[Cob] Cement/adobe plaster and cob floors

[Shawn King]

HI David, Cal-Earth has extensive experience with plastering earth bag wall
systems, inside and out. When I was an apprentice there several years back,
plasters were not used for either water proofing or vapor barriers.
 Instead, the bag material on the exterior of a finished dome was coated
with common tar, and sand tossed on while the tar was still wet to make the
tar rough.  A soil-cement or soil-lime hybrid plaster was applied over this
to protect the tar seal from degradation from the sun.  The hybrid recipe
for a given soil (since soils vary from one build site to the next) was
developed by making sample recipes of plaster in dixie cups, each with an
increasing percent of cement or lime mixed in, starting at 3 percent and
going up to maybe 15% at the most. The samples were allowed to dry and
cure, the cup cut away, then each sample submerged in its own jar of water.
 The sample with the lowest percentage of cement or lime that would not
dissolve when submerged for days was used for the final recipe.

That said, please note that earthen walls work best and last longest if
they breath.  Very few natural or synthetic water proof seals actually pass
water vapor, most will trap it in the wall, causing problems ranging from
mold growth to collapse. In Europe and many places in the SW US and Mexico,
old cob or adobe buildings, traditionally plastered with lime, were
"improved" with cement stucco, only to have the cob or adobe weaken and/or
collapse due to moisture buildup behind the stucco, which trapped moisture
that would otherwise escape from the wall.  If you must waterproof, I would
suggest:
A) make sure your walls are TOTALLY dry before you seal them.  Than could
be A YEAR for thick earthen walls in your climate
B) Seal interior and exterior of walls so vapor cannot get in from either
direction
C) VENTILATE your building very thoroughly. Monolithic domes and earthbag
domes both tend to suffer from heavy condensation in humid climates unless
very well ventilated.

If you want to try and get a permanent plaster that stops liquid water and
lets vapor pass, this is tricky, but can be done. Concerning cement vs.
lime, and coatings to use: According to research by Minke (2001) and Straub
showed lime-sand plasters to be both water and vapor permeable, while
cement-sand plasters to be close to pure vapor barriers. A mix of 1:2:9
cement:lime:sand had most of the vapor permeability of a pure lime plaster.
Coating either of these after they set and dry with siloxone as a sealant
lets the plaster remain vapor permeable but prevents the passage of liquid
water.  Linseed oil over lime plaster does the same thing but not nearly as
well, reducing vapor permeability along with reducing water absorption. I
do not know about the durability of either when exposed to sunlight long
term.

In your rainy/humid climate, my most conservative approach that remains
cement free would be a lime-sand plaster interior and exterior, siloxone
coating on both (or lime-casein paint interior, if you must have a
pigmented paint option).  Note maintenance to seal any cracks will be
needed over time, not hard to to with lime plasters. Also note type-S lime
is notoriously pour quality.  Get the freshest batch you can find and keep
it sealed in plastic until you use it, best yet, slake it in water in a
trash can as soon as you get it (add the lime to water, NEVER water to lime
or it can boil back at you, metal trash can as plastic might melt if you
have a really good batch of lime).  Also look at St. Astiers lime (I think
the website name might be Lime US or USA lime or something) for high
quality lime products that can be shipped domestically.

At home in Sacramento, I use earthen and earth-lime plasters over clay
straw and cob with littler concern from tiny cracks, as any moisture that
might get in can also get out. I add the lime only for walls that will see
wind-driven rain.

Hope some of this is helpful, good luck!

Shawn King


On Wed, Jan 15, 2014 at 3:07 PM, David Thurman <dmthurman at gmail.com> wrote:

> Dorthy This thread brings up something. Aren't there sealers for lime
> plaster either natural or polymer based? I almost hate to bring up polymer
> based sealers here but sometimes they work great are fairly environmental
> friendly compared to what they do.
> On Jan 15, 2014 2:44 PM, "Dorethy Hancock" <dorethy at centeroftherainbow.com
> >
> wrote:
>
> > For Mike Creedy:
> > I'll add my vote with Tys Sniffen on the cement.  If you'll read what
> Ianto
> > Evans says in *The Hand-Sculpted House*, you'll find that the expert has
> > had bad results with cement.  I don't qualify as an expert, perhaps; but
> I
> > put earthen plaster over cob, (about 1:3 clay to sand, plus chopped,
> > strained straw; and then a lime plaster over that.  My cob house has
> > weathered almost 10 years now, and I only have to patch a few cracks in
> > this lime plaster now and then, and they are very easy to mend.  Anyway,
> it
> > sounds like you have good overhang, so you don't have to worry much about
> > water damaging your plaster.  I'll admit, I don't know your weather
> there;
> > but in Kansas we sometimes get rain *sideways*, with heavy winds.
> >
> > As for floors:  *You can't use JUST linseed oil*!  I have a nice hard
> floor
> > which admittedly had some damage by renters who somehow managed to gouge
> > holes in it (they apparently neither knew nor cared anything about them
> > because they didn't own them!); but for everyday normal wear-&-tear, it's
> > good.  It's a "poured cob" floor:  bottom layer (3" deep) is clay and
> sand
> > with rocks up to 3/4", plus chopped straw and a little manure;  next
> layer
> > is 1 1/2" deep, and has small pebbles, and the straw is strained through
> > 1/4" screen; then the third/top layer is about 1/2" thick, with same
> > ingredients (clay, sand, straw & manure) that are strained through window
> > screen and troweled on.  Now, for the protective coat:  The first coat
> was
> > 3 parts linseed oil to 1 part turpentine.  Second coat is half and half.
> > Third coat is 1 part linseed oil and 3 parts turpentine.  You must wait
> > between each coat for it to thoroughly dry, just as you do with each
> layer
> > of cob.  I've read where one builder does 7 coats, and guarantees her
> work
> > to stand up to anything.  Suit yourself on that.
> >
> > I hope this helps.  Good luck!
> > Dorethy from Kansas
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