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Community and Q&A

Drying to the Interior vs. Exterior

doba1400 | Posted in GBA Pro Help on

I thought I read just about every GBA article and comment, and Lstbiburek’s articles and Builder’s Guide Cold Climate, until I just read Martin Holladay’s GBA article “How to insulate a Basement Wall” published in 2012. In this article, Martin states that the interior basement insulation does NOT need to be vapor-permeable, which contradicts many previous articles.  Is this still true?  I want confirmation because I’ve read so many things that say the basement walls and slab should dry to the interior, including Lstiburek’s Builder’s Guide, last re-printed in 2017.

How to Insulate a Basement Wall

The Lstiburek “Cold Climates” Builders Guide I bought last year is copyright 2011 and reprinted in 2017 – and it argues that the basement walls should dry to the interior.  In the article below, Lstiburek states that what’s in the Builder’s guide is wrong (“New Story #2):

Joe Lstiburek Discusses Basement Insulation and Vapor Retarders

Questions:
1.  Does this mean I can use any thickness/perm interior basement insulation?  Foil faced is okay?
2.  Lstiburek’s Builder’s guide advises against using impermeable interior finishes such as vinyl wall coverings and vinyl flooring using the argument that the slab and walls need to dry to the interior.  Since this is no longer the case, is it acceptable to use vinyl flooring (over insulation)?
3.  I’m in climate zone 4a (just north of NYC) in a 90-year-old home w/o exterior waterproofing and w/o a capillary break at the sill plate (not pressure-treated).  At the rim joists, I am adding 2″ rigid insulation.  I was going to avoid wrapping the rigid insulation on top of the sill plate, to allow the sill plate to “dry to the interior” since there’s no capillary break.  Is this a good idea?  Or can the sill plate dry to the exterior (vinyl siding over 1/4″ xps)?
4.  I have an interior french drain.   Is this the latest recommended detail for finishing a basement?
https://www.buildingscience.com/documents/building-science-insights/bsi-110-keeping-water-out-basements

I was just about to pull the trigger on finishing my basement when I read that drying to the interior is no longer the science.  This opened up new questions.  Thank you!

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    Repost from another thread:

    Never let the concrete breathe. Ever. Nothing good can come from that. You want the moisture that is in the concrete and the dirt behind and below it to stay there. Concrete is completely unharmed by moisture. Any moisture that is in the concrete will do a lot more harm in the basement than it will stuck in the concrete.

    So you want a vapor barrier lining the entire concrete of your basement. But this doesn't have to be elaborate, a sheet of plastic under your finish will do. If you have no liquid water intrusion, and a vapor barrier, any moisture that gets into your finished wall and floors will be coming from the interior -- primarily from humid air condensing. Let me repeat that, because it's important: If you have no liquid water intrusion, and a vapor barrier, any moisture that gets into your finished wall and floors will be coming from the interior.

    There are a couple of strategies to keep that moisture out. All of your finishes should be air sealed to prevent the flow of air. An insulation layer keeps humid air off of cool surfaces. But the best way is to keep the humidity out of the basement in the first place by conditioning the air, either with air conditioning or a dehumidifier. You should also make all of your finishes out of materials that don't absorb moisture.

    To the extent that moisture that gets into your finished wall and floor will dry, it will only dry to the interior -- because you've put vapor barriers to the entire exterior. In practice you won't see much or even any drying, and it's better to assume that there will be no drying potential and plan accordingly. The reason there will be no drying potential is that there are two things that drive drying: moisture flows from wetter to drier, and from warmer to cooler. In a basement, year-round, the interior is warmer than the basement walls. Year-round, the moisture drive from heat is toward the exterior -- which you've blocked with vapor barriers. The only way the finished walls can dry is if the wetter-to-drier drive overcomes the warmer-to-cooler drive. Which means that the basement air has to be significantly drier than the interior of the wall or floor.

    Now, do you recall what was the only source of moisture in the finished parts of a well-sealed basement? Humidity from the interior. And what's the only way a basement wall can dry? If the interior air is significantly drier than the wall. So the only scenario in which drying is possible is the one in which it isn't necessary. I'm going to say that again too, because it's important: the only scenario in which drying is possible is the one in which it isn't necessary.

    Build your basement with the assumption that drying won't be possible, so it can't be necessary.

    1. maine_tyler | | #2

      DC, who's words are those?

      1. Expert Member
        NICK KEENAN | | #3

        Mine.

        I am indebted to Joe Lstiburek who helped crystalize my thinking about vapor drive.

    2. Expert Member
      NICK KEENAN | | #5

      Let me add that when I say "never let the concrete breathe" I'm only talking about letting it breathe to the interior. It's OK to let above-ground concrete walls breathe to the exterior.

