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250 cfm differential with radon fan: Why?

Hi. I am working on a continuing problem, and hope to get the advice from the smart minds on here to figure out why we have are having both indoor humidity fluctuations and elevated radon ( GBA links to both topics attached below). Trying to solve both humidity fluctuations and elevated radon ( 9.3 pCi/L) with radon fan running. I did close up the 3 small HVAC supply damper vents as suggested in the previous post ( see link below)... thanks! However, we are still having pressure/cfm differential with the radon fan running, based on blower door test done yet again. The consensus on here has been to look at sealing-at that MUST be the issue. I agree, since the radon system is supposed to be separate from the home and crawl space-located beneath the vapor barrier and terminating above the roof line. I noticed one of the advisors on here mentioned the vapor barrier-which I never considered before. When I looked closer at it (12 mil. guardian liner), I noticed it has multiple pin pricks/tiny holes throughout out the crawl space ( 3400 square feet), damaged from the sharp , uneven gravel beneath. Could this be the possible answer to all our problems with pressure/radon and humidity????

Just to recap, we have an encapsulated crawl space (house built in 2008) with closed cell foam on foundation walls ( open cell on rim band), 12 mil vapor barrier, ducted Ultra Air Dehumidifier with radon piping installed beneath vapor barrier as a precaution. Home tested 1.0 pCi/L before encapsulation. After crawl space was encapsulated radon test came back at 10+pCi/L. We then added a Radon mitigation system ( fan) to the existing radon piping installed beneath the vapor barrier. Immediately after the radon system was operating we noticed a significant increase in indoor humidity. In addition, the dehumidifier in the crawl space was now working/operating 24-7. We realized that if we turned off the radon fan, the home and crawl space humidity were immediately impacted (see 2nd link below on humidity and radon fan). After determining the radon fan was directly affecting humidity, we had a blower door done with both the radon on vs off. Just as we thought, the radon fan was affected the pressure-cfm was 250 less with radon fan on. Additional sealing was done on the vapor barrier seams ( which were gaping and open in multiple areas) with mastic. We had hopes this would solve the problem.

Two months later, we did another follow up radon test, and we were at 9.3 pCi/L with the radon mitigation system running. We then had diagnostic testing done to help determine what and where the problem might be. A blower door test was done again and the results were:

2554 cfm with the radon fan on
2789 cfm with the radon fan off

In addition, pressure testing was done on the radon piping which is beneath 12 mil Guardian Liner ( vapor barrier). Pressure test revealed pressure was good at pipes nearest to start of system (fan located outside), and decreases significantly to only provide suction to the first third of radon piping.

I am at my whits end... have been told they have sealed all they can, and don't know what else to do. I am left with both a radon and humidity problem-which I think are both related, especially considering the cfm differential. The advice on here mentioned to look at the vapor barrier and sealing. The contractor seems to think the pin pricks located throughout the vapor barrier ( which is on top of sharp gravel ) is not an issue.

Apologies for the long sordid story...appreciate any thoughts/ideas??????

Thanks to all!

Asked by hotandhumid
Posted Feb 12, 2018 1:06 PM ET
Edited Feb 12, 2018 1:45 PM ET


12 Answers

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Oops..here are the links to the previous GBA posts

Humidity/Blower door:



http://www.greenbuildingadvisor.com/community/forum/gba-pro-help/104140/... again!

Thanks again!!

Answered by hotandhumid
Posted Feb 12, 2018 1:25 PM ET


If you can see "multiple tiny holes" in the vapor barrier on the floor, it's not airtight. Install a layer of sand, followed by another layer of sealed poly.

Answered by Martin Holladay
Posted Feb 12, 2018 1:47 PM ET


Thanks Martin! One question...why the sand??

Answered by hotandhumid
Posted Feb 12, 2018 1:57 PM ET


As you now know, you have excessive air leaks into the radon system. Yes, small holes in plastic sheeting contribute to this. Have they used a smoke pencil?

What is the pressure in the crawlspace (vs the house interior) and what is the current radon level in the house (since you closed the vents and presumably sealed any duct leaks)? But this is a radon workaround, not the best solution.

Answered by Jon R
Posted Feb 12, 2018 2:18 PM ET


Evidently someone decided to install "sharp gravel" under your poly. According to what you wrote, the sharp gravel left holes in your poly.

The layer of sand will protect the second layer of poly (which I advised you to install) from the sharp rocks.

