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Vapor permeance

Chris_va22 | Posted in General Questions on

Hi All,

I was speaking to a product rep from Canada and asked him what the perm rating on his brick coating was. After getting past the difference in vernacular he stated the product had a vapor permeance of 375g/m^2.

I asked if he had that stated in US perms but the answer was no. I am assuming what he stated was in metric but I am not 100% sure.

After a quick bit of research I found the following types of vapor permeance:
-US Perm (duh)
-Metric Perm

Then a couple of conversion factors followed:
-Metric Perm to US Perm, Multiply by 1.517
-Metric Perm to ng/s*m^2*Pa (Si Unit), Multiply by 86.8127

So my question is, does anyone have experience working with Canadian material vendors that could help me decode what type of permenace rating he referred so I can convert?

Thanks in advance

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  1. Jon_R | | #1

    I suppose there could be some implied (or sloppily omitted) time and pressure values in his units.

  2. Chris_va22 | | #2

    I agree, and I don't have enough technical knowledge about the subject to say that someone would reference this based on a common test spec.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hey Chris,

    I wish I could help with this, but I have only ever used and worked with folks who use US perms to measure permeance. Perhaps your question would get some attention if you posted it here:

  4. CramerSilkworth | | #4

    I don't think the sales rep gave you the full units on that 375. If it was in fact supposed to be ng/sm^2Pa (nanograms per second per square meter per Pascal), then converting it to US perms gives me 6.55, which is in the ballpark for a decently permeable brick coating, I'd say.

    See page three of the attached pdf from Henry for conversions.

  5. Expert Member


    One US perm is approximately sixty ng/pasm2. So the coating is about six perms.

    Hopefully Dana will confirm.

  6. Chris_va22 | | #6

    Thanks Malcolm, I am going to operate under that assumption as it makes the most sense to me as well.

  7. Peter Yost | | #7

    Hi Chris -

    Brian cited the recent blog I wrote on the US perm and the info you need is there, but here is the short answer:

    1 US perm = 1 grain H2O/1 square foot x 1 hour x 1-inch Hg.

    1 metric perm = 1 ng H2O/ square meter x 1 sec x 1 Pascal.

    To convert from metric to US perms, divide by 57 (approximately).

    To do anything with the number he gave you, you need to assume quite a bit, since his units don't make any sense.

    It does make sense that a coating could be in the 6 US perms range using the assumptions that Malcolm makes above.

    But also, coatings are tricky. Vapor permeance is the rate of vapor transmission for a given material at its tested thickness. Vapor permeability is the rate of vapor transmission per a given unit of thickness of the material.

    The vapor permeance of coatings are particularly sensitive to mil thickness so the number the product rep gave you, if it is indeed the vapor permeance of the coating, depends on what mil thickness the coating was measured at. And to achieve the same vapor permeance in your assembly, you need to apply the same thickness as was tested.

    Generally, assigning coatings to one of the Classes of vapor retarder for coatings is about as good as you are going to get and probably all that you need.


    1. Jon_R | | #8

      Also note that coating are sometimes (or always?) tested as stand alone films. But when applied to a porous substrate, they will have very different perm values.

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