• Henri Boyea
    I would like your assistance.

    I have a Client who has a fairly small home with an ASD system, and rents one side of the basement to a tenant. The Renter did a radon test which came back at 5.6 pCi/L and moved out until levels can be brought lower. After speaking with me, Client decided he had more faith in me than original installer and asked for my help. I found and corrected some major errors with the original installation, but have not been able to get concentrations where we want them. Unless noted, there were no significant anomalies with any test data. All test exhibited normal patterns, and all ( with one exception) were done during normal weather conditions for the area. (Please refer to the attached diagrams/tables in the PDF at the bottom.)

    Basement is approx. 1000 sq ft, has crushed stone under the floors, and has no garage/carport/porch slab on the upper level. It is a multi-piece modular home where sections are pre-built and bolted together; the center wall is 10" thick (framed). If I remember correctly, exterior walls are poured concrete. There are no below-grade utility penetrations, and no dry traps/drains. House is on City water. Where wall/floor joint was accessible in storage room, Client caulked perimeter. In the rest of the basement it has 1x4 baseboards on concrete floors. He caulked the top and bottom edges of all baseboards. The one variable with which I am not familiar is a free-standing portable A/C unit in the apartment that exhausts through a window. I am unsure whether the exhaust is for heat only, or whether it actually pulls air from the basement (which seems illogical). In any case, tests were done with the Unit off, as it does not run the majority of the time.

    Original system had an older RP140 fan which was making some bearing noise, but still moving air pretty freely.
    My PFE diagnostics showed that the original system was affecting only 2 of the 4 sub slab areas divide by footers. The PFE did extend across the center footer, (probably because there are plumbing and drain lines that cross the center), but did not reach the front room on either side. As we had pretty good PFE and a relatively small footprint, my solution was to "stitch" the other slab areas to the ones that had good PFE by adding suction points on either side of footers, connected to each other (thanks Bill Brodhead). After stitching slabs, the radon retest at 3.7 pCi/l, but the renter was not satisfied and would not move back in.

    We then swapped the fan since the bearings were beginning to fail. As Client was desperate to get the Renters back in, he wanted to just jump to a very powerful fan even though a more energy-efficient one may do the job, so we tried an Eagle Extreme to increase both vacuum and airflow. PFE increased substantially, but radon concentrations also went up to 4.6 pCi/L.

    After discussion with my fan supplier, we could not think of a specific instance of it having occurred, but both agreed there was a possibility the strong fan was pulling radon in from outside the foundation somewhere. After swapping the fan to a Maverick (1" w.c. on u-tube), the retest came back at 7.6 pCi/L, but this test had an anomaly where the concentrations spiked up to 30 pCi/L and back down over about 12 hours. Dropping that data, the rest of the test averaged 5.7 pCi/L. Client subsequently did an AirChek test which came back at 4.4 pCi/L.

    I admit to being somewhat mystified as to why there is still a radon issue with such good PFE across the slab (-.008 at farthest point from fan.).
    Thoughts and questions?
    Robinson Long (1M)
  • Wally Dorsey Jr
    Are there any exterior foundation walls that are slightly below grade? If so you may have a sidewall issue, if so, are they be the farthest walls from the suction point?
  • Robert Burns
    You seem unsure about the exterior basement walls. Could they be block and not poured concrete?
  • Kevin M Stewart
    By those replies, it looks like the walls are the big suspect.
    I'm only a sidewalk superintendent in this particular area of expertise, so I leave it to your judgment if I know only enough to be dangerous...
    For one thing, I do wonder what the tenant would find acceptable, since it may be possible that even if averages are kept quite low, that may not prevent every possible brief excursion above 4 pCi/L. Also, wondering what the radon level is the tenant's current location elsewhere.
    Curious if you're doing surveys (grab sampling, air movement, not just PFEs) at different points near the walls to get a better sense of source location(s) and pathway(s)?
    The sensitivity of the home to rapid peaks also makes me wonder if there is any overpowering (karst?) phenomenon that is sometimes at work. Did you any see diurnal behavior patterns?
    Even wondering about any unusual direct sources of radon--recalling I heard (or is it not the case?) that radium salts were found to be efflorescing onto the basement wall of the Index House. What is your gamma microR meter showing?
    I'd be tempted to say "Well, just directly mitigate that apartment with a dedicated system installed in the corner diametrically opposite the first," but apart from any impracticalities to doing that, unless you adequately characterize what the real problem is, I admit that would just be "Poking and Hoping."
    In any case, be sure to communicate to the owner and tenant that you are consulting with other professionals about this unusually difficult case, all of whom want nothing more than for this mitigation to end quickly and successfully.
  • Henri Boyea
    Possibly, but we occasionally do block-wall depressurization, and I am pretty sure I ruled that out and just didn't note it on my worksheet.
  • Henri Boyea
    The front wall is below grade, and just a little bit of the side walls.
  • John Mallon
    Most free-standing portable A/C units do exhaust hot air and cause considerable negative pressure. Although, there are units that use fresh air supply exchange to eliminate this they’re much less common. Check the specific manufacturer’s tech spec. Could be a big issue!

