GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Radon, drain tile, vapor retarder

user-6253285 | Posted in General Questions on

Hi guys, 

Here’s what I have.. 1966 rancher In northern Maryland 

Full basement with perimeter drain tile system leading to two sump pumps. One pump collecting the front section and the other collecting the rear respectively.  Planning to finish the basement with 2×4 walls. 

Home has been tested for radon and exceeds the EPA recommended level of  4 pCi/L. Our latest 3 day test was 15.6 PCI/L.  

Radon Contractor recommends a single exhaust pipe that taps into both sump pits with and upgraded fan. 

Obviously a drain tile system, due to its nature can act as a super highway for radon entering the home. The Radon contractor also recommends a Vapor barrier on the basement foundation walls that is tucked inside the exposed drainage system and Air sealed at the top. This would create the necessary vacuum effect needed for the radon system to function properly. 

To me this sounds appealing because i believe that it will also assist in keeping the basement dry. Any moisture that makes its way through the block foundation would remain behind the vapor barrier, be forced into the drainage system thus keeping basement area dry. 

My question is basically does this system make sense? 

If so, what type of vapor barrier would be best suited for my needs? 

Also, as i have intentions to finish the basement… with this system mentioned above, i would then frame with 2×4 walls and attempt to install airtight drywall. 

Is this the best way to go or am i missing something? My concern is moisture  trapped behind the wall causing problems in the future.

I also know many people line there foundation walls with rigid foam. Would standing the rigid foam off the foundation wall accomplish the same thing? 

I know this is a lot, but any help would be appreciated.

Thank you all.

Stay safe. 


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Jon_R | | #1

    Lstiburek says that plastic against concrete creates a mold risk. But the system you are putting in to control airflow should do the same for any mold odor. So it sounds fine, better than most systems (which don't address radon entering from the walls).

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Andy.

    It sounds like your contractor understands the details necessary for effective radon mitigation. One thing that you didn't mention that is important are airtight lids for the sumps. You need everything to be sealed up to keep radon out of the house and to allow the mitigation system to work. You may find this article helpful: All About Radon.

    Among the reasons to use rigid foam against a basement wall are that it is a vapor retarder, can be air sealed well, and is continuous. With a vapor retarder already in place and acting as an air barrier, I guess you could eliminate the rigid foam and go with cavity insulation in your stud walls, but in general, this seems like a risky basement to finish, to me.

    1. user-6253285 | | #3


      Thanks for the Response! Also congratulations on the new gig!

      Could I use rigid foam in place of a vapor barrier that the radon contractor recommends ?

      Or would the continuous rigid foam be installed after the vapor barrier?

      Also, If i use a regular vapor barrier on the foundation wall tucked behind the French drain like the contractor recommends.. what would be the best wall assembly for finishing off the basement? It sounds like cavity insulation would be risky.

      Thank you!

      1. GBA Editor
        Brian Pontolilo | | #4

        Hi Andy,

        I'm having a tough time imagining how the contractor plans to tuck the poly into the perimeter drain, but I'm assuming he has a plan to seal it to the slab or an existing sub-slab vapor retarder. Is that the case?

        Also, basements with open perimeter drains often have them because the walls leak water. Is that your situation?

  3. Expert Member
    Peter Engle | | #5

    If you use foil-faced foam, the foil is an effective air and vapor retarder. Seal the foam to the top of the wall when you spray foam the band joists, and seal the foam to the slab with canned foam. Any water seeping through the foundation will be directed to the perimeter drains by running down between the foam and the wall. Same thing with water vapor and/or radon seeping through the walls. The walls will be "bumpy" enough to allow for gas and bulk water migration down to the drains.

    This approach get you air, water, water vapor, radon and thermal management in one layer and eliminates the potential for condensation on the front side of the foam. You can then add some fluffy insulation to the stud cavities if you want. I'd just use thick enough foam to meet your local R-value requirement for foundation walls. If you use a sheet of poly as an air/vapor barrier, you run the risk of condensation on the face of the poly in warmer weather and you've still got to install foam to control that.

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi Peter,

      I've always been told that if you have leaky basement walls, water leaky, that it's a bad idea to put rigid foam against them without first using a dimple mat or textured membrane. Have you done this successfully before?

      1. user-6253285 | | #7


        Thank you both!

        My original thought was similar to what you said Peter..rigid foam over the foundation wall and that would take care of water and radon in one layer. However, as Brian stated I was thinking of using a dimple mat or standing the foam off the wall a little to allow for water/radon to freely reach the drainage system.

        To answer your question Brian, ( which i should have mentioned earlier ) my basement is relatively dry now. The previous homeowner had water intrusion a few yrs back and had it mitigated with a French drain system installed. (I believe the water intrusion was mostly due to exterior grading issues) which i have addressed. The basement is currently unfinished and has a dehumidifier running 24/7.

        I’m glad to know that the poly isn’t the best option, it made me a little nervous, hence why i brought the question here.

        I’d be interested to know if using a dimple mat or a textured membrane would be better than attaching foam straight to foundation wall. It sounds like either way would work.

        1. GBA Editor
          Brian Pontolilo | | #10

          If you have the budget, I'd spring for the dimple mat. If not, refer to Peter's advice. Just keep in mind that air sealing is the key to keeping radon out of the house and keeping warm indoor air away from the concrete where it can potentially condense.

      2. Expert Member
        Peter Engle | | #8


        Yes, I have, though not by choice. My first option for every basement is to fix the water problems first, preferably on the exterior. If you need an interior solution, I always try to start with dimple membrane, then foam. But budget and homeowner choices often take precedence and the $1k or so for dimple membrane would go so far towards a bigger TV.....

        If the water problems are not frequent/huge, I don't have too much heartburn using just the foam. I usually pin the foam to the walls with plastic anchors, and the bumps really do leave plenty of drainage space.

        1. GBA Editor
          Brian Pontolilo | | #9

          Thanks Peter. Good to know.

          1. user-6253285 | | #11


            Thank you both for the insight. I’ll keep you posted on what i come up with.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |