[permaculture] Question about Earth Homes, thermal mass

Gene Monaco efmonaco at comcast.net
Thu Feb 4 14:33:49 EST 2010

Being a builder I would say that both are correct.

I have seen many many buildings that have water issues, and lots and lots of
mildew issues without even using straw.  The building industry is finally
starting to get the idea that it is best to take away the materials that can
be consumed by fungi, such as the paper on gypsum sheathing. We're seeing
fiberglass exterior sheathing and paperless drywall.  

The issue is that even with the best laid plans, moisture has a tendency of
working itself into wall cavities, both from the inside and the outside.
Even without breaches in the building envelope or drainage systems, humid
interior conditions like shower rooms will let moisture migrate into the
wall unless there is a vapor barrier on the inside, which there almost never
is.  Humidity from the outside will work its way into the wall cavity unless
there is a vapor barrier on the outside.  Most building details do have some
sort of vapor barrier on the outside, but not on the inside.  Even when
vapor barriers are used, they often are breached by penetrations at doors,
windows, pipes, and fasteners.  Cob, mud plaster, cement plaster, and
drywall will all allow moisture to migrate from a zone of higher
concentration to a zone of lower concentration.  

Moisture can even wick up through the footings into the walls.  Who do you
know that puts a vapor barrier under or on top of the footings?  The most
you ever see is a stem wall from the ground level down to the footing, with
waterproofing on the outside of the stem wall and a foundation drain along
the bottom of the stem wall at the top of the footing.  What you have to do
is make the building foundation system like the bottom of a boat so it
floats in the dirt so no moisture can get in.  It can be done but it is not
easy, and certainly most mom and pop types that set out with stars in their
eyes to build their straw bale house will not get it right.  Plus, ask
yourself, why then do they put bilge pumps in boats?

When straw is used, it is already inoculated with fungi from its natural
environment, which is anything but natural the way straw bales are created
(but that is another story).  Plus the bales are generally not bone dry;
they have some moisture content.  They absorb humidity from the air right on
the jobsite.  Some moisture inevitably gets trapped in the wall, and if
enough moisture is present the fungi will multiply and eat the straw.  If
that happens enough you will smell mildew.  I've been in a number of straw
bale structures that despite the best efforts of the builders smelled like
mildew and eventually were abandoned.  For the one case Katey cites, I'll
bet there are dozens if not hundreds of others that were abandoned.

This is not to say that Katey is wrong.  If the structure is detailed AND
built AND maintained, then all CAN be well; but in my opinion you are
stacking the odds against you.  It takes more than an architect to make it
all work right.  All it takes is an overlooked spot during construction, a
crack in the plaster, a leaky joint in a gutter system, nail holes punched
through the vapor barrier, or a leaky window, door jamb, plumbing
penetration, chimney flashing, whatever you can imagine, and in the warm
humid regions your structure will be soon on its way to toast or more
precisely, soil.

A great book to read that describes in excruciating detail how nature takes
apart structures in no time is: The World Without Us by Alan Weisman.  In a
nutshell, it's the water (stupid).

Gene Monaco
Knoxville, TN  

Katey wrote:

Hi Robyn,
Are you a builder or architect?  I am wondering how extensive your
experience is.  I am partner in the architect/design firm Ecoville
ArchiTechs.  We live in the Southeastern US and promote natural building
methods.  Since 1994 we have helped over 35 clients in 9 states build with
straw bales.  I have not found it to be true what you say about humidity and
straw  bale homes.  Humidity is not a problem.  Moisture is a problem if it
gets in your walls, just like with any building.  You detail to prevent it.
In our designs we also add a drainbed under the straw in case the bathtub
overflows.  We use a lime/clay plaster which also performs well in our
region.  And of course the Burritt Mansion in Huntsville AL is straw bale
built in 1936, still in use today as a public building.

We have not yet had a client who wants to build cob but we'd welcome it.  We
have worked on many cob structures in the area that are doing fine but they
are not conditioned living spaces.  Some of these are going on 15 years; so
far so good.


Katey Culver

On Sun, Jan 31, 2010 at 9:18 PM, Robyn Francis <erda at nor.com.au> wrote:

> Hi
> I live in a similar climate to SE USA: hot, muggy, humid and big rains in
> summer and big temperature changes from day to night so condensation can
> also be an issue. Here we need to be very careful using earth or strawbale
> construction. I've seen numerous earthbrick/cob structures in this area
> simply erode away with torrential rains, and know of issues with strawbale
> buildings with humidity entering through the render and the straw
> decomposing and becoming an ant metropolis.

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