    You said the Sub-slab material is “crushed stone” aggregate. If this is typical (without fines) and generally dry, I’d agree with Kevin, put a 2nd ASD system on the renter’s side. If correctly built, the wall between family room and apartment should be an “air tight firewall” Your current system is on the owner’s side and the renter is likely creating even greater negative pressure at times on the apartment side with the AC unit.

    The apartment side should also be a high air volume fan. You said the original fan was a RP140 (too small). I’d use a RP145 at least and change the “stitch pipes” (seems very inadequate) to a 2 suction point system. To further enhance air movement volume: excavate large suction holes and ideally use 4” pipe throughout (suction side and exhaust side).

    My 2 cents, since you asked and sent good comprehensive information.
    Good luck!
  • Bill Brodhead
    Hi Henry
    A common problem with post mitigation elevated radon is trying to locate a source when radon levels are only slightly above the guideline. Because the building is broken into individual rooms multiple CRM testing might be a good next stage. If you can spare 4 CRM's I would locate them in each of the main rooms for at least a day. To reduce the cost of that I have been working with one company Ecosense to develop a package of four radon monitors for the typical price of one CRM that you might consider getting. The package is called Ecotrackers and has four of the home style Radon Eye's that have been modified to go up to 255 pCi/l and can be programmed for 5 minute averages to use in sniffer mode or hourly averages to use as regular CRM's. Getting comparative measurements at the same time might clue you to which area is causing the elevation as all four units display their data on your phone at the same time using bluetooth. Or the home owner can also capture the data and email it to you if you leave the units for multiple days. Of course then you have to figure out why one area is higher. If the foundation walls are indeed poured then that usually rules them out as a source. Sometimes there are hidden abandoned utility pipes behind finished walls that have tripped me up for months until we found them because one room was higher radon than another. Good Hunting to you. Bill
  • Henri Boyea
    Thanks for the A/C info. Logically, I can not figure out why such a Unit would exhaust indoor air, as cooling would be much more effective when recirculating the indoor air. I do know for certain that the portable A/C unit was off during the last two tests. As for the second system: that has been suggested, but I have trouble justifying that when there are good negative sub-slab PFE readings in all rooms. When we had the Eagle Extreme fan on, the lowest PFE reading was -.015, and the radon test result was 4.6 pCiL. When we downsized to a Maverick, we still had -.008, and the retest was 4.4 pCi/L. Thanks,
  • Jeff Tikkanen
    Does the upper house structure truly match the foundation footprint? 3' x 10' crawl spaces can be hot spots and easy to miss.
  • Wally Dorsey Jr
    Good point Jeff,

