[GSBN] Perms and Grains?

Derek Stearns Roff derek at unm.edu
Mon Sep 1 16:33:40 CDT 2014

I think they are the same units, Chis.  What could be easier to calculate in your head than grains over square feet times inches of mercury times hours?  It just rolls off the tongue.  It’s possible that Imperial Perms are different from US Perms (we wouldn’t want to make things to easy).  Sometimes US and Imperial measures are the same, sometimes not.  Neither of the first two links below mention Imperial perms, while the third link doesn’t mention US perms.  However, comparing all three links leads me to conclude the US Perms and Imperial Perms are the same.  

According to this reference, http://www.greenbuildingadvisor.com/blogs/dept/building-science/oriented-strand-board-impermeable-they-say 

So the sources that you cite would seem to be claiming 37 perms and 5 perms respectively, in one of the squirreliest US/Imperial units on record.  Those are very different ratings, with 5 perms being in the middle of the range for a class-III vapor retarder (1-10 perms), while 37 perms is quite vapor open.  

Grains come 437.5 to the ounce, which once again proves the simple, human-friendly, common-sense utility of the English system.  Of course, you could think of it as .04167 pennyweights or .3161 carats.  Or 480 doites.  The conversions to stones, long tonnes, and short hundredweights are similarly straight forward.  Conversion to other useful measuring systems, such as the Chinese units in use between 1930 and 1959, are available at this link:  http://www.convert-me.com/en/convert/weight/grain.html  Spoiler Alert:  A grain is equal to 20.74 hao, among other things.  Old French, Ancient Roman, and Biblical units are available at the same link.  It’s a remarkable units conversion resource.  

In contrast, a grain equals 64.79891 milligrams, in the metric system.  That fifth decimal place could be critical, don’t you think?  

1 US perm  =   0.659045 metric perms  

Before looking this up, I didn’t realize that the metric perm is not an official SI unit.  The SI unit for permeance is kilograms per second per square meter per pascal, and is roughly equal to 1.74784x10+10 US perms.  If I’m calculating correctly, I think that’s over 17 billion US perms.  Perhaps not the most useful unit for builders.  

Stay dry,


On Sep 1, 2014, at 2:28 PM, Chris Magwood <chris at endeavourcentre.org> wrote:

> Hi all,
> I'm trying to compare the permeability of two different sheathing products. One tells me that it is rated at 37 imperial perms and the other says it is 5 grains/h-ft2-in (Hg min).
> Can anybody tell me what the second rating means, and how it might compare to imperial perms?
> Thanks!
> Chris
> -- 
> Chris Magwood
> Director, Endeavour Centre
> www.endeavourcentre.org
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Derek Roff
derek at unm.edu

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