      1. Expert Member
        MALCOLM TAYLOR | | #8

        DC,

        It's small point, but talking about building assemblies "breathing" has caused an awful lot of harm. It confuses air-movement with vapour-movement. A lot of traditional builders believe walls need to "breath", by which they mean not be too airtight. You are using "breath" to mean mean vapour-open. "Breath" is probably a term best let die.

        1. Expert Member
          NICK KEENAN | | #10

          You're right. Everywhere I say "breathe" I should really say "dry."

        2. thegiz | | #11

          So how does this fit with those newer smart barriers like this one

          100 in. x 50 ft. Air Barrier with Smart Vapor Retarder

          https://www.homedepot.com/p/205920791

          Apparently is let’s the wall “breathe” with humidity. Would you want this to happen on slab or even wall? I’m in NY hot and humid summer cold winters. How would it even adjust to seasons? Having poly directly on bare concrete works great in winter, I forget where I read this but only problem you might run into is in summer months and you try to cool the room. Although I would never run my ac in the summer in the basement, I would just run a dehumidifier. The indoor temperature should be more comfortable because heat rises and you are removing humidity.

          1. Expert Member
            NICK KEENAN | | #12

            That product is for above-ground walls in cold climates. It blocks vapor drive in the winter when the drive is from inside to out, and allows it in the summer when it's outside to in. It wouldn't be appropriate in basement walls.

      2. Expert Member
        Dana Dorsett | | #22

        >"Let me add that when I say "never let the concrete breathe" I'm only talking about letting it breathe to the interior. It's OK to let above-ground concrete walls breathe to the exterior."

        That's right. Concrete doesn't need to "breathe" or dry in any direction- it does just fine when saturated wet.

        But if it can't dry at all there needs to be a good capillary break between the concrete and any susceptible materials such as wood.

    3. maine_tyler | | #13

      DC,
      Something about it did strike me as 'Joe-esque' in style. I was just curious because you mentioned it was a repost.

      >"An insulation layer keeps humid air off of cool surfaces. But the best way is to keep the humidity out of the basement in the first place by conditioning the air"

      I might be misreading your point here, but I would caution that one is not a replacement for the other in all circumstances--you may be able to tweak one based on the other however. 'Normal' interior basement humidity levels could still load up a wall with liquid moisture if allowed to contact sufficiently cold concrete. In other words, don't build a poly diaper wall. Sufficiently thick vapor impermeable insulation against the concrete serves a role that active conditioning may not serve as reliably or efficiency.

      >"the only scenario in which drying is possible is the one in which it isn't necessary"

      In this case I would again just caution that one should not build a poly diaper wall (poly sandwich with wood and fluffies on the inside). :)
      Control of RH within the wall assembly is the name of the game, and this can be done by controlling absolute humidity of the basement and/or by adjusting the permeability and thermal profile of the wall. It may be both, but most certainly will be the latter in accordance with the former.

      I suspect many basements oscillate in their environmental conditions enough that inward drying might occur at times (though certainly not simultaneously with the wetting).

      1. Expert Member
        NICK KEENAN | | #14

        Thanks Tyler. I am anti-fluff in basements. Too easy for it to get wet, too hard for it to dry.

        >Control of RH within the wall assembly is the name of the game, and this can be done by controlling absolute humidity of the basement and/or by adjusting the permeability and thermal profile of the wall. It may be both, but most certainly will be the latter in accordance with the former.

        I would quibble and say control of the dew point within the wall is the name of the game. And yeah, you can do that either through the construction of the wall or by managing the interior climate. Either way can fail. I wouldn't rely on either on its own.

        1. maine_tyler | | #15

          DC,
          I assumed as much re your position on fluffy stuff, but I worried one could misconstrue your analysis and think the diaper wall was acceptable if they took things out of context. (No one ever does that here though ;)

          Re dewpoint vs RH:
          Interestingly, I chose to use RH instead of dewpoint intentionally, but I understand the quibble.

          Dewpoint certainly is what's always discussed in terms of wall safety. I chose RH to give nod to the hygroscopic characteristic of wood and the relevance of the RH gradient. Moisture uptake can create a fungus party without ever having been touched by 'dew'. I do wonder sometimes why dewpoint is the only point to be strictly discussed, but I assume it's for simplicity.

          Dewpoint is based on the saturation curve, which is merely an upper bound to the RH curves. (psychro chart)

          Dewpoint, of course, is a temperature. So if dealing with a steady state, dewpoint is also a physical location in the wall. RH is a percentage, but one could as easily ascribe a temperature to any RH value for a given absolute humidity. So it's also a physical location if we pick a specific RH. It's sensible to pick 100 since it's a limit, but for hygroscopic materials, the gradient beneath that limit is also relevant.

