[GSBN] Permeability question

Jim Carfrae jim.carfrae at plymouth.ac.uk
Fri Nov 13 06:07:08 CST 2015

Hi Chris and Derek

I find this one of the most confusing areas of building science!
If in doubt I check a document that has helped me in the past, and I attach it in case it may help (you might have to read all the way to the end….)



Dr Jim Carfrae
Environmental Building Group
Room 302 Roland Levinsky Building
School of Architecture, Design and the Environment
Plymouth University
Drake Circus PL4 8AA

From: Derek Stearns Roff <derek at unm.edu<mailto:derek at unm.edu>>
Reply-To: Global Straw Building Network <gsbn at sustainablesources.com<mailto:gsbn at sustainablesources.com>>
Date: Thursday, 12 November 2015 20:38
To: Global Straw Building Network <gsbn at sustainablesources.com<mailto:gsbn at sustainablesources.com>>
Subject: Re: [GSBN] Permeability question

Wikipedia says:  https://en.wikipedia.org/wiki/Perm_(unit)
1 US perm       = 0.659045 metric perms

However, the units for both kinds of perms are different from what you have:
"The US perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.”
"The metric perm (not an SI unit) is defined as 1 gram of water vapor per day, per square meter, per millimeter of mercury.”

The base SI unit is different once again:
"The base normal SI unit for permeance is the kilogram<https://en.wikipedia.org/wiki/Kilogram> per second<https://en.wikipedia.org/wiki/Second> per square meter per pascal<https://en.wikipedia.org/wiki/Pascal_(unit)>.”
These come out to be the ever so convenient ~17 billion US Perms.  Or maybe the inverse of that.

The symbols associated with the figure that you received don’t make sense to me.  Guessing (badly) at what they might possibly mean, we have milligrams instead of nanograms, hours instead of days, and meters surely must be square meters.  However, this still doesn’t look at all promising.  Maybe “h” is millimeters of mercury.  A web search for the unit suggests that “h” might be Planck’s constant, but I doubt it. The milligram to nanogram conversion is a factor of a million.  The days to hours is a factor of 24.  So the figure that you were given pencils out to be somewhere around 1/250 millionths of a metric perm.  I think we could call that an effective moisture barrier.  Or rather, a complete lack of understanding on my part.

OK, so maybe these aren’t perms of any sort, and “m” really is meters.  Then if you apply this stuff in meter thick sheets, not a lot of moisture will get through.  I’ve probably still got something wrong in my conversion attempts.

I think we need to come at it from another direction.  Can we find any testing of MgO board, to compare with?  On the webpage below, on about page 6 of a non-searchable PDF, on a page labeled “Page 2 of 9”, near the top, we have ASTM E 96 figures for Magnum MgO board.  This looks promising.  Five tested samples show a high of 9.58 US perms, and a low of 7.38.  That’s using "ASTM E 96 (Procedure A)”.  Unfortunately, the same table shows that using "ASTM E 96 (Procedure B)”, the five tested samples have perm ratings from 31.9 to 36.6.  I’m not ready to jump into ASTM testing protocol definitions.  http://aebuildingsystems.com/newsite/wp-content/uploads/2014/05/Certified-Testing.pdf

Searching just the string of units, "mg/m*h*Pa”, Google showed me exactly one reference that they claim contains exactly that string of characters.  You have won the old game of Google Whacking.  That reference is to a Google Books rendering ofa page from “Ecology of Building Materials” by Bjørn Berge.  The book’s table identifies this unit as “Specific vapour penetration”, which is footnoted as “This is the vapour penetration through a 1 mm-thick layer.”  A search for "Specific vapour penetration” came up with zero exact hits on Google.  In any case, we might have to discount this source, because it actually uses meters squared, instead of simple meters, in the units definition.  So much for Google’s exact hits.


In summary, I didn’t get any farther than you did, Chris.  The only thing that comes to mind is to contact the manufacturer, and try to get a perm figure that relates directly to US or metric perms.  Or maybe John Straube will reveal all to us.

Keep us posted,

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

On Nov 12, 2015, at 11:51 AM, Chris Magwood <chris at endeavourcentre.org<mailto:chris at endeavourcentre.org>> wrote:

Hi all,

I'm looking at some magnesium oxide board sheathing that claims it has a vapour permeability of mg/m*h*Pa 0.11-0.14. Can someone tell me what this would be in US perms? Google is failing me on this one...


Chris Magwood
Director, Endeavour Centre

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