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Re: GBlist: Strawbale not good for humid, mixed heating and cooling climates




In a message dated 5/10/97 12:07:17 PM, Fred Lugano and Ron Hughes wrote:

>It is my understanding that the failure was primarily a result of improper
>flashing above windows and doors which allowed rain water into the walls.
>If that is the case, I would think proper flashing and a generous overhang
>could solve that problem.
>
>snip from Fred:
>>The building code would be the least of your problems if you were to
>>construct strawbale houses in NC. The big problem is the "face sealed
>>stucco" exterior finish systems. There is currently over $9,000,000,000
>>worth of moisture damage in new homes in your area that employed this
>>siding.
>
>Ron Hughes
>THE WHOLE HOUSE

It is my understanding that straw bale buildings (some as old as 100 years in
Nebraska) are routinely built with large overhangs, protection at tops of
walls and at openings, breathing surfaces (allowing moisture to "dry to the
outside", as Joe Lstiburek says) and bottoms of walls raised well above the
ground.  Water from the top is the enemy of straw bale.  Water from the side
can dry out.  Joe says when things get wet and are allowed to dry out even in
frequently repeated sequences--that's OK.  The problem is standing water or
trapped moisture.  Face sealed stucco like "Dryvit" is absolutely
inappropriate with straw bale (or most other types of structure).  Some straw
bale buildings have been built with conventional partially permeable building
paper drainage planes covered with wire mesh and stucco.  

Some breathing earthen or cementitious stucco face coatings have been applied
directly to the straw surface without any additional vapor or air retarder.
 Plaster can be applied to interior surfaces in similar fashion.

I have seen and heard of partial building failures with straw bale
construction where foundations are not provided with sufficient drainage or
weep openings to allow accumulated water in the wall to escape harmlessly.  I
have seen failures where tops of exposed flying buttress walls were not
protected from direct rain exposure, allowing water to soak down through
stucco and straw to the base of the bales.  The sun's heat on dampened
material created a furious heat engine driving moisture vapor in the bales
into the adjacent structural building wall (also straw bale).  The concrete
bond beam at the top of the building wall stayed cold in the shade of the
roof overhang.  The hot driven moisture rose in the bale wall until it hit
the cold bond beam at the top.  Moisture inevitably condensed on the bottom
of the bond beam and soaked as liquid water back down through the straw to
the foundation, where the straw began to rot--adding more heat to drive the
vapor back up again to keep the cycle going.  The damage was limited to the
area near where the buttress wall joined the building wall on the South side
(sun heat provided driver here, but not on the identical wall to the North).
 It was discovered by seeing a discoloration on the inside wall, feeling heat
inside the wall, and by drilling to test moisture deep inside the straw.

Solution was quite simple:  Provide a drainage plane at the top of the
south-facing buttress wall.  Provide sufficient weep openings at the base of
the bale structure.  Provide a moisture barrier and thermal break between the
bales on the main wall and the bales on the buttress wall--preventing
sun-driven vapor from reaching the concrete bond beam in the main wall.

Another concern of Fred's was R-value in straw bale contruction.  I have been
given to understand that R-values of upwards of 40 or 50 are common in bale
buildings and that this is one of the strongest arguments in favor of this
type of construction.  It would be a pretty sloppy "wall captain" who would
allow such large unstuffed gaps in a bale wall to reduce its R-value to 12 or
less.  Honestly, I'm not a lurker from the Straw Bale List!

I don't like to see flat-out condemnations of one technique or another.  Each
climate, each design, each set of conditions and each user's needs may
greatly vary within the big picture.  We have much to learn from our past and
from what we experience each day in the present.  Our rapidly dwindling
resources require us to seek better solutions for our future.  Problems in
each different approach can be opportunities for greater learning.  People
make mistakes.  People learn from mistakes.  This list is a wonderful example
of how we share these learnings and amplify each other's experience.

David Kibbey, Environmental Building Consultant
1618 Parker Street
Berkeley, CA 94703
Phone or Fax:  510-841-1038
IAQKibbey@aol.com

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