Well Doug, you may have whacked the bee hive with this one.

In part, the rationale is that the mitigation system is drawing in a lot more radon than would normally be found in the house since it is sucking directly on the source. If the system is going to leak, it will be on the pressure side of the fan, which is why fans cannot be located in or under conditioned space. Leakage could be caused by a leaky fan, poorly sealed connections (including couplings and pipe fittings), damage to the pipe, etc.

So the mitigation standards have always guarded against concentrated radon being dumped back in the house, which could create higher concentrations than pre-mitigation testing. That was also a factor in the venting location and blowing it out above the roof.

I'm sure you'll get more replies, so I'll grab some popcorn.

So I will bite too.

I think there was some rationale in preventing a pipe under pressure with dangerous gas from being present inside a living space. However, there is also concern that furnace exhaust under pressure should not be inside my living space, yet my furnace is in my basement and not outside. Somewhere along the way some very smart people figured out how to vent the exhaust from the house with minimal risk due to problems with the pipe (and that exhaust is hot, which radon exhaust is not). Why don't we do that with radon? I would love to put radon fans inside rather than on the roof. This is particularly true here in Minnesota where the cold weather can wreak havoc with the fans in the winter due to freezing of the ever present moisture in the air being drawn out of the ground. If I'm not mistaken, Canada allows fans inside the house in their radon code.

Is it time that US radon experts take a fresh look at this?

When I first started in 1989 fans were leaky and people were using dryer vent tubing with the vent out of the side of the house into the children's sandbox. The fan placement, pipe material and venting protacals developed in the 90s made sense. However fan construction has improved and studies in Canada indicate venting at ground level dose not result in elevated levels in the house.
Venting protacals still make sense in new construction but can be a challenge with existing homes.
Fan placement outside the conditioned space can raise the cost, because of the need for an electrical outlet, and creates fan issues because of extremely cold or hot locations. Astetics is also an issue. One builder asked why don't we put the toilets outside because the waste pipes may leak.
Another collateral damage issue is that certified mitigators lose business because builders and homeowners turn to someone who will install in the basement or crawlspace when there are no local building codes stating otherwise.

Doug, your reasoning is correct. Conditioned attic spaces are relatively new (in our area) and are not addressed in Appendix F of the Building Code, which is what the Builders go by. It is my understanding that work is being done to reconcile Mitigation Standards with the Building Code, but Code changes take years even when unopposed.
A very logical solution to this concern is to require a remote radon sensor, such as the Airthings Wave, in the same attic space with the fan to warn occupants of any leakage.
I am with Bruce as far as wondering why it is ok to vent carbon monoxide from gas appliances through a living space and exhaust it below windows.
What I have learned from trainings, articles, and colleagues about rim-joist venting (as is done in Canada) is that safety studies show mixed results.

30 or so years ago a trainer (I believe it was Terry Brennan) in an EPA course told us that radon fans could no longer be in the basement or the exhaust at grade. He said he did a research project in a home with a radon system exhausting at grade. He injected a tracer gas into the soil below the home, and then found that he could detect the gas in the home, having been reentrained.

Later, the EPA's first mitigation protocols prohibited fans in basements and exhaust at grade, to a large degree due to his research. He protested to the EPA folks that his study predicted a reentrainment health risk in homes with unusual, extreme radon levels, but the rest of us were fine with the fan in the basement. The EPA response was essentially "You're right, but we want one standard for every home".

Since Bob Wood, seems to be sleeping.
I have over 3000 Canadian Radon systems, no reintrainment and every home gets a AIR THINGS monitor, just as a we have smoke and Carbon Monoxide detectors.
Today’s fans don’t leak, so long as you stick to the big 3
PVC pipe, is highly robust and it has proven the test of time, with millions of HE furnace installations- leaks = death, literally!
So we have chosen to eliminate the major failure that -40 deg temps, impose on a Radon system-using indoor fans.
But 0 failures in 14years.

Bob Wood might be sleeping, but i couldn't resist. I think that when we, as Canadians, developed our mitigation guidelines, we had the benefit of learning from our southern neighbors successes and failures. I can certainly see why the EPA would be overly cautious when developing their original standards, but technology certainly has changed as have material standards since the original US standards were implemented.

I know that Health Canada did extensive testing before giving the thumbs up to sidewall discharge. Personally, i am with Rob on this one. I haven't had any issues with re-entrainment with sidewall discharge. That being said, we also are not allowed to use anything less than sched 40 pipe and fittings or fans that are not certified for use indoors (must be sealed).

