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Breathable foundation and ceiling?

user-1042388 | Posted in Green Building Techniques on

I am designing a house for myself and my wife. I have a bad mold allergy, and my wife has multiple-chemical sensitivity. I have seen some knowledgeable forum members advocating the benefits of a breathable house envelope. I am in agreement for the walls. I plan to use Hemcrete, clay plaster and lime stucco. For the foundation floor and the ceiling, I am not sure which way to go.

For ease of cleaning of the floor, I am leaning toward ceramic tiles with epoxy grout, which out-gasses in a few days. This acts as a vapor barrier. I would choose a more breathable floor covering if I could find another that didn’t stink, didn’t grow mold when fruit juice spills on it, didn’t serve as food for pests, didn’t burn and was easy to clean. Given that I live in the Southeast US where high humidity is a problem during three seasons, I don’t see how allowing humidity to rise from the soil through the floor would be better than a vapor barrier as a floor covering. As long as the foundation materials are not damaged by mold, why would advocates of breathable house envelope say my floor covering is a bad idea?

My roof will have soffit and ridge vents. For my ceiling, I plan to use clay plaster over MgO boards and air krete insulation. None of these materials provide food for mold or termites and they all provide good fire resistance. There are several companies offering MgO boards, only one of which I found specified the perm value, which is 2.46. This is about the same as concrete, which is also a vapor retarder. Thus, the MgO boards are not in the spirit of a breathable ceiling. I haven’t been able to find a highly breathable wall board that is code approved and cost effective in the the US. However, if I have good breathability in the walls, would this cause any problem?

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  1. GBA Editor
    Martin Holladay | | #1

    For readers who have never heard of the product, "MgO boards" means "magnesium oxide wallboard." (You can Google it; that's what I did.)

    First of all, I agree with your analysis concerning the tile and grout. (I'm confused by your reference to the "foundation floor," but I assume you are talking about either a basement slab or a slab-on-grade.) There is no reason to encourage moisture to pass through a slab into your house. That's why it's important to include a moisture barrier (usually 6-mil poly, or something stronger) under your slab, along with a continuous horizontal layer of rigid foam insulation.

    The foam will keep your slab warm, thereby limiting condensation in the summer. Limiting condensation in the summer is good, because you don't want to encourage mold or mustiness. A slab without a layer of foam underneath will be cold during the summer, sometimes leading to condensation. That's unhealthy.

    So now that I've concluded that you have done a good job thinking about your slab, you can see that the analysis raises another question: why is it that you want your walls and ceiling to "breathe"? I really don't know of a single reason why a builder would encourage the flow of moisture through your wall or ceiling assembly.

    After all, once your indoor conditions are pleasant -- something you can ensure with a heating system or a cooling system -- why not separate your indoor conditions from the outdoor conditions? Why encourage the flow of moisture?

  2. albertrooks | | #2


    It sounds to me that your are looking to create a vapor profile of a permeable wall and roof system that is capable of drying both ways. Perhaps your in the right climate for that, I don't know. However, look at cellulose treated with borate as a dense packed insulation. It's got some good mass and storage capabilities. Consider a vapor retarded near the center or more to the exterior.

    Be creative and inventive... but check the final plan with a good quality WUFI study done by a pro.

  3. user-1042388 | | #3

    I first learned about the advantages of a vapor-permeable envelope by reading the "Breathing Walls" book by Swanson et al. Here is the web site for the book, which has a FAQ on this issue:

    I don't have the intention of trying to convince folks who advocate vapor barriers. I am hoping to dialog and learn from with those advocate a highly-breathable envelope. One such contributor I have seen in this forum goes by RiverSong.

  4. GBA Editor
    Martin Holladay | | #4

    The article you linked to is full of half truths. Obviously, no one wants to build a wall that permits condensation and mold. But it's a simple matter to pay attention to airtightness and thereby avoid condensation problems. To achieve a wall that performs well, there is no need to encourage water vapor to move from one side to the other.

  5. user-460363 | | #5

    I realize that this in an old thread, and the OP is probably long gone, but for the benefit of those looking at breathable walls, I hope this helps.

    I don't see these "half truths" in Swanson's article. What he says seems to be in agreement with what I've seen at Building Science Corp/BSC/Straube and Lstiburek. They all say that it's not really a question of "if" moisture will get in your walls, but more "when" and "how much". And that the generous use of plastics in a structure (foam, 6 mil poly, building wrap) can & often do become serious problems. These are well-respected people in the building science world.

