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Irregular Basement, Interior Perimeter Drain, and Radon Mitigation

doba1400 | Posted in General Questions on

I am planning to finish the basement in my home and I would like some input about how to tackle adding insulation, the existing interior perimeter drain, and radon.  The home was built in 1930, no exterior insulation or vapor barrier, climate zone 4A (Westchester county, NY).  I have radon readings of about 4.0 pci/L.  There are rock outcroppings in the basement – about 20 linear feet where the interior drain and dimple mat follows the rock outcroppings (not the basement walls).  See attached picture.

Does it make sense at all to try to seal the dimple mat where I can to prevent radon from coming into the basement, even though I can’t seal the dimple mat at the rock outcroppings?  And if so, how to seal the dimple mat to the EPS?  Or instead of sealing the dimple mat to deal with the radon, perhaps it’s better to actively or passively venting the perimeter drain with a pipe to the roof?  I’m looking for the simplest/most economical and effective solution.

Thank you!

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  1. krom | | #1

    AFIK radon enters from below the floor, not the walls.
    Most abatement systems depressurize the area below the basement slab.
    This is often done by cutting the floor putting pipe and gravel in trenches dug, running all the pipes to a main stack that goes through the roof, with a fan.

    1. Jon_R | | #2

      Radon enters both ways, but floor mitigation (with a powered fan) is much more common and will probably be sufficient.

      Spray foam can be used in difficult air sealing situations.

      1. doba1400 | | #4

        Thanks Jon R. If you can take a look at the photo, you'll see some rock outcroppings in the corner of my basement. I don't really want to spray foam the dimple map to the rock, as it'll cut off a pathway for water to get to the interior drain. Is there any SIMPLE solution here to air seal the dimple mat to the foundation wall while maintaining a way for water to get to the interior drain? Or is a radon mitigation system connected to the interior drain a better way of dealing with the radon, over trying to air seal the dimple mat? Thanks!

        1. Expert Member
          Michael Maines | | #5

          You can run a vapor retarding membrane over the top of the dimple mat and seal it to the wall with tape. I like Siga Fentrim for this type of thing. There are heavy-duty white vapor retarders available that are meant to be left exposed.

          1. doba1400 | | #11

            Thanks Michael. So this vapor retarder can also just be draped over the rock outcropping as well and I would run it up to the top of the foundation wall? Could you share a specific vapor retarder you recommend? And I assume it’s not a vapor barrier correct? Thanks!

          2. Expert Member
            Michael Maines | | #13

            Doba, these are two brands I spec regularly:

            Yes, just drape it over whatever you have. I've seen it done in many irregular crawlspaces.

            I have not installed either one myself but they are both reputable brands. I try not to use the term vapor barrier, since even these products allow some vapor through, but for sub-slab and foil products, they are essentially barriers.

          3. doba1400 | | #18

            Michael Maines,

            Reading back on your response with puttting polyE on the concrete walls, between the concrete foundation walls and the insulation - Martin Holladay seems to disagree, saying polyE doesn't belong anywhere inside basement walls. Or was this negated by Lstiburek's admission that the basement walls no longer need to "dry to the interior"? Both articles are below.



    2. doba1400 | | #3

      Hi Trevor,
      The dimple mat runs up the foundation walls 3" and is connected to the perimeter interior drain. The dimple mat creates a pathway for water to drip down to the interior drain, but also creates a pathway for radon below the slab to travel above the slab to the basement space.

  2. _jt | | #6

    I have about the same levels. Adding a radon fan off thr sealed sump pump cuts it down about 30%. Opening a window cuts it about 85%. I suspect good erv or hrv would do the same. That is probably my next step.

    Try using a radonaway meter to watch the levels. Airthings also has a good real time meter.

    1. maine_tyler | | #7

      Jay, do you know what the aggregate is like beneath your slab? Is there an entire perimeter drain hooked to the sump? I'm surprised there's only a 30% reduction.

      1. _jt | | #9

        1920 home with a fieldstone foundation, No perimeter drain hooked into the sump - so its all a bit if a science experiment. There does seem to be gravel beneath the slab. But the slab is old and the stone foundation is porous.

        RadonEye says I am between 2-3 but then spike much higher after rain storms. Opening the window after a rainstorm seems to be the most efficient way to clear the build up. (Pretty much instantaneous)

        I also have an air things, I highly recommend the time resolution in the RadonEye, you can see when things happen. But the app is not intuitive nor does it store data in a useful way. The readings match up between the two, and I seem to be below 1 on my main floor. I also did a lab test and received basically the same results.

        Not Radonaway - that's the fan - I meant RadonEye!

    2. doba1400 | | #8

      Hi Jay,
      Did you add the radon mitigation pipe to your sump pump and you're still getting radon levels around 4.0 pci/L so you're thinking of going with an HRV/ERV next? I haven't done anything for my radon issues yet and wonder if it's best to try the HRV first? Seems like it would be visually less obtrusive than having a pipe go up the side of my house, but probably more operational costs? Any insight you can provide is appreciated!
      Yes, I've been using an Airthings meter for the past month or so and got the higher radon readings since it's gotten colder here. I also verified with a short term lab test.

      1. _jt | | #10

        Just for clarity I had levels around 3.0-4.0 before a radon mitigation pipe and it dropped to 2.0-3.0 with one. I don't really have a need to go lower. I would be interested to know if anyone has real world ERV experience. I've read they can drop levels about 50%.