In cold climates, the outdoor fan and piping simply isn't an option for most houses (for many reasons, cost, esthetics, not to mention freeze-ups). We want more people to have these systems installed, so we have to make it more practical for it to be implemented. If i told all my clients that i had to run the pipe above the roof line, 90% would likely tell me to hit the road (simply because it would mean running the pipe through most of their living space; cannot run outside due to freeze-ups). Not to mention the costs would be prohibitive to many middle class working families.

Obviously, we don't discharge the system in areas where people gather outside (kids sandbox, decks etc). Not just because of radiation levels, but also because of the fan noise. We don't want people to turn these systems off because its bothering them.

I really think its time the EPA looked into changing their guidelines. It might encourage more people to test/mitigate as well as reduce mitigation costs. Especially in colder climates like Minnesota.

If you blow radon ourside, how does it come back into the house in high concentrations? It doesnt.

Terry Brennan did do a number of studies on radon in NJ, including at least one that looked at detailed measurements of radon in soil levels around houses. The start of moving away from band joist exhaust was before most of those however.
During an EPA Office of Research and Development study in Clinton, NJ that Terry worked on they were unable to get the levels below 4pCi/l and finally decided there might be re-entrainment. So they put a hose at the outlet of the band joist and extended the exhaust beyond the foundation plantings and away from the house. We could actually watch on the real time monitor as they levels in the house dropped. These houses were quite high - with many in the 6-800 pCi/l range.
In public meetings, Terry used to describe the attractive nuisance of a ground level exhaust imagining a kid playing with their parachute man there.

Unfortunately up until recently there has not been any studies done in the US that measure the effect of rim joist exhaust versus roof line since the mid 90's. This year we may get the final report from Bob Lewis and Brad Turk that did an extensive test in several homes. I myself have been amazed how quickly radon diffuses from a source in a basement when I'm trying to locate it.
I just submitted a paper to try and measure radon diffusion from a rim joist exhaust. So maybe in Nashville we will have some actual data to reconsider this long held position. I think a good comment at this point is maybe we can all learn from the example of our brothers to the North that we would all be better off making decisions based on science rather than our own opinions.

I will reach out to Health Canada, to find the dispersion testing they did on one of my 20,000 Bqm3 homes, it was really great.

Ok i wasn't sleeping i have just been focused on this covid thing ( i think my son has it after a trip to NY state to pick up radon materials).

Rob will tell you that way back in 2005-2008 when we were getting started in the radon industry. I would not install a SSD system with pressurized piping in the building. as a plumber i was concerned not about the quality of my work or my teams work but if we opened the door to interior fans pushing radiation through the interior of a home what would happen when somebody who doesn't have my skill set and knowledge gets in the industry. (Rob said we already install natural gas, propane, furnace exhaust and plumbing in the interior of homes)

Rob quite calmly listened to my rants and said we'll see. Offer it to a hundred customers and we will talk. So i did. slowly the percentages climbed 30% of homes then 50% then..... i think in the last 200 systems we have installed maybe 3 garage style systems with insulated piping in garages and heat tracing (high end homes who did not want noise or no place to exhaust that made any sense). All the rest have been interior fans and sidewall discharges. exterior system just freeze up with our ground soils moisture content and -20 days.
I freely admit i am a convert. Reluctantly only because i dislike swelling Rob's head about him being correct again.

I will freely admit i am a very strongly opposed (in first draft it was Hate) to one other practice that i am told continues to happen in the US. That their are a few (or many) mitigators in the US are still using SDR 35 pipe and that it is a practice that needs to be stopped. it is criminal to use a pipe that is not certified by the manufacturer to be used above ground to convey radiation laden air anywhere. in a country (US) that claims to be risk adverse for fear of legal damages how this practice survives I do not understand.
Gentleman and Ladies it is time to come indoors. Interior fans and sidewall discharge works.

Agreeing with following the science, of course. Would that we had more of it.

* Since not all houses are the same, I'm most interested in what the science says about
- houses with subslab soil gas concentrations / volume production rates in the higher orders of magnitude. This raises the question of whether an installer (climate-permitting) may need to make use of some thresholds for decision-making--i.e., certain values above which they really ought to do exterior fan / above roof discharge. (I make no prejudgment here; I'm only asking a question, and would someday like to know what the science tells us.)
- certain geometries where ground-level exhaust points may create greater propensity for local build-up outdoors or opportunities for re-entrainment. (Certain row-house arrangements come to mind.)

