARST. It is unfair that only very few companies bear the burden of AARST work, and on the other hand, unfair that only a few are making the decisions and setting the direction for the entire radon industry. And we, the radon community, have no idea how many people are involved on all the various committees, since only the chairs are listed, but I think some companies are sacrificing significant manhours to AARST while others are left out. I think making this transparent, with all committee meeting minutes published, would help us truly appreciate the work of AARST.

Hopefully Dallas can commit to making these administrative changes, and that the AARST, NRPP and the Standards Consortium will be a partner with radon professionals instead of an adversary that uses policies and procedures to ignore valid complaints and concerns.

I skimmed though this and concluded if, from the beginning, the USA had been using ground level discharge like our hockey loving friends up North, there would be no compelling evidence to switch the standards to above roof discharges for radon.

Page 35 paragraph 3 says what I've always suspected and Canadian already know: "In summary, this study of 10 houses near Harrisburg, PA found that ground level discharge of ASD mitigation system exhaust did not increase indoor radon concentrations when compared with traditional roof top exhaust."

The other interesting tid bit I took away was very low level of air a radon fan pulls out of the building envelop as measured by ACH50 blower door tests. The worst house at 14.3 CFM with most being way lower. Not a big number! A Clothes dryer can pull more 10X that amount of air. Again statements like radon fans contribute significantly to air/energy loss, or 50% of the operating cost of radon fan is the homes energy loss, are not even close to be proven by this study.

We need more work like this, real building science based and industry experience.

Delete this post please, repeat of above

Delete this post please, repeat of above

As a participant representing public health interests on some of the ANSI/AARST standards commmittees (not Mitigation), I do want to underscore the following:
- We are all in the business of saving lives from radon, and strong yet reasonable standards of practice adopted and implemented across the country not only can ensure that lives are indeed saved, but also can help protect the radon industry from legal and commercial challenges.
- In my experience, the process of developing standards has always been one that seeks input from beyond the committee membership, particularly if that includes illustrative science and evidence from practitioners. As Dallas has shown above, standard-setting is intended to be open for all to make contributions at https://standards.aarst.org/public-review/ and I do strongly encourage anyone who has recommendations for what the standards should say to provide specific language for such changes along with a cogent accompanying rationale.
- The principle of the standards requiring only what is needed for safety and effectiveness is something that has always been a touchstone for our work. We certainly recognize that a standard that's "perfect"--and hence, too complex or too onerous, and so is too difficult to follow or is not even used--is a worthless standard. That's why, in our meetings, standards committees are most eager to hear from stakeholders who represent those in the industry who would actually have to live and work under the standard. I have seen proposals simplified and improved after someone active in the field made exactly that sort of commentary.
- If any radon professonals want to be more involved in the standard-setting process, such as by devoting time to advise standards committees, that kind of interest is certainly welcome: https://standards.aarst.org/participation-in-aarst-consortium-radon-standards/

My two cents on ground level discharge.
I did not work on any of the first PA houses, but there were some pretty funky internal walls that were entry points in those houses. I was involved in all of the next set of houses in Clinton NJ. Those houses were in a neighborhood where houses were in multiple 100's. All homes were built by same builder on a few floor plans so information from one house was then used on the others and there were public meetings so homeowners could use the research houses to mitigate there own. We were in the basement of one of these houses where the mitigation was not getting the radon levels down below 4. There was a concern that there was re-entrainment with discharge behind the shrubs outside the house. Someone put dryer vent on the exhaust to run it past the shrubs and away from the house and we watched the radon level drop on the CRM.
Bruce Henschel did an amazing job developing the encyclopedic Mitigation Manuals, but models are only as good as the data input into them and need to be used with caution. For almost 20 years the VI folks did not look at pressure driven flow into the buildings even though the radon folks knew it existed. Their models did not show it and as late as 5 years ago the standard radiation models used on contaminated sites still did not include it - only included diffusion through the slab.
I saw re-entrainment from a high level house with shrubbery around it - it is clearly possible. Will it happen in a house below 10 - maybe not that we can measure, but what radon level is safe for ? Is it a function of the inside radon level or the soil gas number?
If there is a lot of sealing that results in only slightly elevated radon levels in the home, will we have any idea if the soil gas levels are very high. As with much of radon - it all depends.
It seems to be an area where significant research could lead to an answer, which might be one thing for radon, where we have good options for real-time monitors and a different answers for VI where we don't. And as discussed above, that may only apply to single family homes with larger lots and not urban/suburban areas where homes are closer.

It is nice that our Canadian friends who are also CARST board members are taking the time to advocate for the ANSI-AARST standards. Does everyone know CARST is part of US main AARST? https://carst.ca/BoardofDirectors?&tab=2

The 5 years that I was a Member of the School and Large Building Standard and chaired the Multifamily Mitigation Standard, the committees debated most of these issues. When we were finished, the Standards then went into public review cycle. I was proud to be asked by CARST to join Dr. Bill Angell and Jack Bartholomew (May he rest in peace) to teach in Canada. This was when some of the new Canadian Guidance was in development. In doing so was glad to get to know many top notch Radon Mitigators (Some on this thread).
“Reducing Radon Levels in Existing Homes: A Canadian Guide for Professional Contractors”, is a fine Publication / Standard but not a perfect set of instructions. Neither are the AARST-ANSI Standards. Problem is that structures vary. Soil, aggregates, foundations, climates, geologies air issues etc. etc. etc., create their own unique problems. Products, instruments, tools and methods are evolving. There’s no silver bullet for success, instead an accumulation of experiences and observations that hopefully will move us towards best designs and installations.

Not sure how change is addressed in the Canadian Guides, but Dallas (in this thread) has shown the way to address changes in the AARST-ANSI Standards. For those who are passionate, you will be considered and replied to in the review cycles. The AARST-ANSI Standards are fluid and designed to change as necessary.
John Mallon

Michelle, CARST is an independent organization. I was proud to be one of the co-founders of CARST while I was AARST President. Getting CARST up and running as an independent organization was a personal and professional priority. An early priority of my/our Canadian colleagues was creating a certification program, C-NRPP. Pam Warkentine was one of the leaders at establishing and operating C-NRPP. We, U.S. radon professionals, can learn from our CARST/C-NRPP professionals and it has been a great pleasure to see how the radon profession has grown with collaboration with Health Canada.