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Green Building Blog

Vapor: Pressure, Permeance, and Permeability

A few mental visuals to help understand that elusive moisture menace

Illustration by Don Mannes, courtesy of Fine Homebuilding magazine

Welcome to this month’s installment of “Ben bumbles an analogy.” This is the place where I attempt to take something that people go to graduate school to understand, chop it into pieces based on half-informed Internet searches and a few well-placed phone calls, and hopefully squeeze out some kernel of shareable knowledge. So hop in, let’s take this ride . . . 

Big perm

I’m feeling masochistic this month and going straight for the cliff dive. We’re talking vapor, permeance, permeability, wet cups, dry cups, rates, pressures, oh my! I’m not going to go into why water vapor is something to pay attention to—suffice to say, moisture causes problems—but we’re going to talk about how vapor does its thing. It’s a convoluted and nuanced subject that often leaves me—and, as I discovered while researching this post, many others—flummoxed.

Back to pasta

For a brief explanation of water vapor, I’ll revisit my boiling pasta analogy. Essentially, vapor is the gas form of what we use to cook pasta. The steam coming off the pot is vapor in a concentration so strong we can see it. Ultimately, it spreads out to the point that we can’t see it, but it’s still there—all around us, bouncing off and through everything. Unless we’re in the vacuum of aerospace, there’s water vapor in some quantity everywhere. We measure it with the common metric of humidity, which is a measurement of the relative vapor pressure in a given space. If we put a lid on our pot of pasta, the space under that lid is going to have a higher vapor pressure than the space outside of it—you with me?

Vapor is like a box of bouncy balls

Vapor always wants…

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  1. Jason S. | | #1

    Might I be so presumptuous as to add a takeaway? :)

    6. When in doubt, choose the most vapor-open (higher permeance) materials allowed by code for your climate, and install them in the most airtight way possible. Vapor diffusion is more often friend than foe with regard for wetting vs drying of building assemblies. Control bulk water and airtightness first. Airtight plywood, airtight OSB and airtight latex-painted gypsum board can often sufficiently control vapor diffusion in all but the coldest climates and highest interior humidities, such as with greenhouses or indoor swimming pools.

    This is tough subject that seems difficult to reach consensus even among experts, so I appreciate the article and furthering of the discussion.

  2. Tyler Keniston | | #2

    Very enjoyable and witty writing.
    Watch out for Big Perm, they're only in it for the curls.

    I have no such qualms about googling (embracing the hive mind), so I googled 'why can we see steam' since something struck me as odd about being able to see water vapor. Turns out that what we see is actually vapor turned back into liquid form (small droplets) when it hits the colder air.

    This becomes a neat (and loose) analogy for buildings during heating season: inside the building we have sources of moisture and sources of heat which in turn increases the temperature and the vapor pressure of the container, just like in the kettle.
    So water vapor is driven from inside the vessel to outside. As it exits, this vapor can condense into liquid phase as it contacts colder objects.

    RE the box analogy. It is stated that "the more balls we add to the box, the higher the vapor pressure."
    I would change that slightly to say that the more balls BOUNCING around in the box, the higher the vapor pressure. Vapor pressure depends on temperature, and shaking the box is analogous to increasing temperature (adding energy to the system). The balls sitting idly on the bottom of the box can be considered to be in liquid form and won't contribute to vapor drive.

    Joe has some interesting things to say here about that elusive thermodynamic law:

  3. Tyler Keniston | | #3

    I'm curious about what criteria defines an assembly that is not subject to concerns about vapor (takeaway number 3). Likewise, it was brought up in the recent Q&A about longevity of air barriers that perhaps assemblies that rely on air tightness to survive shouldn't be chosen.

    I think these are interesting points; it would be interesting to see a list of assembly types that goes from higher vapor resiliency to lowest.
    Obviously near top of the list is an uninsulated yet heated structure, but since most discussions here involve higher levels of insulation, I'm curious how the major assembly types would rank in terms of indifference to vapor profiles and air leaks.

    Is it basically any system that keeps moisture sensitive elements warm?

  4. Mark Parlee | | #4

    Great article Ben, and you are just starting!

  5. Wiscoguy | | #5

    My comment is specifically geared towards cold climates and techniques I've seen used.

    This article leaves me if nothing more confused. If this is true all home builders using 4 inches of polyfoam on there outside walls and zip sheeting will have mold fairly quickly. Kudos for the article but then to say at the bottom of the article it all depends on the situation for building really doesn't teach us much in respect to our wall structures. Id also like to know why the wall assembles that seem to break these rules are successful.

    There's many article on this very website that state the way to mitigate moisture issues using 3-4 inches of polyfoam on the exterior which breathes almost at 0 at that thickness so are all these articles then flawed? Maybe I'm missing something and I'm really just interested in learning as much as possible about all the different nuances that are discussed on all these topics. I would like to think that at the bare minimum we should be able to form a consensus but then an article like this throws into doubt what I have been reading all over this website and building sciences website. It would be excellent if you expand further on your reasoning and to explain if vapor will react like this then why it doesn't on wall assembles that are completely the opposite of vapor open to the outside.

    Thank you for making me think about these things I feel like articles like these create positive conversations that lead to better understanding of all these topics.

    1. Expert Member
      Malcolm Taylor | | #6


      What has caused you to worry about the exterior foam causing mold?

      1. Wiscoguy | | #7

        I’m not but the article the way it is written leads me to believe that being vapor permeable both ways is necessary. I wasn’t asking. Those questions from an argumentative stand point more like devils advocate. Trying to figure things out.

        1. Expert Member
          Malcolm Taylor | | #8

          No knock on Ben, I'm an admirer of his, but I agree the Takeaways bounce around a bit.

          #1 answers your concern. You only need one side to be vapour-open.
          #2 As a general rule makes sense, but would perhaps of been better prefaced by saying "Unless you are going to compensate by using a thick enough layer of foam on the exterior...."
          #3 Might be paraphrased as something like "Marginal assemblies are risky assemblies. Don't use them" . Ben's formulation is a bit confusing because all assemblies rely on the vapour performance of their parts.
          #4 I draw a slightly different takeaway from. Rather than caring whether we know what's going on, we should again not use assemblies so fragile they won't work under certain site-specific situations - especially as these can change over the lifespan of a building.

          My feeling is that the current confusion and proliferation of building assemblies will shake out in the next few years and each climate zone will end up with a couple of dominant ones that perform well without needing to be customized for each project. That's already started to happen in many areas, driven by building code changes.

          1. Expert Member
            Michael Maines | | #9

            Kiley's interview with Ben, featured today, provides some insight into Ben's recommendations.

          2. Expert Member
            Malcolm Taylor | | #10

            Thanks Michael,

            That fleshes out where he is coming from and explains his comments 0n impermeable exteriors and foam. Everything he says makes sense, I just didn't understand it.

  6. Tyler Keniston | | #11

    "A general rule of thumb: As the thickness doubles, the permeability is cut in half."

    I think this one slipped through the cracks. In fairness, the way permeance vs permeability is discussed nearly EVERYWHERE is really confusing. Does it depend on thickness or not? What does it mean if it does?

    Permeability is a property of the material, and does not change with thickness.
    Permeance is the ABSOLUTE number, meaning if you didn't know how thick the material was, it would still be useful because it ACCOUNTS for thickness. Permeability is WHAT it is. Permeance is HOW much of that 'what'.

  7. qofmiwok | | #12

    There still are mixed opinions about permeability and solar vapor drive, especially when it comes to reservoir coatings. Thoughts on that?

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