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How is that a material can be both airtight and vapor permeable?

Hayden Robinson | Posted in GBA Pro Help on

Can anyone point me to an answer that explains the physics? Obviously to be vapor permeable a material must have penetrations that are bigger than vapor molecules; why doesn’t air, driven by an air pressure differential, flow through the same holes? I’ve heard it said that the pores are big enough for H2O molecules but too small for O2; is that it? Or is it simply that the rate of air leakage is too low to matter given the grosser methods used to measure air leakage compared to vapor diffusion? Or is there some force binding the molecules in a volume of air that prevents them from convecting through very small openings. Or perhaps a force greater than air pressure that retards them within the pores? I discovered in Straube’s Building Science for Building Enclosures that liquid water tends to exist in clumps of about 80 molecules and that this phenomena allows for materials to be at once watertight and vapor permeable. Is there some similar clumping phenomena occurring in air?

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  1. John Brooks | | #1

    Hayden, check out comment #16 & 19 by Riversong at this blog
    There are also some relevant videos at KhanAcademy...
    I will post a link later for the Khan explanation

  2. GBA Editor
    Martin Holladay | | #2

    I'll defer to any chemists who want to chime in. But I think it's easy to have an intuitive understanding of the difference between an air barrier and a vapor barrier by considering a piece of gypsum drywall, which is clearly an air barrier but is also vapor permeable.

    We've all seen what happens to drywall if you put it in a hot, steamy environment: it gets damp. In fact, the dampness pervades the material -- it just takes on moisture. If you have a steam room with drywall on the walls, and no other vapor barriers or insulation, clearly the backside of the drywall will evaporate moisture. So drywall is an air barrier but not a vapor barrier.

  3. Stuart Fix | | #3

    Hi Hayden,

    I believe that certain amounts of air can move through membranes like goretex, tyvek, etc, but as the post above mentions, there isn't a large enough driving force for it to be really noticeable.

    The materials are 'breathable', because water vapor can be driven through by vapor pressure differential, which can be quite strong.

    Bulk water is kept out because there is a lack of driving force, and because surface tension keeps the capillary tendrils from going too far, if the holes are small enough. I think that you could drive liquid water through these materials if you set up enough pressure differential.

  4. Hayden Robinson | | #4

    It makes sense to me now: typical vapor pressure differences are much bigger than typical air pressure differences.

    Also, I wasn't aware of KhanAcademy and was glad to find that resource.

    Thanks everyone!

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