    There is a gigantic difference between airtightness and breathability or vapor/moisture permeability. The example that's "closest to home" for most people is their SKIN. I don't think many would question that skin is airtight, but highly breathable. And for our human body, that's a very desirable thing. You wouldn't live very long if all your skin was coated in plastic cling wrap, or sprayed with a layer of foam, and wasn't allowed to breathe.

    Swanson (and BSC) say that you should definitely do your very best job in ensuring water doesn't get in from the outside, and it doesn't migrate through from the inside. In other words, make sure that rain/snow/condensation/storm driven rain/ground water does not penetrate. And make sure you are not encouraging condensation from the inside out, by making sure the warm, moist interior air does not have free-passage into your walls (aka "airtightness" around penetrations like electrical, HVAC, etc).

    But then you have to be smart about any moisture that makes it past your best efforts. Because it will. They have proven it over and over again. It's a problem in today's "plastic" homes. And using more plastic is probably not the best answer.

    That's where having an assembly that can "breathe" is beneficial. Because it can handle the stray amounts of moisture that seem to be inevitable - in a healthy and natural way - by "breathing". And in many/most cases, it can also handle bulk moisture much better than "plastic homes" (flood damage, 100-year storms, errors in exterior detailing, extreme vapour pressures like hot sun/sudden rainstorm/hot sun again, etc).

    I think that humanity doesn't have to look very far to find excellent examples of good design. Because nature is filled with it. It's sad that we've been trained to look at petrochemicals and plastics for the answers to problems that nature figured out a long time ago.

    I am NOT saying that there should be no under-slab moisture barrier. That is ridiculous. Of course you should prevent moisture from wicking through from the ground and through the foundation walls. And that may be a place where plastic is good (like dimpled drain board). But the plastic should go on the outside, while still allowing the inside to breathe. Like wearing a loose raincoat for your skin.

    And there should be other measures below your slab, or next to your foundation walls that subvert much of the moisture in the first place - like drainage layers, drain tile systems, etc.

    I wonder what the aboriginal people used as a moisture barrier? Surely they noticed that it sucked to sleep on damp earthen floors?

    Anyways, the problem is with a comment like this: "To achieve a wall that performs well, there is no need to encourage water vapor to move from one side to the other". Because the principles behind breathing walls isn't that you WANT a bunch of water vapor moving around.

    If you take the comment and reverse it, you get "To achieve a wall that performs well, there IS a need to STOP water vapor from moving from one side to the other." One one level, this is very true. You should definitely prevent water from getting in in the first place. So yes, it's partly true - a "half-truth". But what about the water vapor that DID make it into the wall - past the building wrap from the outside, or past the drywall layer from the inside. Now it's trapped, and I cringe at this, because it is the very definition of a water/mold/rot problem. One that a "breathing" wall could handle infinitely better than a "plastic wall". With a breathing wall, it dries out before becoming a problem (healthy). With a trapped plastic wall, it turns to mold or rot (unhealthy).

  6. GBA Editor
    Martin Holladay | | #6

    It sounds like you understand the basics. We need plastic moisture barriers, especially under slabs, because "breathable" building assemblies aren't a universal virtue. Inviting moisture into or through our building assemblies is dangerous. It's rolling the dice.

    Sometimes, a thick layer of rigid foam -- something that is far from breathable -- is just the ticket to avoid moisture problems. The classic example is inward solar vapor drive through permeable exterior sheathing -- an undesirable phenomenon. Thick exterior foam can stop that problem in its tracks.

    Good building assemblies have to be designed to dry out when they get wet, as you point out. Some layers have to be breathable. And other layers should definitely be vapor-impermeable.

    You have to use good design principles rather than slogans. "Walls have to breathe" is a slogan, not a principle.

  7. user-460363 | | #7

    I have a 2-part question for you:

    a) Do you have a copy of George Swanson and Oram Miller's book "Breathing Walls"?
    b) If so, have you read it?

    It addresses your comment "I really don't know of a single reason why a builder would encourage the flow of moisture through your wall or ceiling assembly." Because it turns out that there are many reasons why one would want a wall assembly to "breathe". And it carefully defines "breathing". And no, it doesn't say that walls have lungs and that's why they have to "breathe". But it does go into great detail about important aspects of breathability, such as permeability & diffusion, hygroscopic adsorption/desorption and capillary absorption/desorption.

    I'm very curious to know if you've read it.

  8. GBA Editor
    Martin Holladay | | #8

    I haven't read it.

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