* It's important to pay attention to the "sociological and political science" in the States vs. Canada: When "somebody who doesn't have [good practitioner's] skill set and knowledge gets in the industry" and uses poor piping, poor techniques, for interior fan and/or sidewall discharge and causes problems, how will that perception affect the perception of the industry as a whole for the public and lawmakers? How will not wanting to risk ending up with such a problem affect the public's perception of even the basic value of testing for radon and mitigating it? (Again, I don't prejudge the answer. This is not a rhetorical question. I'd honestly like to know what any scientific approach to the sociology and psychology of risk perception might have to say here.)

What a great question: Here in Canada as a group of mitigators and team teachers we see the ones that excel at learning as the average mitigator not because it is true, but because those are the ones who ask us questions. Marcel and Colin more than Rob, Bruce, Jeff and myself (i use first names because for those that know us that will be enough for those that need last names contact me off this forum), because they are the mitigation instructors, We are the group of the old guard and i am sure that many new people in the industry would not want to ask us for fear we would want to steal their new idea and they haven't yet seen that we have the philosophy that rising tides raise all ships. little do they know that I regularly ask questions of my radon guru's and when it comes to the tough ones i ask a lot of questions of our radon guru's.

When you are doing radon mitigation you are running a lot of radiation through a pipe, it is not unusual for radon concentrations in that pipe to be in the realm of 10-50 thousand radioactive strikes per second. Yet it is extremely common for builders to say i can get my labourers to do that. Lack of knowledge about what we do all the parameters we consider and dismiss, is ignorance not bliss. i do not know how to change the value that others (public, builders, health professionals, government) put on our work of radon mitigation, i do believe that it not understood therefore undervalued. After all it is just nuclear science, and building science that gets run through a pipe. done right it has a great value to society, done wrong it is a pipe sucking on dirt.

The Health Canada dispersion study was the game changer for me when i looked at the data sidewall discharge has very little risk associated with it. Unless someone places little sally's sand box in front of it.

So, this is a wonderful discussion among some knowledgeable practitioners. I think I see a common thread, i.e., it's okay to allow fans inside buildings. But, this is a very small group. By my review of the authors of SGM-SF Consensus Body Members listed in SGM-SF 2017, none of the comments in this thread were from members of that group.

So how do we make a change? Do some of us need to contact members of the SGM-SF Consensus Body? I assume that some of you know some of those members. Who wants to run with this?

Indeed, this has been an interesting discussion and one that has come up every two years going back to the beginning of Bill Field’s Listserve. Allow me to pontificate as I have been involved with this all the way back to the original mitigation standards.

The purpose of having fans outside or in the attic is to prescriptively reduce the potential for the leakage of radon laden air into the occupied space for two reasons: 1.) Leakage from the fan housing and 2.) Leakage from poorly connected vent pipes on the discharge side of the fan. During the late 1980s and early 90’s fan housings and passage ways to the electrical handi-box could and did leak. Also, people were using sheet metal rather than PVC piping, or if it was PVC, some were using thin wall pipe that even a moderately 5 year old with an attitude could break.

If we fast forward to present times, fans that are considered to be “Radon” fans have definitely improved since 1991. Also current standards cite the use of schedule 40 PVC or ABS as Mr. Wood has pointed out for years as being the appropriate thickness.

Yet, we still occasionally run across installations with non-compliant piping and hear anecdotal stories about installers not gluing their pipe. So the question comes down to: Do we have standards that guard against non-compliance and poor workmanship, or do we recognize advances in the fan industry and assume compliance with minimum standards?

One recommendation might be that a standard be developed by the fan manufacturers that specify what a radon fan is. This certainly would address several elements such as moisture resistance, UV resistance, electrical, etc., as well as leakage. Without a prescriptive fan standard I would assume that fan manufacturers would be reluctant to guarantee 100% leak proof, but rather meet and verify a maximum leakage rate that would not present a significant increase above background (0.4 pCi/L).

The second aspect would be verification. Going back in time again to the early years, tests were often deployed on multiple levels of a home rather than in just the basement. This more thorough testing identified re-entry problems that led to the current prescriptive approaches. However, today’s approaches only test the lower level and hence do not pick up higher measurements upstairs that could result from disjointed pipes or improperly located discharge points.

Fast forward to today, we have relatively inexpensive consumer testing devices that although not necessarily approved for certified tests do a pretty good job of identifying problems.

So, consideration could be given for alternate approaches if additional testing and surveillance measures are employed.