          1. Expert Member
            MALCOLM TAYLOR | | #16

            Tyler,

            I'm not advocating for it, but the standard way of insulating concrete crawlspace walls here are those batts encased in plastic, and surprisingly to me anyway, they seem to work fine.

          2. maine_tyler | | #17

            Malcolm,
            Yeah, I suppose reality and theory don't always align. It's likely a matter of risk and covering for a large situation variance when it comes to building science recommendations.

            I can't claim to be familiar with the method you describe. Are they enclosing any wood within plastic? I would feel less squeamish about fiber-glass wrapped in plastic than a wood wall wrapped within a continuous sheet of it.

          3. Expert Member
            MALCOLM TAYLOR | | #18

            Tyler,

            The batts come encased in a vinyl bag you just staple to the sill-plate above. I expected to find mold on the concrete when you move them, but haven't found any or heard it's a common problem - which is quite strange considering your cat would grow mold here if it sat still for long.

  2. Expert Member
    NICK KEENAN | | #4

    I'll add that basements are different from above-ground exterior walls, where the expectation is that the warmer side will be the wetter side in all weather. So you put a vapor barrier on the side that is more commonly the warm side -- the inside in cold climates, the outside in hot climates -- and build the wall with the expectation that it will dry to the cold side.

    Rim joists should be treated like exterior walls -- vapor barrier on the warm side, dry toward the cool side.

    1. qofmiwok | | #31

      How in the world will it dry to the outside if the outside is covered with a waterproof membrane? I've never seen one of those that is vapor permeable.

    2. andyfrog | | #58

      " So you put a vapor barrier on the side that is more commonly the warm side -- the inside in cold climates, the outside in hot climates -- and build the wall with the expectation that it will dry to the cold side. "

      In a cold climate, would there be an appreciable difference of an exterior facing WRB of 4-11, 29, 38, or 68 perms assuming your interior air barrier and vapor control layer was taped plywood (4-11 perms)?

  3. thegiz | | #6

    I also have an old 100+year old basement which means most def you have no poly or insulation in your slab unless work was done. So technically you can apply a 6mm poly directly to bare concrete, if there is anything on it you would need to remove it because that’s how mold can grow. You wouldn’t want anything drying to the interior and then being trapped by something like vinyl. Same concept for why they use to not recommended vinyl so air could escape. Same reason why you would still not want a vapor barrier underneath your drywall trapping all your stud walls in moisture. Basically keeping vapor out of entering from the concrete. Overall though I think an air barrier is more effective than a vapor barrier

    1. Expert Member
      Deleted | | #9

      Deleted

  4. Deleted | | #7

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  5. doba1400 | | #19

    Thanks everyone for the conversation.

    To return to my original questions, it sounds like the consensus is that Lstiburek used to advise to "dry to the interior", but now the direction is to stop moisture from coming into the basement in the first place. Since this is the case, does this mean there's no longer a limit to the thickness of insulation on the basement wall? If I'm using EPS or XPS taped at the seams, this would serve as an air barrier itself, no?

    1. Expert Member
      NICK KEENAN | | #20

      I would agree with that.

    2. Expert Member
      Dana Dorsett | | #23

      >"If I'm using EPS or XPS taped at the seams, this would serve as an air barrier itself, no?"

      Rigid foam shrinks over time and isn't a very good primary air barrier, but yes, tape the seams.

      But DON'T use XPS, easily the least-green insulation material in common use today:

      https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

      If there isn't much exterior exposure and no capillary break between the foundation and foundation sill, using 1-1.5" of UNFACED EPS and an UNFACED batt insulated studwall tight to the wall foam may be more moisture-safe overall. Even at 1.5" most Type-II EPS is running about 2 perms- that's about twice as vapor-tight as interior latex on drywall, but still a reasonable drying path to moderate the moisture content of the concrete to protect the foundation sill.

      If the basement has a history of seeping (liquid) water, install a dimple mat between the foam board & foundation to let bulk water find your perimeter drain.

      Installing an inch of EPS under the bottom plates of any studwalls provides a reasonable capillary break, and keeps the bottom plate temperature above the summertime indoor air dew point.

      1. maine_tyler | | #26

        >"still a reasonable drying path to moderate the moisture content of the concrete to protect the foundation sill."

        Dana, are you thinking mostly of summer drying? Regardless of the exact permeance of the insulation/wall layers, there has to be decreasing vapor pressure from the concrete surface towards the interior, correct?

        If the concrete is WET and WARM (exposed to a heated interior), then certainly it can dry inward (both being quite warm, but one being much wetter, there is an obvious inward drive). The heat from the interior lifts the water molecules into a higher energy state, off the concrete.

        But if the concrete is WET and COLD due to the insulation, how likely are the conditions that would create a higher to lower VP gradient from outside to in? Perhaps the above grade portion in summer? That is the portion closest to the sill... so I could see the value in that maybe.

        1. maine_tyler | | #27

          It'd be interesting to measure actual conditions and see how often the potential for drying exists.

          If I'm thinking about this correctly, looking at a psy chart: concrete saturated at 35 degrees F, and an interior basement temperature of 60 F, inward drying can occur at interior RH levels below roughly 40% (just drawing the horizontal line).
          Ground temps won't get down to 35, but strangely I'm measuring temps not much above that on my basement walls (uninsulated portion below spray foam top) perhaps due to conduction through the concrete, and a large exposed above grade.

        2. Expert Member
          Dana Dorsett | | #39

          >" Regardless of the exact permeance of the insulation/wall layers, there has to be decreasing vapor pressure from the concrete surface towards the interior, correct?"

          Not correct.
          If the temperature of the below-grade soil is 50F and the entrained soil gases at nearly 100% RH, whenever the indoor air's dew point is below 50F the vapor drive is toward the interior. When the dew point of the indoor air is above 50F the vapor drive is toward the exterior.

          The boundary zone just below grade will vary quite a bit in temperature seasonally, but in summer if the indoor air in the basement is kept at a dew point lower than the average temp of the foundation, there will be some drying toward the interior.

          1. maine_tyler | | #44

            Dana,

            I don't see how what you said is any different than what I said.

            If one reads 'vapor pressure' to mean 'equilibrium vapor pressure' (which admittedly is probably the more common interpretation?), than you would be right that my statement was incorrect. EVP only depends on temperature for a given substance.

            I used vapor pressure to mean 'partial vapor pressure' i.e. actual vapor content. Does that clear it up, or am I missing something else?

            Here's an example of a document that also uses 'vapor pressure' as shorthand for 'partial vapor pressure.' Though perhaps that is also in bad form. I really don't know the convention on this.
            https://www.bdcuniversity.com/sites/sgc-university/files/AIA_BDC_April2016.pdf

          2. Expert Member
            NICK KEENAN | | #45

            Moisture transport between dissimilar materials is a complicated subject, you can't just treat the moisture as entrained gas.

            Just as a for-instance, check out this page of equilibrium moisture content for wood at different temperatures and relative humidity:
            https://www.woodworkerssource.com/wood-moisture-content.html

            Note that the values are given as a percentage of weight; i.e., they are absolute and not relative moisture contents. The saturation point of wood is 28% moisture content, so values could be converted to relative humidity by dividing by 28%.

            Note also that at all humidity levels, wood holds more moisture when it is cold than when it is warm. This is the exact opposite of air. At 5% atmospheric RH wood holds about 30% more moisture at 30F than at 130F, and at 98% RH it holds about 10% more.

            Note also that in a sealed environment, if the temperature drops the RH in the air will increase, which will lead to an increase in the moisture content of the wood, even though the total amount of moisture hasn't changed -- it's moved from the air to the wood. And when the temperature increases, moisture will move from the wood to the air.

            I don't know how concrete behaves, or soil, but I suspect they behave more like wood than like air.

          3. maine_tyler | | #49

            DC,

            I'm all for following one into the forest of science, but I think you may be entering Mirkwood here with that wood EMC/temperature quirk. :) (See Lord of the Rings)

            >"you can't just treat the moisture as entrained gas."
            –True, if it's not gas.

            >"saturation point of wood is 28% moisture content, so values could be converted to relative humidity by dividing by 28%"
            –This wood moisture is more accurately a liquid phase, so that conversion is not really meaningful. It's intra-cellular water (up until fiber saturation, then it's inter-cellular, but still not gaseous).

            That effect of temperature on EMC of wood is interesting (and unexpected) but it's overall quite slight and not really in contradiction to the fundamental principles of vapor drive discussed here.
            Note that the predominant effect of a temperature change in a closed environment on the EMC of wood will be the change in RH borne from that change in temperature (a point you make).
            I suspect the odd behavior shown in the relationship between a given RH and temperature has to do with interactions at the cellular level (I have no idea), but for our purposes here I think it clouds the matter. (I say this fully aware of being responsible for clouding matters on occasion ;))

            The reason wood displays 'reverse' characteristics of air is because they 'hold' moisture in different phases (two sides of the coin, if you will).

            I.e. Wood is the object the lower energy vapor molecules condense/adsorb to, while air occupies the space higher energy vapor molecules evaporate to.
            Higher temperatures lead to an overall more gaseous state; lower temperatures lead to a more liquid state. Therefore lower temperatures lead to higher EMC in wood for otherwise similar conditions (same absolute humidity).

            I agree with your points below re soil moisture reservoir, and capillary action can transport this moisture across a bare concrete wall.

            I suspect the most likely scenario in which interior basement moisture can actually dry to the exterior is if the above grade portion is exposed, in which case water can be transported via capillary to that surface and then evaporated by the above ground air movement. Perhaps some prefer above grade foundation coatings to be vapor permeable for this reason.

          4. maine_tyler | | #50

            Sorry DC, I had a realization that I didn't understand your point about dividing %EMC by fiber saturation %EMC to obtain %RH. I thought you were implying the water in the wood could be discussed as a sort of relative humidity in its own right.

            I imagine you are just pointing out that for 100% RH, the EMC is not 100% but rather around 29% EMC (approximate fiber saturation point). That makes some sense. Though note that the RH/EMC curve is not linear but curving upwards, so that conversion doesn't truly hold; though the broad point that 100%RH corresponds to the 'fiber saturation point' holds. (Once in equilibrium, there will be time lags).

            See figure 1 in this article:
            https://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange

      2. doba1400 | | #29

        Thanks Dana. On the exterior, the foundation wall rises up about 12-"18" above the grade. Is this enough exposure for the wall to dry to the exterior w/o worrying about the sill plate or trapping moisture on the interior?

        Also, if I do extend the dimple mat on the interior foundation wall fully height, then install EPS, isn't the dimple mat essentially the air and vapor barrier, therefore making the thickness of the EPS irrelevant?

        I can also just stop the dimple mat or poly and EPS short of the sill plate like in the attached sketch - what do you think?
        Thanks!

        Thanks so much.

        1. Expert Member
          NICK KEENAN | | #40

          Code requires both a capillary break between the mudsill and the foundation wall, and that the mudsill be made out of pressure treated wood. And that any wood in contact with the foundation be pressure treated. I don't think you can ever assume that the concrete will not be a source of moisture.

  6. maine_tyler | | #21

    Doba, you cannot have too thick of foam, though the argument can be made that you could have too thin of foam (the need is to keep the interior of the foam above average dewpoint). And foil faced is fine.
    Taping foam is a good idea for air control.

    Part 3 from your original question is a bit different. If you have xps on the outside of your sill and there's no capillary break, in this case I think you would be prudent to consider leaving an inward drying path.

    I can't comment too much on the vinyl floor, but would ask what it's being laid on? Wood?

  7. GBA Editor
    Martin Holladay | | #24

    Doba1400,
    Your question was answered in an article I wrote in 2012 ("Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.") Here is the link:
    https://www.greenbuildingadvisor.com/article/joe-lstiburek-discusses-basement-insulation-and-vapor-retarders

    The short version: Joe Lstiburek changed his mind. He admitted that his earlier statements on basement walls needing to dry to the interior were wrong.

    Click on the article link, and scroll down to this heading: "News story #2: Basement walls don’t need to dry to the interior."

    1. doba1400 | | #30

      Thank you Martin, much appreciated. I've been following your work for some time now and appreciate your response! I read your article from 2012, which is really what lead me to posting this question, since there's still a lot of information out there from Lstiburek about drying to the interior. Thank you for clarifying.

      If you could take a look a this sketch and let me know if you have any comments or thoughts please? Since I don't have a capillary break at the sill plate (old home), I want to make sure I minimize risk for mold/rot at the sill plate. Exterior insulation is minimal (1/2" of XPS) and the rim joists are already insulated with 2" xps. Also with this new understanding of the system not needing to dry to the interior, if adding the sheet of poly to the foundation wall and going with a Kerdi and porcelain tile on the floor is acceptable.

      All feedback is very much appreciated and thank you.

  8. thegiz | | #25

    So is it ok to place poly over bare concrete floors and walls. Do you have to worry about mold under the poly if concrete doesn’t need to dry to the interior? I heard you can do it and I also heard you can’t. Is there a real life example of mold growing under poly?

    1. Expert Member
      Michael Maines | | #28

      The kinds of mold that grow in or on houses need three things to thrive: a source of food, a source of moisture and temperatures between roughly 50°F and 90°F. Mold can find food pretty much anywhere so ignore that one. If you put poly over the concrete and the temperature is higher than 50° (or a little less, to be safe) there is risk of mold growth, because all of the conditions that allow mold to grow are present.

      Does mold growing under sealed poly sheeting, under other layers of flooring, hurt anything? Probably not, as long as any seams remain sealed. But like everything we have discussed relating to your basement, it's not a black or white situation--there are degrees of risk.

      1. qofmiwok | | #32

        This is actually one of the top causes of mold that remediators find when searching for "hidden mold" when a house is making people sick but a cause cannot be found. Mold is eventually found under vinyl floors or behind poly sheathing. Therefore the building mold experts recommend not putting anything that is not permeable above a concrete slab or on the walls.

        Yes, I suppose that in theory a thick enough completely impermeable perfectly taped poly layer under the slab might keep the moisture out, and then if you wait a few years for the concrete to dry before applying a non-permeable product above the slab you might be okay. But it is risky.

        1. maine_tyler | | #34

          "Therefore the building mold experts recommend not putting anything that is not permeable above a concrete slab or on the walls."

          Really? I'm finding that hard to believe. As has been discussed here, that won't really allow for that much drying anyways, at least not if it's insulated inside the concrete. There are certainly a few variables at play here though.

          The issue I have heard with sheet VB's (from the likes of Joe L.) is that it forms an air space, which can then communicate with the floor/wall assembly via air movement rather than just vapor movement. I have no idea if this is a valid concern, but it's one he apparently has. His recommendation is not to use permeable materials, but to use an adhered vapor barrier, like an epoxy coating or similar.
          One ingredient left out of the list of what molds need to grow is Oxygen. Though I think the point of the adhered VB is eliminating the air space for communication reasons (not cutting off Oxygen to the mold, but maybe that too to some degree).

          1. Expert Member
            NICK KEENAN | | #41

            I agree with this.

            If you have a sealed vapor barrier, the fact that there is mold growing on the other side of it should concern you no more than the fact that the dirt on the other side of your basement walls is chock full of mold. The problem is if the vapor barrier is poorly sealed, in which case the mold is exposed to the interior of the house. The solution is to seal the vapor barrier.

            Painted on vapor barriers are inherently sealed, but foam or sheeting can be sealed.

  9. thegiz | | #33

    So this is really confusing me. I have been posting constantly about my basement project to the point where Maines gave me a yellow card. I drilled through my slab today to put in a post and found no water lurking underneath. So basically I have a dry slab or at least liquid dry. I have been going back and forth with what to put on an unfinished basement floor and wall to keep out dampness and mold. I know eps with wood would be best but I can’t finish everything yet (planning an addition and on top of that like ceiling height) I’m limited to doing floors and sealing walls. Roger pointed me on how to waterproof my walls with thoroseal, not that I have liquid water flowing but I want to keep dampness out. On my floor I was going to put this

    7/16 in. Thick Waterproof Premium Plus Carpet Cushion with Air Channels and Ultra-Fresh

    https://www.homedepot.com/p/312019782

    With the theory that it can dry because it can breathe. Then I was convinced to keep everything sealed and not have anything permeable. I guess as Maines said not everything is black and white and each situation is individual. However there has to be some kind of consensus on wether items should be permeable or not. I do know that most basement systems like thermal dry and drybarrier delta etc. have the idea that you need a air gap.

    1. maine_tyler | | #35

      "consensus on wether items should be permeable"

      Ha! It's practically politics. Ultimately, you'll decide. Unless you hire someone and get a warranty.

    2. maine_tyler | | #36

      Hammer, see this: https://www.greenbuildingadvisor.com/question/vapor-barrier-under-basement-floor
      for some of the heavy hitters giving opinions on this matter.

      1. Expert Member
        NICK KEENAN | | #43

        I like this Joe Lstiburek article, a little off-topic because it deals with wood in basements, but it has some wisdom:
        https://www.buildingscience.com/documents/insights/bsi082-walking-the-plank

        "When we had concrete slabs on grade or basement concrete floors old guys learned to install a continuous layer of bitumen over the top of the concrete surface. The bitumen would be both a vapor barrier and a capillary break. It was sticky, icky, gooey and smelly and otherwise really good. It needed to be continuous. To keep it from getting over everything it was covered with felt paper. Sometimes it was installed in two layers. This was nothing more than a single ply or double ply built up roof installed on a concrete slab. This was an inside roof. A roof. Roofers did the install. Roofing technology."

        While a basement floor doesn't need some of the properties of a roof -- like sunlight resistance -- the basic idea of making the floor remorselessly waterproof and airtight is sound.

    3. qofmiwok | | #37

      I guess you have to remember all this building science stuff is still fairly new, the knowledge doubles every year or two, and it's complicated by the fact that it can be different in every climate zone and situation. Also specifically when it comes to mold, the vast majority of even knowledgeable people have no knowledge or understanding of how little mold it takes to make a person with the right genetics and history extremely sick. What I said comes from an architect Cheryl Ciecko whose family had mold illness and did a ton of research and consults with people to identify and fix moldy houses as well as build mold-safe ones. Can I guarantee she is right? No, but I have personally seen a lot of photos of mold underneath vinyl floors and behind walls that others felt were done correctly. And she has worked with a lot of mold remediators who have seen these things for themselves. But you could consult with her on your specific project.

