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RE: GSBN:Moisture Problem



Hi Guys
Sorry I have not had time to think about this together with you.

I am still not clear about the wetting observed.
The one source mentioned, the condensation on the underside or roofing, is
clearly due to air leakage and not protecting the top of the bale walls.
The wetting around low-density gaps, the recurring wet spots, and the top of
wall problems mentioned, are all unclear as to source or transport
mechanism.

Some observations.

The discussion on drying out is very important. We cant make general
conclusions about the viability of SB unless we are starting with the best
practise in construction drying -- heat the space generously in cold weather
and ventilate.  During operation, venting moisture sources plus general
background ventilation is VERY important in cool to cold weather (less than
55 F or so). I think Habib's comments summed this up well, and we all likely
know what should be done, and Paul's experience may emphasize how important
it is.

Everything I know about straw, plaster, and buildings screams at me that
vapor diffusion through the plaster skins is not a problem.  Until we have
clear evidence, I must warn against pursuing this.  The exterior portions of
walls during cold weather will have higher moisture content because moisture
in the SB and the stucco will not dry in cold weather, and the average RH
(which is what determines the MC of protected straw) rises in New England in
the winter versus the summer.

The observations of moisture damage Paul is making at joints and low-density
parts is compelling evidence of convective air loops and redistribution of
vapor within the bales.  This behavior is not unexpected, especially in
walls exposed to more extreme temperature differences (say 40 or 50 F
differences, due to cold air or hot sun).  Hot air rises, cold air falls,
and moist air at the same temperature as dry  air is significantly lighter.
If the bales are not well packed, and/or gaps not sealed, air can rise and
fall and move around. This flow is what was observed during the bad ORNL
R-value testing, where the flow transported heat. The same type of flow will
transport tremendous amounts of vapor from wet parts of the wall to dry
parts, and from warm parts to cold parts.  It is quite conceivable for vapor
to rise from the concrete slab base to the roof by this mechanism under
daily temperature cycles: the sun heats walls, evaporates vapor, which rises
due to its heat and moisture content reducing its density, and then vapor in
it condenses as the exterior of the wall (and especially the roof) cools at
night. Note: Roofs will often cool 10 to 20 F below air temperature on cool
clear nights. The goal is to provide a void free wall, and to maintain as
many bits of the wall at constant temperature. Embedded concrete or steel
lag or lead the temperature swings too much AND cant store vapor fast enough
to prevent condensation when cooling occurs.

And finally. I see rotting failed houses all the time. Often for the same
reasons failures occur in SB.  Examples given on this list show, however,
that even careful, intelligent, and skilled people will have a problem with
SB, whereas the lack of same is often the problem in wood frame houses.
That said, I know several great builders who have also problems that could
not be foreseen. Check out this poor women's experience in Philadelphia --
it is not uncommon it is just rare to see it so on the web.
<a  target="_blank" href="http://imageevent.com/platow/defectivehouse";>http://imageevent.com/platow/defectivehouse</a>

PS. I would also like to recommend Graeme's excellent paper.

Dr John Straube
Associate Professor
Dept of Civil Engineering &amp; School of Architecture
University of Waterloo
Waterloo, Ont. Canada
<a  target="_blank" href="http://www.civil.uwaterloo.ca/beg";>http://www.civil.uwaterloo.ca/beg</a>