[permaculture] Question about Earth Homes, thermal mass and humidity

Robyn Francis erda at nor.com.au
Thu Feb 4 02:07:58 EST 2010

Hi Katey
Your work sounds interesting. I've tried a few different lime-clay plasters
and renders over the years but not found any to withstand our wet seasons. I
collected a few new recipes last year from Penny Livingston which I'm going
to experiment with.

You asked about my experience...
I'm an owner-builder several times over and been involved in following,
observing, researching and teaching natural building, solar design and
bioclimatic building for over 30 years. I collaborate with a number of
builders and architects with long experience in this and other climates. In
our subtropical climate we receive around 70-75 inches (1750-1850mm) average
rainfall during the summer/autumn wet season, with extreme humidity and
storms delivering 4+ inches in an hour driven horizontally by strong winds.
Much of this occurs as monsoonal and cyclonic rains. In between rain fronts,
temperature and humidity levels soar. Winter and spring are dry with huge
temperature extremes, over 30degC difference between max/min in a 24 hour
period, hence a lot of condensation on hard surfaces.

My observations are based on the performance of natural buildings in this
climate since the '70s. This part of Australia is a hot-spot for
experimental and natural building for the past 30+ years, so there's lots of
examples--I'm not referring to isolated cases. Strawbale and earth buildings
have a sad track record here, even with excellent damp coursing. The
building code requires a 1.5m (5 ft) overhang for earth walls, but this has
proved inadequate for severe weather events.

It's is also usual to lay several courses of fired bricks or stone wall on
top of the foundations (i.e. above floor level) with a second damp-course
and termi-mesh (prevents termites but not the small black ants that colonise
straw wall interiors) before commencing to lay any earth or straw wall
materials, as an insurance against flash-flooding (bathrooms are required to
have floor drains as a matter of course) and rising damp.

Our valley soils are highly plastic, reactive clays, not well suited to
earth construction, or supporting heavy wall structures. Even with
over-sized foundations, the movement of soil (expansion/contraction) from
the extremes of wet and dry seasons, inevitably results in cracks in
plasters on strawbale and cracking in earth wall structures (cob,
earthbrick, rammed earth, also cement and regular fired/cement brick walls
as well). I agree that strawbale with a timber frame has inherent degree of
flexibility in response to foundation movement, but thick plasters don't
have the elasticity to cope with much movement without cracking. Once the
integrity of the render is compromised, moisture can enter...

Most traditional houses here are timber on stilts with big verandas and
corrugated iron rooves. This is ancient volcanic humid subtropical
rainforest country. Local hardwood timber in this climate has continued to
hold its own as an exceptionally appropriate and durable natural building
material. The saw-ment in timber frame also works and lasts very well.

BTW I'm currently building a studio with corrugated roofing iron external
wall cladding and will experiment internally with bamboo wattle daub, cob,
mud-straw and other earth wall techniques for insulation and thermal


On 4/02/10 10:48 AM, "katey culver" <newtribe at hughes.net> 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

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