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RE: GSBN:Moisture Problem



Paul,
Iâm a bit confused by an assessment of the sources of the moisture.  In
the first part of your description, you describe an external source of,
apparently, a good deal of water, coming from condensation around the
top of the roof.  
 
ãThe extreme wetness on the surface of the plaster was caused by water
(clearly condensate- the standing seam roof could not be leaking in
every bay) running down the proper vent, hitting the blocking at the
exterior of the wall, and running down the back side of the blocking,
into the outer 2ä of the bale, and into the plaster.ä
 
But then, after tearing open the walls, you remarked that:
 
ãThe patterns he found speak clearly of interior moisture sources, and
also correlate to the long-held idea that areas of lesser density are in
greater danger of moisture damage.ä 
 
Itâs clear that the bales were subjected to lots of moisture.  First,
during the wet summer of plastering, when apparently drying of the
considerable moisture introduced through plastering was very slow; then
during the following winter, when the house was unheated, and therefore
unable to dry out; and finally because of the roof vent condition.
 
So I wonder whether what you are taking to be evidence of ãinterior
moisture sourcesä isnât just an observation of the pattern of moisture
migration (and collection) as the moisture laden walls (attempted to)
dry out.  
The relatively humid interior conditions, in this case, wouldnât be the
cause of the excessive moisture, but would have resulted in a sluggish
drying process which, given the amount of water the bales had stored
during construction and the winter, and the input of water from the
roof, was insufficient to dry the bales before damage occurred.
When bales are moisture laden, the moisture will transport and migrate
all around the walls, in and out, up and down, depending on Îclimateâ,
even on a daily basis.  So perhaps you are seeing evidence of the
dominant pattern of moisture migration in the walls, but not of the
introduction of moisture from the interior. 
This would indicate that the lack of barrier on the interior is actually
working to dry the wall: The gradation of humidity from dry at the
interior to moist at the outside may be evidence that the
humidity/temperature/pressure differential between inside and out is
actually working to drive moisture out of the walls, but was
insufficient to do the job before damage occurred.
But then again, I havenât seen the walls·.what do you think?
 
John
 
John Swearingen
Skillful Means
 
 
 
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Subject: GSBN:Moisture Problem
 
Hello all,
Well, we have now had a significant failure.  The owner of one straw
bale house here (on which I served as a consultant and led the plaster
and bale work) has removed his straw bale walls after only 2 years,
because of moisture problems.  I wasn&#xE2;t part of the deconstruction (this
is both merciful and unfortunate) but I have a good idea of what was
going on beforehand, and I&#xE2;ve spoken to Jim McSweeney, the owner, about
what he found.  I&#xE2;ll attempt to convey it here.  At the end of this
email there will also be some questions- I hope you all will have some
ideas to share.
To begin at the beginning:
Last February, after a particularly cold spell, I noticed a pattern of
wetness at the outside top of the north wall, which appeared as if water
had been poured along the top edge of the wall, and allowed to run down.
Jim removed the blocking at the end of the rafter bays, and found ice on
the underside of the sheathing, and also on the underside of the proper
vent that was installed at the upper part of the rafter cavity, to
maintain a ventilated air space.  The extreme wetness on the surface of
the plaster was caused by water (clearly condensate- the standing seam
roof could not be leaking in every bay) running down the proper vent,
hitting the blocking at the exterior of the wall, and running down the
back side of the blocking, into the outer 2&#xE4; of the bale, and into the
plaster.  The outer 12-24 inches of cellulose was also quite wet.
This house is of post and beam construction, with bales wrapped around
the structure, and dense-packed cellulose in cathedral ceilings.  The
walls had approximately 1&#xE4; of lime stabilized clay plaster, with a lime
finish coat, and limewash.  Plaster and finishes were the same inside
and out, except for the bathroom, where a conventional paint was applied
to the interior, in place of the limewash.  (I doubt that it was an
especially vapor-retardant paint, though I&#xE2;m not sure.)  The ceiling is
&#xB8;&#xE4; drywall with latex paint, poly, and dense-packed cellulose.  The
cellulose ends atop the bale.  Interior butt joints between plaster and
timber framing were backed with 15 lb felt, in an attempt at tightening
the joint against air leakage.
In late February, Jim took a set of moisture readings which showed
alarmingly high moisture levels in the outer 3-6&#xE4; of the walls, often in
the range of 25-40%.  Unfortunately, I don&#xE2;t have any records of these
readings; they seem to have been only loosely correllated to depth. I
then took a set of readings (in different but nearby holes) in mid-June.
By this time there had been considerable drying; while there were many
readings still in the high teens and 20&#xE2;s, there were none in the 30&#xE2;s,
and all high readings were in the very outer inch or so of the straw,
just behind the exterior plaster.  The inner third of the bale was
consistently at 8-12%, and the middle third at 8-15%.  Even the outer
third of the wall, except for that last inch, ranged from 10-20%.  Jim
had found that the highest exterior numbers were on the east wall, which
is most exposed to rain, and receives some spray from an adjoining roof.
This still seemed to be the case in June.  I also pulled straw from the
drill holes, and found some minor discoloration, but no evidence of
fiber breakdown.
In October, Jim tore down the walls.  The patterns he found speak
clearly of interior moisture sources, and also correlate to the
long-held idea that areas of lesser density are in greater danger of
moisture damage.  The places where the straw was in worst shape were at
joints between straw and wooden members, and also at the corners of
bales.  In some of these areas the fiber breakdown was such that he
could crumble the straw in his hand. Some of these areas would be places
where air leakage was a possiblility- around windows and timbers, for
instance.  (The wettest point was clearly one of these-a large joint
between bale and timber that was imperfectly sealed with spray foam,
because it was impossible to plaster this area.  Some plywood in the
outer section of this wall was saturated and delaminating, and carpenter
ants had moved into the structural timber, at the inside of the bale
wall.)  But direct air leakage could not have been possible for
depositing moisture in all of the problem places- many of the wood/straw
joints and essentially all of the bale/bale joints would be buried in
the field of the wall, with a continuous skin of plaster over each face.
This seems to corroborate the theory that internal convection loops- or
some pattern of air motion- are causing water to be deposited in the
outer section of these less dense areas.  By contrast, the main body of
each bale (or at least the great majority of bales) was relatively
intact, with only a thin layer of damage at the very outer surface.
Additionally, the wall which received more wetting from rain did not
show a greater degree of decomposition than the others.
For some time now, I have been musing on the possibility that the
plaster system that we use- equally permeable on the two sides of the
wall- may not be suitable for this climate.  I know of three of our
projects that show the annual damp patches in the upper areas of the
exterior walls.  I have been taking occasional moisture readings in one
(buried wood block sensors) and these seem to indicate a pattern of
increasing moisture levels through the wall section, from inside to
outside, during the cold season.  Of course, there are also many houses
that show no visual (or olfactory!) signs of moisture accumulation.
Only one is anything like the extreme of Jim McSweeney&#xE2;s house- but this
was pretty clearly a different situation, a hilltop site where a
combination of a failed finish coat and severe windblown rain caused
water to enter the bales at the corners of the building.
(Interestingly, in this house, it was only the areas of 30+% mc that
showed any damaged straw; the 20+ areas felt damp to the touch but were
bright and intact.)
I think the McSweeneys&#xE2; house showed such severe signs so quickly
because of two factors-
1-   Higher than usual (for SB) moisture production within the house.
The McSweeneys had not been venting showers, were drying laundry in the
house(including diapers) with no ventilation, and had a pot of water on
the woodstove. The McSweeneys reasonably felt that their practices were
safe, because 3 humidity guages in the house were reading consistently
in the area of 40%.  But the extra moisture had to be going somewhere.
It seems that it was exiting in large amounts through air leakage at the
tops of the walls, at the plaster/timber intersection.  But the
generally high moisture levels in the field of the wall also indicate
that it was working through the plaster by diffusion, and that the
resulting condensation at the back of the plaster was not drying fast
enough to prevent accumulation. The fact that other houses are showing
damp patches in late winter indicate that this is not an isolated case,
even if it is extreme.
2-   Weather.  The summer in which this house was built (2003) was very
wet and not very hot.  The base coat of plaster took 2 weeks to stiffen,
and weeks passed before the surface appeared dry, on any wall other than
the south.  Finish coats were applied in late summer (exterior, in the
rain) and in early winter (interior.)  The owners then left for much of
the winter of 2003/2004, leaving the building unheated.  During the
summer of 2004, the red oxide colored exterior limewash developed a
splotchy character, with whitish surface deposits.  In retrospect it
seems reasonable to think that this was caused by moisture migrating out
through the walls.  For a long time I have been saying that &#xE3;plaster
adds a totally unacceptable amount of water to bale walls.&#xE4;  This is
mostly a way of cautioning people against applying the interior finish
coat too quickly, before the bales have a chance to dry off the water
from the base coat.  In this case, the walls may never have dried
completely.
 
So now the big question- what to do?  Jim McSweeney is understandably
convinced that straw bale construction can&#xE2;t work in this climate.  I
tried on that idea, but it doesn&#xE2;t seem right.  What is clear is that a
system of equal permeablity rates on both sides of the wall is not the
best choice for colder and wetter climates.  It appears to be working on
lots of houses- and it&#xE2;s definitely working on at least two that have
sensors installed- but it is just as clearly not working on the houses
with damp patches.  (From Jim McSweeney&#xE2;s experience  I feel it is safe
to conclude that seasonal damp patches are not OK- they are almost
definitely causing some degree of deterioration of the straw.)  The
difference may have to do with usage patterns.  But if straw bale houses
are supposed to last indefinitely (250+ years, by the standards around
here) we cannot reasonably expect every future family to be obsessive
about a dry indoor environment.
I hate to admit this, but I will- for some time now, I have been
thinking that with a clay/lime/ limewash or silicate paint exterior
system, it would be wise to use a commercial vapor-retardant paint on
the interior, to reduce the permeability of the interior wall surface.
But the problem is that no painted bale wall ever has the same timeless
feeling as a limewashed bale wall- and this feeling is a big piece of
what make straw bale houses such wonderful spaces.  So I&#xE2;ve been
resisting this idea.  But what other realistic options are there?  We
aren&#xE2;t going to come up with a way of dramatically increasing the
permeability of the exterior.  Increasing the thickness of the interior
plaster is a nice idea for a whole set of reasons, but applying ~1.5&#xE4;~
as we currenly do, is already a lot of work.  Trying to double or triple
that amount (Would this dramatically change the permeability?  Anybody
know?) seems wholly impractical, for any building larger than a shed.
Interestingly, we usually do paint kitchens and bathrooms, because of
the unusually high moisture production in the these rooms, and also for
cleanability.  In the two houses where I have taken readings, I have not
found appreciably lower moisture readings in the walls behind these
painted bathroom surfaces.  I have assumed that this is because the
exterior bathroom walls are typically pretty small, and so the moisture
level will tend to equalize with that of adjacent wall areas.  Also, I
don&#xE2;t believe that either of these bathrooms is painted with a
particularly aggressive vapor retarder.  Any ideas, here?
I understand that there is also a danger in dramatically reducing the
permeability of the interior surface- in the case of major water damage
(roof leaks, etc) the ability to dry to the interior can be very
important to the health of the walls.  But we know, from good quality
conventional construction, that less permeable inside, more permeable
outside is what works for everyday conditions.  Why should bales be any
different?  And aren&#xE2;t everyday conditions ultimately more of a driving
concern than individual events?
Thanks for reading through this, and for any thoughts that you may have.
I recall musing, at the time of Danny Buck&#xE2;s repair project, that
failures mean straw bale construction is finally coming of age.  Now I
feel like I&#xE2;ve aged a few years, as well- though not nearly so many as
Jim McSweeney, who is paying quite a lot to have his walls rebuilt with
studs and cellulose.
Thanks again for any thoughts you may provide.
All the best,
 
Paul
 
 
--
Paul M. Lacinski
Sidehill Farm
GreenSpace Collaborative
Mail: PO Box 107
Packages: 137 Beldingville Rd.
Ashfield, MA 01330 USA
+1   413 628 3800
 
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