[GSBN] Vapor Barrier

[Derek Roff]

I'm sure Jeff has already explored trying to educate the code official, 
but that seems like the only safe approach to me.  I would hope that 
the book "Design of Straw Bale Buildings" by Bruce King and friends 
would be convincing.  But of course, that would require that the 
building code official read it and evaluate it on its merits, which 
some will refuse to do.

Here are a few quotes from the book that I consider relevant to this 
important question:

"I just don't want to make the wrong mistake" (Bruce quoting Yogi Berra)

"The best moisture control strategies always involve designing problems 
OUT - not solving them after they have been needlessly designed into 
the enclosure."  p. 144, John Straube (emphasis in the original 
included bold type for the entire quote, and the capitalization of the 
word "OUT").

"Some building codes have required vapor barriers for framed walls. 
These requirements, based on a limited amount of research conducted 
over 50 years ago, have recently come under question.  The 
International Residential Code is being changed to remove this 
requirement for all but the coldest climate zones of North America. 
Straw bale walls are quite different than framed walls... [Their] 
behavior is similar to that of traditional solid masonry or adobe 
walls, which have never required a separate low permeance plastic vapor 
barrier."  p. 156, John Straube

"For a typical residence the vapor permeance of the interior plaster 
skin should be kept as high as possible to encourage inward drying, but 
low enough to avoid a significant amount of cold weather diffusion 
wetting."  p. 156, John Straube


Since the winter interior humidity level for most houses in Colorado is 
likely to be too low, rather than too high, I think the first half of 
John's last quote is the operative one.  The building official is 
endangering the structure and the health of the occupants (through 
decreased indoor air quality) by requiring this vapor barrier.

That puts Jeff in a very difficult position.  I'm thinking that if Jeff 
follows this requirement, primary responsibility for any problems will 
still fall on him.  I'm guessing that in the case of future moisture or 
respiratory problems, it would be very difficult to transfer the 
responsibility for this poor approach back to building department, even 
though that is where it belongs.  I hope Jeff can find a way to avoid 
being pushed into building the wrong way.

Derek

--On Thursday, July 17, 2008 12:06 PM -0600 Jeff Ruppert 
<jeff.ruppert at gmail.com> wrote:

> Hello Everyone,
>
> We are dealing with a picky building official and are interested in
> options to seal interior earthen plaster with a sealer that offers a
> low  permeability and that is also non-toxic.  The building official
> is  requiring the use of a vapor barrier on the interior and we are a
> little  stumped as to what to use.  Code requires something with a
> perm rating  of 1 or less.  Anyone tackle this particular one yet?
>
> Jeff


Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu