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GSBN:Re: Danny Buck's moisture NM

I agree with David.  It seems to be a common dream of builders and
designers to keep all water out, for ever.  I suppose that we have a dream
of building perfect houses.  Perfection might not be bad target, but it's
impossible to attain.  The problem comes when designing for perfection
decreases quality, rather than increasing it.  That has happened many times
on many buildings, when people believe they can keep all the water out.
These houses are the most likely to suffer catastrophic failures.  Examples
include the many centuries-old adobe structures, which were re-roofed and
plastered in modern materials, in order to "protect" them.  A few were
destroyed, and most suffered severe damage over the course of a mere ten or
twenty years, before the new materials were removed.

Ahhh, but that was twenty years ago.  Now we know better, and have better
materials.  We are really going to protect those buildings now...

This relates directly to Danny's project.  I suggest that there are many
advantages to leaving the interior mud plaster as is.  I think that sealing
the plaster would do more harm than good.  I doubt the theoretical
supposition, which Danny quoted, that the interior plaster should be less
permeable than the exterior plaster.  Neither laboratory research nor field
monitoring support that approach.  Rather, my reading of the research tells
me that having good permeability in all plasters is the safest course for
strawbale walls.  Research by the Canada Mortgage and Housing Corporation
indicates that even in Arctic conditions, the amount of moisture diffusing
through a continuous air barrier, from the living space into a wall, is
small.  On the other hand, the amount of moisture moving with air through
small flaws and holes in the air barrier can be very large and have severe
consequences.  While we should be vigilant in reducing these holes and
flaws, as David points out, it is hubris to suppose we can attain
perfection in this regard.  Therefore, we need the broad expanses of
permeable plasters to ameliorate the effects of the inevitable moisture

If it were my house, I would not seal or modify the mud plaster.  I think
it is a big plus for moisture handling.  Removing the elastomeric from the
outside would also be an advantage, in my opinion.  I wish I knew a simple
way to do it.  John Straube's testing indicates that vapor permeable, water
repelling coatings, such as siloxane, can be quite effective over permeable
exterior plasters.  John Swearingen has reported using this material with
success and satisfaction in California.  I don't know if the silicate
paints that Chris refers to offer similar advantages.  Independent testing
is needed.


--On Thursday, November 11, 2004 7:11 PM -0500 Strawnet@...:

Hello from Portland - a wetter place than my customary climate (Tucson),
where I am attending the USGBC Greenbuild conference.

I just need to reiterate a point I've made many times on this subject -
that is the impossibility of keeping moisture out of these assemblies. If
we can't keep moisture from getting into double glazed windows - two
pieces of glass separated and sealed with the highest tech materials we
can invent, built in a factory and put into a frame - why would you think
we could build an assembly in the field, with the variability of
materails, designs, workmanship and all that happens to those assemblies
over time and water won't get in? It will. Take that as your base
assumption and make sure it can get out. I can't say this strongly enough
- things get wet, make sure that the drying capacity is stronger overall
than the wetting mechanisms...

Gotta go...


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@...