Answered by Martin Holladay
Posted Feb 12, 2018 3:01 PM ET


Thank you Martin, good idea, especially since this area will need to be accessed at times ( termite, etc).

I'm not sure what the pressure in the crawl space was-didn't test. We just did a blower door on the house alone-first with radon system on, then off. The blower door test revealed: 2554 cfm with Radon fan on/ 2789 cfm with Radon fan off. A pressure test was done on radon piping in several areas-which revealed the radon system was only pulling from the first portion of the radon piping. I should probably also mention we turned off all the HVAC equipment so that wouldn't have a bearing on the blower door results. Hopefully, ducts shouldn't have affect, as we had those sealed.

The radon test was 9.3 pCi/L. I am hoping if the vapor barrier is sealed that will take care of radon fan being able to draw from below, rather than house/crawl????I was told the fan is properly sized.

Answered by hotandhumid
Posted Feb 12, 2018 3:20 PM ET


Another idea to solve problem: would it help if radon system ( piping) were above vapor barrier-so that any tiny holesin vapor barrier that may arise over time would not affect system ???

PVC piping would replace the existing corrugated piping that is beneath the vapor barrier. It would be hung from the joists in the crawl space, terminating at several points ( elbowing down) below the vapor barrier into the gravel beneath. The PVC piping would be connected to the existing Radon Fan system ( Fan tech) which is in place on the exterior of the home, and vents above the roofline.

Thoughts/ ideas much appreciated.

Answered by hotandhumid
Posted Feb 12, 2018 4:19 PM ET


The point of the radon mitigation fan is to depressurize the soil under your vapor barrier / air barrier. You don't want to depressurize your crawl space.

Answered by Martin Holladay
Posted Feb 12, 2018 4:28 PM ET
Edited Feb 12, 2018 4:29 PM ET.


i agree, I don't want to depressurize the crawl space-which is what we are currently doing and trying to correct.

We were thinking if the radon system is comprised of sealed pvc pipe above the vapor barrier-but terminating, and sucking/collecting from beneath several areas in the gravel below ( seal where it penetrates the current vapor barrier/liner), it would eliminate the current problem with the vapor barrier being compromised by small pin holes/pricks. I know you mentioned installing sand and another layer of poly to correct current issue. We were just wondering if this might be simpler fix.

However, since the vapor barrier is compromised and the blower door results confirm-would we still have the same pressure problem if the radon system is instead collecting from just a few areas vs piping running the entire length of the home beneath an unsealed vapor barrier???

I'm thinking there are no shortcuts to fix, and the only correct way to fix is what you suggested-sand and another layer of poly. Just a thought.

Answered by hotandhumid
Posted Feb 12, 2018 5:07 PM ET
Edited Feb 12, 2018 6:05 PM ET.


I've attached a few photos showing the current vapor barrier ( 12 mil poly liner) for reference so you can see the small holes I'm referring to.

vapor barrier-2.JPG vapor barrier-1.JPG vapor barrier-3.JPG
Answered by hotandhumid
Posted Feb 12, 2018 5:21 PM ET
Edited Feb 12, 2018 5:23 PM ET.


Man I hate crawlspaces. Sorry that you are being punished for attempting to do the right thing. Someone asked in your previous thread if you have had a resnet consultant out yet? Was this radon system done by a dedicated radon contractor and what about the encapsulation work? Is the holey plastic sealed to the walls and piers?

I know you were not the specifier, but has the increased accessibility of a crawlspace been worth it for you?

Answered by Brian Knight
Posted Feb 13, 2018 11:00 AM ET


We encapsulated the crawl space in order to correct humidity issues. Unfortunately, this led to another problem-radon. Oddly enough, we were at 1 pCi/L before encapsulation, and 10 pCi/L after. This led us to having the radon mitigation done ( by the same company), which led to humidity problems again. After radon mitigation we could feel hot humid air coming down from can lights, intake vents, bath fans...all upstairs ( below attic) . We were told to air seal the attic, improve bath fans and remove/replace can lights to fix this issue-which we had done. Sadly, we have spent a ton of money, time and worry over trying to fix. Worst thing is we still have radon and humidity after all this.

The company who did all the work is LEED certified and does HERS rating...also listed by our local electric company as a contractor for energy program ( although they don't certify their work).

We just want it fixed and don't know what to do.

Answered by hotandhumid
Posted Feb 13, 2018 12:50 PM ET
Edited Feb 13, 2018 1:05 PM ET.

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