[GSBN] Perms and Grains?

Derek Stearns Roff derek at unm.edu
Mon Sep 1 17:38:44 CDT 2014

Thanks, Lance.  As usual, I do my best proof-reading after I hit Send.  Sorry that the links longer than one line will need some cutting and pasting in order to work.  In my previous posting, “to easy” should be “too easy”, and “this reference” should be “these references”.  Plurals and homonyms give me more trouble with each passing year.

And while I think I read correctly the Wikipedia conversion for the SI permeance unit, an analysis of my mental back of the envelope calculations suggests that it ought to equal one 17.5 billionth of a US Perm, rather than 17.5 billion.  I hope that error of 20 orders of magnitude hasn’t caused anyone any trouble.  I will try to get Wikipedia to correct their text.


On Sep 1, 2014, at 4:07 PM, Lance Kairl <sabale at bigpond.com<mailto:sabale at bigpond.com>> wrote:

If I had to work with it would drive one loopy,
But to read the response bought a smile to my face,
You have got to love science.......at least once in a lifetime.
Great work.
still smiling lance.

-----Original Message-----
From: GSBN-bounces at sustainablesources.com<mailto:GSBN-bounces at sustainablesources.com>
[mailto:GSBN-bounces at sustainablesources.com] On Behalf Of Derek Stearns Roff
Sent: Tuesday, 2 September 2014 7:04 AM
To: Global Straw Building Network
Subject: Re: [GSBN] Perms and Grains?

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,

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

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?




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Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

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