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How to calculate assembly perm values when combining different materials?

BrunoF | Posted in General Questions on

I am looking to determine the perm rate for a floor assembly that will consist of Advantech 23/32 subfloor, solid white oak hardwood floor and an underlayment of some type (TBD).  The Advantech folks don’t publish a perm value but are telling me that it is less than 1 but let’s assume that its value is 0.87.  If I put a layer of Aquabar B on top of the subfloor, and that perm value is also 0.87 what is the perm rating of the floor system likely to be?


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  1. Expert Member


    This article explains the calculation:

  2. shedworkshop | | #2

    Not sure about a composite perm value, but will let you try different assembly types to determine relative humidity and dew point.

  3. relztes | | #3

    Think of perms like a U factor. Take the inverse to get a resistance, add them up, and take the inverse again to get back to permeance.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      Or (from Martin's article):

      Total permeance = 1/ [1/(permeance of layer 1) + 1/(permeance of layer 2)]

  4. BrunoF | | #5

    Thx folks. So in my case if I end up with Aquabar B as a hardwood floor underlayment I would end up with a subfloor assembly perm of somewhere around 0.43 which is still in the same category as plain Advantech (semi-impermeable). I don’t know how to assess this but it seems like with a conditioned crawl under this assembly I would still be ok and be able to dry to either side.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      If the crawlspace is part of the conditioned enclosure it doesn't matter whether the floor above can dry to either side. We only worry about drying paths for building assemblies that separate the inside from the outside. All sorts of things inside the enclosure have no drying route.

      1. BrunoF | | #7

        Makes sense; thank you!

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