      Regarding carpet, while it sounds good because it is permeable, it is also impossible to clean well and dust/mold spores will continue to collect over time. Remember that while some molds need a lot of water to grow, many molds need very little.

      1. Expert Member
        MALCOLM TAYLOR | | #38

        I think where things go off into the weeds is when we begin to discuss what works to remediate p0orly built or old houses. I'd say there is a pretty good consensus on how to deal with basement floors that have a good sub-slab vapour barrier and insulation.

    4. Expert Member
      NICK KEENAN | | #42

      When people say that concrete "dries" it's not the same as saying it "dries out." While "breathe" doesn't accurately describe what is happening, "dry" also doesn't completely convey what's going on because what we're talking about is the transmission of water vapor, not the drying out of anything.

      You have to think of the soil on the other side of your basement wall as an infinite reservoir of moisture. You are never going to dry that soil out. When moisture travels through that concrete and into the air of the basement we call that "drying," but the concrete never gets dry. Unless there is some sort of barrier, that moisture will continue to flow until the air in the basement reaches equilibrium with the soil on the outside of the basement, which will normally mean the air reaching 100% relative humidity. You want to block that moisture from coming through your walls and floor, and you also need some mechanism for removing moisture that does get into the basement air, either ventilation or dehumidification.

      Any solution that relies on the concrete "drying" just means an effectively unlimited supply of moisture flowing into your basement.

  10. thegiz | | #46

    So just hear me out on my thoughts, this may or may not make sense

    We started out using eps against foundation, then stud walls, then a vapor barrier before drywall. That turned out to be a mold problem because the studs became trapped in a “dirty diaper.”

    So now the best practice is eps directly against foundation then sealed then stud wall in front followed by drywall no vapor barrier. So why does this work so well is moisture actually traveling through the entire wall to “dry to the interior” Meanwhile besides DC I don’t hear much about attaching a vapor barrier to your concrete wall. Even those who believe firmly in impermeable surfaces will only apply this theory to the floor by either adding poly under slab or over followed by vinyl flooring or just vinyl directly on concrete.

    So the real question is what is the sealed insulation layer of eps doing? Is that just taking care of everything forget about permeable and impermeable surfaces. And if that is the case then would something like this:

    75 sq. ft. 3 ft. x 25 ft. x 3.2 mm Waterproof Premium Plus 10-in-1 Underlayment - Vinyl Plank, Laminate, Engineered Wood

    https://www.homedepot.com/p/204700858

    Which is basically tape sealed foam, your eps layer on the floor. Need to have a permeable or impermeable surface on top to work or it doesn’t matter. Is the insulation, maybe too thin in this scenario, doing all the work and I’m just obsessing over permeable surfaces?

    1. Expert Member
      NICK KEENAN | | #47

      The reason that wall assembly works is not because moisture travelling through the wall dries to the interior. It's because moisture entering the wall from the interior air is able to leave the way it came in when conditions are more favorable. The "dirty diaper" let's moisture in from the interior but doesn't let it out. The EPS against the wall keeps moisture from coming out of the wall.

    2. maine_tyler | | #48

      "So the real question is what is the sealed insulation layer of eps doing? Is that just taking care of everything forget about permeable and impermeable surfaces."

      1) not rotting if its wet
      2) blocking air flow
      3) maintaining the temperature of any wood components such that they stay in safe temperature zones (stay dryer).
      4) retarding vapor (even if not blocking entirely)

      Number 4 will vary with the type of foam, but pretty much 1-3 apply equally to any foam (in theory, depending on execution of things like taping). Other insulations can do 1 and 3 (though perhaps not as well), but likely won't do 2 very well and 4 probably quite a bit less than eps.

      That said, I do think it is sometimes recommended to apply a dimple mat or at least poly on walls when they are dripping wet.

  11. qofmiwok | | #51

    Talk to a good mold inspector and you will find that basement walls with internal vapor barriers cause illness in many mold sensitive people. It is simply not true that the mold there is fine because it will "stay behind the barrier". Eventually, at least some of the time, it gets through. We all know now that products fail so for example, we design our envelopes for the eventuality and give the moisture a way out (rainscreens, permeable barriers, etc). I don't know why people think it's different in this case.

