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Re: GBlist: Poly, straw, and moisture

Hi,

I have two points to make on the poly, moisture and straw bales here.  The
first is that I believe polyethylene film can be used to make buildings
durably airtight.  I know this because of the number of fan door tests I have
made on buildings over the years.  I also know that there have been reports
of poly failing inside wall cavities and under slabs due to polymerization.
 As a remodeler, researcher and in the course of numerous forensic
investigations, I have removed decades old poly from walls, crawlspaces and
beneath slabs which was flexible and sturdy as new.  I have only seen
polymerization a few times, always as the result of continual exposure to UV
or to corrosive atmospheres.  Having said that, I also have always designed
the air tightness elements of structure to have defense in depth - that is
redundant air tightness and moisture control features.  When I've used it,
poly is simply one piece of a layered system.

I am uncertain about the moisture dynamic in a stucco coated straw bale wall
in a mixed heating-cooling climate.  It works on concrete block, concrete and
stone buildings (masonry walled buildings have last centuries in virtually
every climate).  It works fine on impermeable foam sheathing (rain absoption
and capillary suction through the stucco are controlled by the foam
sheathing).  But straw bale is not the same as any of these systems.  I need
to think about the straw wall moisture dynamic, maybe make some tests.  Could
be that a straw wall provides a drainage layer.  Could be that it has a large
enough vapor storage capacity to get damp and dry out without getting high
enough relative humidity to support fungal growth.  I simply do not know.  -
Terry
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