[Greenbuilding] Difference between an Air Barrier Strategy vs. AB materials (was Re: Water Barrier)

[Corwyn]

On 3/5/2013 12:07 PM, Eli Talking wrote:
> However, with no affective vapor barrier, the vapor should
> be able to dry to the warm side of the assembly, easily passing through the
> air barriers.

This is often bandied about.  I would like to know if anyone has done 
the chemistry and math on it.  Let's say you have a 68°F building, at 
50% relative humidity, and an outside at 20°F and 50% RH.  Assume for 
simplicity's sake that there is a uniform temperature gradient through 
the wall assembly, which is also evenly permeable.  Additionally, we 
will assume that the moisture is transported to the assembly by the air 
(so not through vapor pressure), and condenses there.  In order for the 
assembly to 'dry to the inside' are the conditions listed above 
sufficient to achieve a positive vapor pressure from the assembly to the 
building (for some perm rating)?  If not, what do the conditions have to be?

My gut intuition is that under those conditions the partial pressure of 
water vapor is going to be pushing more vapor into the assembly rather 
than the other way around.


Thank You Kindly,

Corwyn

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Topher Belknap
Green Fret Consulting
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