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Community and Q&A

Drylock or similar before EPS or XPS Rigid Panels in wall basement

AngeloDs | Posted in Energy Efficiency and Durability on

Good morning,
I’m trying to understand what’s the best way to insulate the basement walls before finishing the basement.
My walls look dry, never had running water problems. some effloresce here and there but nothing more.
Would it make sense to paint with a sealer before applying the rigid foam panels?
What’s the best practice? In some articles they suggest to do it as a peace of mind in other somebody says that it doesn’t make sense. I’m really confused.
Thank you

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  1. Expert Member
    BILL WICHERS | | #1

    As long as your walls are dry, you don’t really have much to worry about. If you use any type of rigid foam that can deal with moisture on the face (EPS, XPS, foil faced polyiso), the insulation itself acts as a vapor barrier so the drylok is redundant.

    In my case where I used reclaimed roofing polyiso with a fiberglass (vapor permeable) facer, I just ran some poly between the insulation and the wall. In the crawlspace, I ran the liner up the wall to a little below the rim joist and insulated over the liner. No drylok. I have one screwy basement wall with a bump of halfway down, so I just put some poly up there. The poly protects the insulation if any water seeps through the wall, but I expect little if any moisture seepage since I watched the wall for several years before doing the insulating work.


    1. AngeloDs | | #2

      Thank you Bill,
      Yes my wall looks dry but I thought just in case the paint would give an extra layer of protection.
      Yes I'm gonna probably use EPS board with a reflective foil probably. I'm still deciding because there are no too many info about this kind of board. From the specs they look great, but if I look online almost everyone suggests XPS.

      1. capecodhaus | | #3

        Ask your doctor how to get a better nights sleep.
        He recommends dryloc and xps.

      2. Expert Member
        Dana Dorsett | | #4

        >"...but if I look online almost everyone suggests XPS."

        It's not a democracy, and like a democracy, following the online crowd doesn't lead to the most satisfactory result (just sayin'...)

        DON'T go with XPS, even though everybody and their sister (and their cousin's sister's dog) thinks of XPS as the go-to insulation for anything below grade. Consider them victims of the industry hype.

        XPS is the LEAST green insulation material R-for-R of anything in common use today, and the warrantee is only good for 90% of it's labeled R-value (read the fine print!) That's because it's higher R/inch compared to EPS of similar density is an artifact of it's climate damaging HFC blowing agents, which slowly diffuses out over the decades, eventually ending up at the same performance of EPS.

        EPS is blown with a variant of pentane, which has only 0.5% the global warming potential of the HFCs used with XPS. Better still, most of the pentane escapes the foam and is recaptured at the factory, not vented to the atmosphere, often burned for process heat. The thermal performance of EPS is stable over the decades, not dependent on blowing agents. Graphite loaded EPS has the same labeled-R of XPS, but also has a stable performance.

        The difference in 100-year greenhouse impact isn't very subtle- XPS is a comparative disaster:

        At R8 or higher EPS doesn't need a facer to be sufficiently vapor retardent against ground moisture diffusing through the foundation (it's more vapor tight than interior latex paint on drywall at those thicknesses even without the facer), but it's easier to air-seal EPS if it has a foil or plastic facer using tapes.

        1. AngeloDs | | #5

          Thank you Dana,
          Yes I would like to go with EPS for environmental reasons too. What do you think of this one:

          1. Expert Member
            BILL WICHERS | | #6

            Nothing wrong with it, but it’s not very high R value since it’s not very thick, and it’s type I EPS with the associated crumbly issue when you cut it.

            If you don’t limit yourself to box store material, you’ll have a lot more options. We all like polyiso around these parts too, Dana was actually the one to turn me on to the stuff. If you get the manufacturer seconds or reclaimed stuff you can save money, and places where it will be hidden from view is a great place to use reclaimed material!

            Note that the box stores can order in other materials if you want to keep your orders with stores that you know. I know Home Depot can order in Dow thermal, for example, which is a very high grade of polyiso.


  2. AngeloDs | | #7

    Thank you Bill,
    I never thought about Polyiso, I read somewhere that it’s not supposed to be used below grade.
    That’s why I was concentrating on XpS or EPS. But for environmental reasons EPS.
    I like the idea of the foil reflective faced EPS if you consider a very good choice.

  3. Expert Member
    BILL WICHERS | | #8

    Polyiso isn't supposed to be used IN CONTACT WITH the ground. The reason is that polyiso will absorb water if it's sitting in something moist like moist earth. Basically you can't bury polyiso in the ground.

    There is nothing wrong with using polyiso "below grade" when it's inside of a structure like a basement where it won't actually be buried in the earth. The benefits you get are better R per inch, and a greener material.

    Foil faced insulation doesn't necessarily help you depending on how you're using it. The foil facing needs to stay shiny and clean and be facing an air gap to be of benefit, and even then the benefit is pretty small. The foil facing won't hurt you though, unless you need a vapor permeable insulation since the foil facings are usually vapor barriers (there are some that are perforated to make them permeable).


    1. AngeloDs | | #9

      Thank you Bill,
      Thank you to you all I have a better understanding now.
      The foil facing thing is the one that is confusing me more. I have a feeling that Vapor retarder would be better than a foil facing that acts like a vapor barrier. Even if I’m gonna paint the walls and in the future the paint would crack or I decide not to paint probably an unfaced EPS would be better for the vapor reterder reasons.
      Sorry for all this doubts but I’m really scared to make a wrong choice.

      1. Expert Member
        Dana Dorsett | | #10

        >"The foil facing thing is the one that is confusing me more. I have a feeling that Vapor retarder would be better than a foil facing that acts like a vapor barrier. Even if I’m gonna paint the walls and in the future the paint would crack or I decide not to paint probably an unfaced EPS would be better for the vapor reterder reasons."