As far as discharge points, I look forward to additional data from Mr. Lewis and Mr. Turk, assuming they are continuing with the study in New York, but preliminary results appear to agree with what our Canadian friends have observed.
Doug Kladder

Doug, well stated. I think it is important that we attempt to keep the bar a little higher rather than lower. To this day I continue to find fans in the crawl space, mounted sideways to save 2 90's with black flexible drain pipe as the exhaust that discharges at he foundation vent, below a window. And people have been living in the home for years thinking they were safe from radon. If some people in our industry fail to follow the standards that have been in place for years I would hate to see the results of a relaxed standard.

Let me add a relevant experience.
I have several thousands of multiple branch active radon mitigation systems installed.
There were two documented failures by me of activated systems of passive systems that had originally been installed in the USA by others, where discharge of radon vent pipes were under soffits that also have (soffit) vents. The ultimate proof that this was the cause for high indoor radon levels was that after several other modifications that did not work and a pressure field that was complete from the beginning by moving the discharge to above the upper roof in one case and in the other case more than 10 feet away from the soffit vents solved the problem. I gave a talk about this at the San Diego AARST Symposium with an article that can be found on the AARST website.
Soffit vents at regular distance along all soffits are very common in the area of the USA where I live and work due to potential Summer attic overheating issues. It is my understanding that soffit vents are not common in Canada because due to its northern climate this overheating issue does not appear to be much of a problem, (although they have their function also in moisture removal, if entered). Conversely, I have been told the danger of these vents in the climate North from where I live allows snow blowing into attics via these vents. This latter argument was used to me by a builder in Wyoming for not having installed roof vents on his house (where that Summer the attic overheated and the 150 W fan died. An IR-temp meter registered 155 degree F in that attic (Yes fans stop running at 140 F having a themal protection, but this fan really was fried) when I replaced the fan, which is why I urged the owner to talk to his builder).
Unless a rim-joist discharge study includes Homes with soffit vents that are directly above the rim joist exhaust it does not reflect the stock of houses built in our area, and I have documented the potential of a negative effect of radon reentrainment on discharges under soffit vents.
Leo Moorman
Radon Home Measurement and Mitigation, Inc.
Fort Collins CO, USA

Here is the conclusion

Soffit vents are pretty much standard on all homes in Canada, as with gas code, the minimum clearance for a furnace is 3 feet from a soffit vent, however the rim joint is the top of basement foundation wall, where the main floor , floor joists sit and are banded, with “rim joist”
You can’t get much further away from a soffit. 10-20feet.

I would encourage mitigators to check systems that they service, to see if the rubber couplings are still tight when they arrive.
I find that systems that have been installed for 3 years or less generally can be tightened by 2-4 turns of the wrist with a nutdriver. Yes they have loosened over time.
I find systems that have been installed for 5 plus years are generally loose enough to require 5+ turns of the wrist.
Since I have been collecting this data for quite a few years, and have thousands of sets of documented system checkups and examinations, I can document that this is true (in Northern Illinois)
Additionally when fans are installed in a Radonaway/RCI Fan housing signs of condensation and moisture inside the housing is often evident in these situations. There have been several situations where these has been enough moisture created inside the housing that there a giant ice cube was formed in the bottom of the housing. Yes the top of the housing was weathertight and properly sealed.

My conclusion is that over time rubber couplings begin to leak if they are not retightened.
This causes me to wonder at the wisdom of unattended fans in attics for years and years.
Thus if the fan is inside, I think there needs to be a new way of connecting the fan to the PVC Pipe.
The Fan and schedule 40 PVC may not leak, but the attachment coupling can and does.

Rich a great point but were those couplers leaking? i over the last 40 years of being a commercial plumber have or caused to have literally 1,000,000's of mj joints to be installed on cast iron plumbing systems. if they do not leak on installation test they do not leak later. plumbing systems experience much more pressure than our radon mitigation systems do.
i don't know how many fans you have exchanged but my experience is that once a fan is in for even a few days, it is difficult to break that seal even with the stainless steel band removed. While i will acknowledge this is anecdotal it is my field experience.

Doug the big 3 radon fan manufacturers did set up a committee and did develop a method or system for indoor fans for the CGSB standard that was developed in Canada for both existing residential building mitigation and new construction of residential buildings.

Please do not mistake my belief, I believe that a radon mitigation system with only negative pressure inside the building is a better system, because it prevents errors in the installation from overcoming the good a radon mitigation system does. i just don't happen to believe that it is the system that is required if you have competent professional installers. that are having the quality of thier work confirmed by multiple layers of testing. Soap test fan and joints radon test the building short term and long term.