    We were working with a mitigator on a problem house and the brick front stoop was about 4'x8', completely outside of the homes envelope. Every time they tried something new on the basement or crawl space, it seemed to work for a day or 3 then back to 90s in the basement and 60s in the living area above the crawl space. When they finally pulled some grabs from the front stoop void, they were hit with a 600 pCi/L and rapidly climbing burst. They never really found out what the true number was because they knew they'd hit pay dirt and pulled the grab sampler out of there! Once they addressed that space everything came down to below 1 pCi/L. They couldn't access the space so they popped a 2" suction point directly through the sidewall and pulled suction on the whole area. Voila!
    Same thing is true of those sidewall issues when the home may only be 6" sub grade. Once the radon is into that block wall void it's outside the PFE of the fan and goes wherever it is pulled.
  • dcasciato
    Wally - thanks for sharing your experience with the entry "stoop". Interesting case.
  • Matt
    Are you testing the PFE with all appliances discharging air to exterior? Example:Dryer, range hood, bathroom exhaust fans, water heater running, furnace blower on and off. Are the heating and cooling units in the attic of the house? I have found these to really depressurize the basement. Sounds like you may be loosing PFE. I like to see -0.02 "WC or - 5Pa at all test holes. What is the pressure difference between the basement living area and the exterior? Is it possible there is a footing between the closet and the owners family room restricting flow? How is the owners family room and apartment living room stitched together? What is the pipe size and cfm you are moving? Have you tried flipping the fan and run a crm to see what radon levels do? Run a short test and stop test if levels rise higher than starting levels. Are there any exterior structures, patios or steps that could be causing an issue? I would say its probably not the ideal location for the suction point but I understand how some clients do not want it visible in the living areas. I would like to see a suction point mid wall on one of the long walls of the basement or next to a plumbing riser. Just my opinion.
  • Robert Burns
    Is there n attached garage? We had a home that had a draintile installed under the basement floor so we had suction everywhere but the radon level would not go below 10 pCi/l. The garage slab was poured level with the main floor level but instead of butting up against the basement walls as is typical it was poured on top of them. The basement utility room was unfinished so I was able to place a CRM on top of the poured basement wall adjacent to the garage slab. The level was 150 pCi/l.
    A second suction point in the garage connected in the attic to the system we already installed solved the problem.
  • Henri Boyea
    I have had some similar cases (with much lower numbers). Thanks,
  • Henri Boyea
    No attached garage, carport, patio or porch. Thanks,
  • Henri Boyea
    UPDATE: Homeowner confirmed that exterior walls are filled (not hollow) concrete block, and the center wall is framed with 2x10s. Thanks,
  • Henri Boyea
    Are you testing the PFE with all appliances discharging air to exterior? we have not. Are the heating and cooling units in the attic of the house? Unsure. I like to see -0.02 "WC or - 5Pa at all test holes. -.008 at weakest point. Was stronger with larger fan, but radon concentrations were also higher. What is the pressure difference between the basement living area and the exterior? Don't know. Is it possible there is a footing between the closet and the owners family room restricting flow? No. How is the owners family room and apartment living room stitched together? They are not, but bathroom and laundry are in middle of basement, and excellent PFE numbers are most likely due to plumbing penetration through the central footer. Have you tried flipping the fan and run a crm to see what radon levels do? Not yet, but it is under consideration. Are there any exterior structures, patios or steps that could be causing an issue? No.
  • dcasciato
    Robert - wondering if you could attach a sketch(hand-drawn is fine) showing the condition. Not sure I am understanding it fully. Thanks in advance - Dan
  • Dick Kornbluth
    Where was your testing done?
  • Robert Burns


    On the left. In our area when the owner does not want steps from the garage to the main level hanging trusses are used and typically the garage floor is poured against the basement concrete wall and the interior walls are constructed on top of the concrete basement wall. Any radon coming from under the garage floor enters the garage where it is usually not contributing to the interior house level. The drawing on the right shows the garage floor poured on top of the basement wall and the interior walls constructed on top of the garage floor which allows radon to flow into both the basement and upper level.
    I was fortunate that the basement utility room was unfinished and I could see the garage floor sitting on top of the basement wall.
  • Steve Green
    I've been following this discussion, all very helpful information. Bob I never would have though of that, very informative information.
  • dcasciato
    Thanks Bob, the drawing was helpful in my visulizing the case. Dan
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