[Strawbale] Strawbale Digest, Vol 3, Issue 11

[Mary Niles]

Andrew Morrison wrote:

> I keep getting questions about building with SB in high humidity
> environments.  I live in the dry Southern Oregon region and do not have
> to worry about such things in my own homes.  My suggestion is to install
> a dehumidifying system in the home to help remove excess moisture from
> the structure.  This, coupled with the standard rain protection design
> features should mitigate the risks associated with the humidity.  I know
> of several homes in the Portland, Oregon and Seattle, Washington regions
> that see to be doing quite well.  I also know there is SB construction
> happening in the Deep South.  I would like to hear some feedback on what
> you all are doing to ensure that moisture does not ruin the bales
> through humidity.  Thanks.

I live in northern Vermont, and have been involved with a variety of
projects that rely upon vapor migration through a wall system
("breathable" wall systems).

You are right in recommending "standard rain protection design features";
depending on the site and prevelance of wind-driven rain, super-standard
protections, such as vented rainscreens or covered porches, may be
required as well.  Certainly choosing the correct plasters is important;
while lime may be required for exterior protection, the use of clay-based
plasters on the interior will help in terms of being able to uptake
significant amounts of ambient humidity without transferring it to the
bales.

Atmospheric humidity alone isn't the only problem, however; in this part
of the world, where there are significant temperature differentials
between exterior and interior conditions, set the stage for condensation
issues, which in my opinion, can be far more insidious, as rather than
simply dealing with vapor-borne moisture, you are now dealing with
liquid-borne moisture.  Ambient humidity in the winter is bone dry
outside; however, since everyone moves inside, and is breathing and
sweating and cooking and drying clothes in front of the fire, and often
using hydronic or other non-drying heat sources, the interior humidity
levels will be high enough that where interior and exterior conditions
meet, condensation developes.  Simply having dehumidifiers in the house,
however, is insufficient to address this issue, because a) it does nothing
to address the uncontrolled air migration that would lead to an
in-the-wall condensation occurance in the first place, and b) to be
effective in mitigating a moisture source from the interior environment,
it would effectively make the interior environment uninhabitable, or at
least very uncomfortable (that's why most people have humidifiers in the
winter, not dehumidifiers.  What's good for us humans isn't always so good
for the bales).

That said, the key as far as I'm concerned in addressing condensation is
to be able to control airflow patterns in the house.  This is accomplished
by the use of mechanical ventilation systems (or at least
strategically-placed operable windows that actually get operated
frequently enough to ensure positive air exchange) to provide a path of
less resistance than the cracks around the posts, and by sound, tight
construction practices (i.e. no cracks around the posts) and routine
inspection and maintanence of plaster skins, top- and bottom-of-wall
details, and other potential locations of air entry or exit.

I've not yet heard of any incidences of bale wall failure due just to
being in high atmospheric humidity conditions; that said, the inability of
the wall to dry out due to other moisture sources (construction moisture,
wall leaks, etc.) can be a problem.

Hope this helps,

Jacob Racusin
Montgomery, VT