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Community and Q&A

Vapor Retarder for Basement Walls

DustinReed | Posted in General Questions on

Hello, I’m in zone 5 upstate NY. I do get water on the inside of my block foundation interior basement walls after heavy rains which is directed to my whole basement perimeter drain system (plastic tray impeded in concrete, directed to a sump pump). Short of excavating outside of the basement wall, can I stop vapor intrusion and direct water into my drain system with a vapor barrier? Ive seen a few sources, Martin Holladay being one of them, saying you can’t use polyethylene against basement wall but you can use a vapor impermeable insulation (rigid foam board, closed cell spray foam) on the inside of basement walls. I don’t understand why there is a difference besides the ability for moisture to condense on a cold wall with a vapor barrier- but isn’t the basement wall damp anyway? I obviously don’t want to encourage a mold growth situation.
I was hoping to stop vapor permeance as a first step and maybe eventually insulate.
Thanks for your help!

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  1. Expert Member
    KOHTA UENO | | #1

    Using interior drainage on a leaky foundation wall (as you suggested) is a detail that is commonly done--this is what Joe Lstiburek (Principal of BSC) shows as a typical remediation of rubble stone/field stone walls:

    BSI-041: Rubble Foundations

    The first layer on the wall is a water control layer/drainage layer: it could be roofing membrane, dimple mat drainage sheet, etc... it directs water into your perimeter drainage system. Then, spray foam to cover it; this deals with potential condensation of interior air on cool surfaces of the drainage sheet/dimple mat.

    Leaving the dimple mat (or polyethylene, or roofing membrane) exposed to interior conditions (omitting the spray foam) will increase risks of condensation at the top of the wall in winter (cold wall surface), and at the bottom of the wall in spring/summer (cool ground, humid air).

    The recommendation against vapor barriers in below grade wall assemblies in general is due to trapping moisture in a moisture-vulnerable wall assembly, like a wood stud frame wall with batt insulation. More background on that front here:

    BSD-103: Understanding Basements

    1. Jon_R | | #2

      > The recommendation against vapor barriers in below grade wall assemblies in general is due to trapping moisture in a moisture-vulnerable wall assembly
      > spray foam ... polyethylene

      I'm curious, why in the recent BSI-125 (Fig 3 and related text) and here and in BSD-103 is there clear advice to not make the foam a vapor barrier, even when the remainder of the assembly doesn't trap moisture? Isn't "polyethylene + spray foam" contrary to this?

      I expect that taped EPS against the concrete could direct bulk water down to the drain and be consistent with BSI-125's "approximately 1 perm" (see Fig 4). So why not follow the "dry inward" advice?

      1. Expert Member
        KOHTA UENO | | #6

        Short Version: Using materials below 1 perm on the interior of a basement wall isn't the end of the world, and in fact, it is the recommended solution if you are doing an interior drainage foundation system.

        Longer Version: I'm tempted to preface this comment with the Pirates of the Carribean line: “The code is more what you'd call 'guidelines' than actual rules.”

        First off: the exact wording from BSI-125 is:

        Figure 3 illustrates a concrete basement foundation with interior spray polyurethane foam insulation. Polyurethane spray foam insulation can be directly applied to the interior of concrete foundation walls. High density closed cell spray foam should be used. The recommended permeance of the interior spray foam insulation layer is approximately 1 perm.

        The goal behind this concept is: if you can allow a slight avenue of inward drying at the concrete wall, it probably moves things in a safer direction.

        However, all of the assemblies shown in BSI-041: Rubble Foundations (linked above) are trying to stop bulk water leakage through the wall. In those cases, we are using some type of waterproof membrane--roofing membrane, dimple mat, adhered membrane, etc.--all of which are Class I vapor barriers. So the foundation wall gets wet, and stays wet... which is still okay, if you see below.