  12. thegiz | | #52

    So reread this whole thread over carefully. Obviously there are still 2 theories out there on if materials should be permeable or not in a basement. In terms of floors I still believe a vinyl floor on concrete is not going to have any problems. I’d like to hear though about these mold experts saying they find mold under vinyl floors and poly and that it it seeps into the air. Is there some kind of photo of this happening or case study? So if these mold experts are against vinyl or impermeable surfaces what do they use? I’d like to hear if there setup is different then the eps with wood subfloor. I think most permeable materials have a higher chance of growing mold then vinyl which is plastic. Yes we reviewed many times the circumstances in which mold can grow but if something happened unexpectedly like a flood, seepage through floor, uncontrolled humidity wouldn’t you feel safer if your floor had vinyl rather than something organic. I found this video where they testing different materials in a basement being exposed to water. This is mostly to advertise their product, but they also had PVC flooring, which is the same as vinyl right? Nothing happened go it after being soaked for weeks.
    https://youtu.be/e3vtxO9oo3g

    1. Expert Member
      NICK KEENAN | | #53

      I watched the video. They're talking about a whole other level -- they take various building materials, weigh them, soak them in water for a month or so, and weigh them again. That's really not what anyone is talking about in this thread. That level of liquid water intrusion -- or any liquid water intrusion -- is a problem, and there is no finished construction where that isn't going to be a problem.

      Liquid water intrusion is a completely different problem from vapor management. The solution is getting the water out, and keeping it out. Drains and pumps, not coating and sealing.

    2. Expert Member
      NICK KEENAN | | #57

      Permeable, as people are using it here, has a very specific meaning. Actually, there are two types of permeability people talk about -- air permeability and water vapor permeability -- and they are different. I don't think anyone doesn't agree that walls and floors should not be air permeable. And I don't think anyone disagrees that a wall or floor needs to have a vapor barrier -- impermeable -- between the concrete and the finished space. Where people disagree is when you have a wall assembly, whether there should be a vapor barrier on both the concrete side of that wall and the interior side of that wall, or just on the concrete side. The argument is whether it is better to try and keep water vapor out of the wall altogether, or allow it a path out if it gets in.

      Note that permeability is not the same as being waterproof, or absorbent, or porous, or airtight. Those are all important characteristics, and they're all different. Concrete is waterproof but it's permeable. Wood isn't waterproof but it's airtight. Drywall is airtight but permeable and porous. Rockwool isn't porous or airtight but it's permeable.

      Vinyl plank is impermeable, waterproof, non-absorbent and non-porous. The fact that it is impermeable to both the outside and the inside isn't really relevant, because it can't absorb or trap water vapor, so it has no ability to dry to the interior, and no need to dry to the interior.

  13. thegiz | | #54

    Yes this is different than dealing with water vapor. They also talked about mold which was a big part of the conversation. However we are concerned with water vapor because finished material that is not moisture tolerant can become moldy if it remains wet. Foam board, poly, concrete can all sit in moisture forever and be completely fine. I just want to know then how is vinyl on concrete a mold trap if pvc can literally sit in water which is much worse than water vapor and remain fine. I think certain materials don't need to dry because they are unaffected by moisture. That is why foam board can be attached directly to concrete and concrete can remain soaked and be fine. Certain materials don't need to dry that is where I'm going with this. Vinyl works similar to foam board and concrete, it can sit in moisture. Just want to see the evidence that this takes place a picture of mold under vinyl or foam board. Sometimes I can be skeptical of mold remediation experts. Yes some are true professionals but then there is a group that wants to find mold because then they would have to rip everything out which creates a job. So the argument is if material is moisture tolerant than in that case drying to the interior does not need to take place but if the materials can not remain moist then it needs to escape. Technically that is why you could literally put carpet directly on concrete as long as you can control moisture in the room, and moisture can escape the carpet.

    1. Expert Member
      NICK KEENAN | | #55

      Mold can grow on any wet surface, even if that surface is unaffected by water. I've seen mold grow on concrete, metal, even glass. Dirt is full of mold and water certainly doesn't hurt dirt.

      The way to prevent mold is to prevent moisture -- keep it out, and get it out when it comes in.

  14. qofmiwok | | #56

    To answer your question about what "is" recommended, it's either concrete, or tile (which can dry through the grout). Nobody is saying the vinyl itself is going to mold, it could be the backings, glues, dirt and dust that was trapped beneath the vinyl or goes between the cracks over time. As an example, people say concrete can't mold but there are thousands of photos of moldy concrete. Because it's not the concrete itself that molds (although it could if the mix contains organics), but typically it's organics like dust and dirt that land on the concrete and receive moisture from it.

    A point I'd like to make is that if you ARE going to put vinyl on your floor, at least wait a couple years until the concrete loses much of its moisture. If everything else around your foundation is done right then you might be fine.

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