        Foil faced polyiso would make the interior paint (if any) LESS likely to crack, not more likely.

        Foil facers more completely blocks moisture migration from the foundation wall into the finished wall. That could in some cases raise the moisture content of the top of the foundation to where it might affect the foundation sill, but the difference in risk between 1 perm EPS and 0.05 perm foil is almost academic, since it's the lower TEMPERATURE at the foundation sill due to the interior side insulation (no matter what insulation is used) that is the primary risk factor to the foundation sill in an otherwise dry looking foundation, assuming at least some above grade exposure on the exterior.

        1. AngeloDs | | #11

          I’m thinking that at this point the best way to go would be
          Not painting the wall with any sealer
          Using EPS unfaced
          Metal or Wood frame on top and
          Unfaced drywall.
          In this way if any moisture will come from the wall the EPS will let it dry and air from inside the basement will not condensate on the wall.
          Am I right?

  4. Expert Member
    Dana Dorsett | | #12

    >"In this way if any moisture will come from the wall the EPS will let it dry and air from inside the basement will not condensate on the wall.
    Am I right?"


    The verb you're looking for is "condense". But humid basement air won't condense on the wall- it will take up the moisture IN the wall as adsorb, not liquid. And that's just fine- concrete & concrete block are highly tolerant of moisture (even submersion), which is why the are used as foundation materials, which are constantly subjected to ground moisture.

    Most of the year the moisture migration direction would be coming out of the foundation wall, toward the basement, not the other way around, and it's just fine block that moisture drive with low permeance foam.

    To avoid interior moisture drives from reaching a cold foundation wall (if for some unknown reason that is deemed desirable) a foil or plastic facer does a better job than unfaced foam. The only common EPS with facers is usually Type-I, which needs the facer to limit damage when handling. It would be totally fine in 99 out of 100 basement wall applications, and it's easier to air seal well using tapes on fully bonded facers than with raw EPS.

    It's totally fine to install polyiso (fiber faced or foil faced) on the interior side of foundation walls, but the cut bottom edge should not rest on the slab. It's not usually a good idea to install polyiso on the exterior side of the foundation, between the foundation and soil.

    It's worth applying a sealer to the wall, which cuts down on efflorescence and total moisture transfer, but not water vapor. Something like Drylok is fine- it's vapor permeable, but severely limits capillary draw.

    BTW: What is "unfaced drywall"? Exterior grade fiberglass faced gypsum board is extremely moisture tolerant, but in an otherwise dry basement any drywall will work.

    Metal framing passing through the insulation layer is a huge thermal bridge. The thermal conductivity of steel is hundreds of times higher than wood, so it doesn't take a very thick steel to severely undercut the performance of the insulation.

    1. AngeloDs | | #13

      Thank you Dana to spend the time explaining me everything. I really appreciate it.
      And now I have a better understanding on the sealer too. I thought they completely isolate the wall, I didn’t realize they are not vapor barrier. So now I feel better applying it to the wall.
      And then I would like probably to go with faced EPS on top of it.
      After the framing No paper faced Gypsum board (this is the one I meant before, sorry I did t remember the name)
      MY basement is made with cinder blocks by the way.
      Do you think this is the way to go?

  5. Jon_R | | #14

    Below suggests that basements and conditioned crawlspaces are similar and that faced foam is best avoided.

    It appears to be a recent reaffirmation of what was published here:

    Others came to similar conclusions about inward drying being beneficial:

    1. AngeloDs | | #15

      Thank you Jon,
      I just finish to read everything. I thought I was almost there with the best way to go but not yet.
      I looked everywhere on buildingscience and they don't mention anywhere about painting or not with a sealer first?!?

    2. Expert Member
      NICK KEENAN | | #17

      Lstiburek has since walked that advice back:
      See: , see "News Story #2."

      Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”

  6. PLIERS | | #16

    So what happened in this situation. I’m at the same road block here. About to use 2inches of rigid foam on foundations walls, do I need a sealer on bare concrete or is the insulation enough. I’m using 2in rigid foam, 1x4 furring strips, and paperless drywall

  7. Expert Member
    NICK KEENAN | | #18

    If the seams in the foam are taped and the edges are sealed it should be sufficient to prevent water vapor from the concrete from entering the living space, and it should be sufficient to prevent warm humid air from the interior coming in contact with a cool wall and causing condensation.

    If liquid water is penetrating the wall, the foam may or may not be sufficient to direct it someplace harmless. A sealer will do a better job for that specific issue, but there is no guarantee it will be successful.

  8. PLIERS | | #19

    It sounds like I could skip the sealer in my situation, when you say liquid water penetrating the wall you mean actual water coming down correct? I don't have any signs of liquid water at most maybe water vapor and/or condensation.

  9. Expert Member
    BILL WICHERS | | #20

    The general rule is that you only need a sealer if you have bulk water (seepage) issues to deal with prior to insulation. You're really better off correcting bulk water problems from the EXTERIOR side of the wall though. In my case, I had issues with bulk water after rains. The solution? Put in better rain gutters, and very long (full 10 foot sticks) of downspout to carry the water out away from the walls. My ultimate solution will be to put in a storm drain system, but I haven't gotten to that yet (and won't until PVC prices get back closer to normal!).

    You want the exterior ground to slope away from the walls, and you want rain gutters to prevent the roof from acting as a water collector that concentrates rainfall right next to your foundation. Doing those two things will solve most "water comes through the wall" issues. Drylok is more of a band aide than a real solution.


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