        Martin Holladay had a discussion with Joe about this 'basement wall drying' back in 2012, cited below:

        Joe Lstiburek Discusses Basement Insulation and Vapor Retarders

        In November 2010, I wrote, “It is illogical and unnecessary to encourage a concrete wall in contact with the damp soil to dry inward.” Similarly, in December 2010, I wrote, “The experts at the Building Science Corp. believe that a concrete basement wall should be able to dry to the interior, but I think that the importance of this detail is exaggerated. After all, the soil on the outside of your basement wall is usually damp. If you encourage a basement wall to dry to the interior, you are inviting a lot of moisture into your home.”

        The question popped up again in March 2011, when I wrote, “Some experts come up with a logical explanation for why it’s useful for basement walls to be able to dry to the interior, but I don’t buy it. … Masonry and concrete walls aren’t hurt by a little moisture.”

        Here’s the news story: Lstiburek now says that the advice he gave in the Builder’s Guide was wrong. 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.”

        Lstiburek’s former recommendation to use vapor-permeable foam was due to a concern that imperfections in the foam installation might allow some warm interior air to contact the cold concrete, leading to condensation. However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall.

        1. DustinReed | | #7

          Thank you for detailed response, Kohta. Can a basement be treated the same as an encapsulated crawlspace? There seems to be a lot of confusion about moisture and vapor barrier with basements but with crawlspace the advice is pretty clear.

  2. DustinReed | | #3

    Thank you all for replying. So it looks like you need at least 1 perm in any interior basement wall assembly? Is it possible to add too much rigid foam or closed cell spray foam and not meet that 1 perm criteria? Would this be true for crawlspace as well?

  3. maine_tyler | | #4

    This issue needs a thorough hashing with clarifications from all the experts that have made (seemingly) contradictory statements. The only thing apparent to a casual reader of GBA is that no one agrees—including experts.

    Maybe GBA could contact a myriad of BS experts and try to shed some light on this ever present question of basement interior-material vapor permeance, and the related issues (mold?).

    Since the answer is probably 'it depends' we need clarity on exactly what it depends on.

    Closed cell foam adhered to poly would have no air space between the poly and the solid air barrier of foam, which leads me to think it passes the test on that side. What about the air space / drainage plane behind the poly? Seems fine doesn't it, so long as no air passes to the interior?

    So can well taped XPS over poly, or foil faced polyiso even, also pass the test or does that interstitial air space fundamentally differ from the drainage plane behind spray foam? Concerns with air exchange? A better environment for mold?

    1. DustinReed | | #5

      You are describing all my same questions well, thank you. I was concerned about mold from the start- is there a solution that doesn't let mold grow? Is conpletely sealing of an air source the solution? What conditions keep the moist dirt from growing mold on the outside of the basement wall that can't be replicated on the inside?

  4. zac2944 | | #8

    Ddreed, I'm also in upstate NY (Rochester) and just posted a similar situation:

    I'm not sure if mold will grown between moist block wall and poly sheeting/XPS, but I've got everything sealed up tight enough that it shouldn't matter.

    1. DustinReed | | #9

      Zac, thank you very much for the insight. I'm near Albany, btw. So I have a drain system like yours but not quite as nice. The plastic lip that comes up is flared out more. How essential is it to get the vapor barrier and foam over that drain tile all the way down to the floor? Do you think I could tuck the vapor barrier behind the plastic lip and then end insulation right above it, seeing as tho I can't go all the way to the floor. Or I could cut the lip off since the vapor barrier will force any water coming through downward into drain.

      Also, do you feel confident that if all ends of vapor barrier are sealed then there is no chance for mold? Or it won't matter because it's sealed

      1. Jon_R | | #10

        See Akos' response for a case of mold growing behind a vapor barrier.

        From below, one can infer what Joe L thinks about the idea of non-fully adhered vapor barriers inside of basement concrete being harmless because the smell won't make it into the interior. SPF may change this.

        One can achieve a little more consistency on the issue with something like "approximately 1 perm of inward drying is best, but lower is probably OK".

        1. DustinReed | | #11

          Thank you, Jon. It looks like only fully adhered methods are recommended. Is there any other options besides closed cell spray foam?

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