  3. Expert Member
    Peter Engle | | #12

    doba, Michael's suggestion is a good one. Any impermeable plastic film on the walls will not only reduce radon entry, but also moisture vapor passing through the walls. And yes, it is generally a true vapor barrier. It should be sealed to the wall at the top of the foundation. At the bottom, it should be tucked behind the dimple mat so that any water leaking through the wall goes into the space behind the mat. You can seal the seam between the two with caulk to make it airtight. Once this is all done, a suction system should draw a vacuum under all of the membranes and this should be pretty effective for radon reduction. I would run the plastic over the rock outcrop and tuck it behind the dimple mat just like the walls.

    ERV/HRV systems are the newest tool in the radon mitigator's toolkit. One of the labs I used to work with is moving more and more towards using an ERV as a primary radon remediation, not just a backup if a suction system doesn't work. The fan on the ERV probably uses about the same amount of energy as the radon fan does, so energy costs will probably be similar though there will be some other energy costs with both. The ERV also provides fresh air to the basement area and that has some other benefits as well. The ERV may be a somewhat more expensive solution.

  4. doba1400 | | #14

    Thank you Peter and Michael for the helpful replies! I’m trying to decide between the 2 strategies below before partially finishing my basement. I’m leaning towards Option 1 as it will also supply fresh air, though I am a little concerned that if it doesn’t work in reducing radon, it would be very difficult to open the walls back up again and air seal the perimeter drain. I very much appreciate your feedback, what do you think?
    Option 1:
    Leave the dimple mat that runs up the foundation wall by 3” unsealed. Install 2”EPS above the dimple mat on the foundation walls and metal stud walls+1/2” green board. I’ll also be topside insulating the 1” thick mud/rat slab with 1” Wedi board (1” EPS) plus tile on the floor. I’m hoping this floor will further seal the thin slab from radon. If Radon is still an issue, add an exhaust fan (per Matin Holladay’s article below) or add an ERV or HRV. And if it’s still an issue after that, adding a radon mitigation system to the interior perimeter drain.

    Option 2
    Add the vapor retarder and tuck into the dimple mat at the bottom and at the sill plate at the top. I thought adding an interior vapor barrier anywhere was a big no-no? Perhaps I’m missing something. If I went with this strategy, how would I attach the EPS insulation to the wall without penetrating the vapor retarder?

    Thank you!

    1. maine_tyler | | #15

      Doba, don't take this as replacement for an answer from Michael or Peter, but my opinion is that you should take the opportunity now to seal the dimple mat.
      You ideally want your floor / wall corner to be continuous with insulation, so option 1 of starting the insulation above the mat isn't great anyways, unless you scab in an insulation cove or something.
      If you don't want to buy and run all that poly, you could seal the space off with just the foamboard itself. (edit: forgot about your rocks, probably wouldn't work in that case. but if you ended up leaving only a small portion of the dimple mat unsealed, IMO it's better than leaving it all unsealed.) For example, seal the bottom of the foamboard to the face of the dimple mat, then seal the top of the foam to the wall, and tape the foam.
      Don't run horizontal glue lines behind the foam, except at the top, to allow drainage. If using poly, attach foam with tapcons, etc. Don't worry about small holes in vapor barrier since it'll still be 99-odd percent effective.

      Vapor barrier rules are of the thumb. Concrete could potentially be laden with moisture. You want to BLOCK this moisture from coming to the interior, hence barrier is good. And concrete can stay wet. What you don't want is interior moisture-laden air condensing on cold surfaces and causing mold/rot, which is why poly against a concrete wall followed immediately by a wood framed studwall filled with fiberglass batts is NOT a good idea. The foam in your assembly is key to keeping potential condensing surfaces warm (note: if you insulate a stud wall interior to your foamboard, you must maintain certain ratio's of foam to interior insulation R-values).

  5. Expert Member
    Peter Engle | | #16

    If you intend to insulate with foam, I would skip the vapor barrier. Use foil-faced polyiso foam (like Dow Thermax), as the foil provides air/water/vapor barrier and also a fire-resistant surface. In fact, their Ag Board has a white textured surface that is entirely suitable as a finished surface in a rough-finished basement. Tape the seams to make the surface airtight and seal the top to the foundation with caulk or foam. The transition to the dimple mat is tougher. If you keep the foam tight to the wall, any moisture running down the back of the insulation will drain behind the dimple mat. The challenge is to seal the bottom of the foam to the top of the dimple mat in a way that is reasonably airtight. This is probably best done with a trowelable/brushable waterproofing mastic, potentially including fabric reinforcement. The various EIFS manufacturers make good products for this (Sto Gold Coat would be one). Something from the Prosoco family would also work well. The goal is to provide a continuous air/water/vapor barrier that keeps radon and moisture behind the insulation and dimple mat.

    FWIW, I doubt that all of this work will have an appreciable effect on radon levels. At best, these sorts of passive interventions sometimes reduce long-term levels by .5 pci/l or so - not a published study but my own experience. Add an ERV/HRV to the rest of the work and operate it at a slight positive pressure, and you will probably see a dramatic reduction.

    1. doba1400 | | #17

      Thanks Peter for all the great information, much appreciated.

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