<div dir="ltr"><div>A number of the straw bale houses I have built have had a cob (clay soil+sand+straw) scratch coat. Some of them had a sprayed on clay slip to the bales first, before the scratch coat. Natural Hydraulic Lime is the usual brown and finish coats for exterior plasters. As Jacob and others has said, the "key" seems to be adequate key in, with a good physical connection by scratching. But getting the moisture level just right in the scratch coat seems to be the other critical piece, with multiple mist passes to give adequate time for the water to absorb in without running on the surface. Not enough water makes getting good hydraulic suction very difficult, and the plaster goes off really quickly, even with a two pass application. Too much water and the surface becomes mud which reduces bond and mixes into the plaster as well.<br>
<br>I have attempted to put lime on both troweled earthen plasters or smoothed cob walls, and the results are always variable. Sometimes, I have applied the lime wash as mentioned first, to provide an initial lime penetration and bond, but if it is too heavy in lime, it seems to actually reduce the moisture absorption into the base coat, and make getting suction more difficult. I have seen delamination when the base was too smooth. An application of Weldcrete or similar bonding product works wonders on smooth surfaces, allowing NHL to stick to electrical cover plates, sheetrock, and previously troweled surfaces. I have seen a repair on a lime over cob interior plaster that had delaminated in places. The areas that were obviously not adhered well were removed, but the remaining plaster got re-plastered with a thicker than normal finish coat that seems to have unified the whole well.<br>
<br></div><div>On cob ovens in the great outdoors, without cover, the transition from earthen to lime or lime/cement plasters seems to be the most problematic, where moisture seems to find a way behind the more impervious skin of lime and into the cob behind it causing delamination. If that is any indication of what would happen to a wall system getting exposed to water, I would agree with Graham North that first protecting the wall is best, but a rain screen or other system would be in order.<br>
<br></div><div>With regards to Sarah's question, I agree with all the others about the use of hessien (which I'm assuming is similar to burlap or even more open weaves). I have used a number of fabrics for various purposes, and while they can be great to join dissimilar materials (like fiberglass tape at corners between plastered straw wall and a sheetrock wall), they definitely can create a barrier between the two coats. I usually let it dry like the base coat it is attached to before applying the brown coat. I also agree that when the fabric gets bigger than just strips, it can get really heavy under the weight of itself, to all sorts of other problematic situations. The one house that I worked on that had to be plastered under less that ideal conditions, we attached 2x4s to the eaves and ran them to the ground at a slope. Horozontal 1x4s joined them, with tarps tensioned and attached with cleats. This was impervious to serious winds, and really worked well to retain the humidity for plaster curing.<br>
<br></div><div>Misha Rauchwerger<br></div><div><a href="http://www.builtinbliss.com">www.builtinbliss.com</a><br></div><div><br><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Apr 22, 2014 at 7:45 AM, Jacob Deva Racusin <span dir="ltr"><<a href="mailto:buildnatural@googlemail.com" target="_blank">buildnatural@googlemail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
"Lime over clay" is a topic just about as heavily-debated as
"drainage under bale walls", it seems...<br>
<br>
We've seen mixed results of lime over pure clay, but too many
variables at play to draw any conclusions. Our regular practice,
however (when not building in a rainscreen), is to apply a pure lime
finish plaster over a lime-stabilized clay base coat, very similarly
to your procedure with similar experiences (makes for an incredibly
durable base coat). We started doing this, based off the precedence
set by Paul Lacinski and Andy Mueller of Greenspace Collaborative,
because it would allow the base coat to set up more quickly, with
less likelihood for molding (we frequently use manure in our
formulations) particularly in late-season jobs, and would ensure the
base coat could make it through a winter without wear as we were
rarely able to get a finish lime coat on with enough time to cure in
a single season (we're in the cold and wet northeastern US). We
strongly favor the use of clay-based renders as base coats to help
control moisture (a big issue for us in our climate), and lime is
favored as a finish for its durability (relative to clay - high
exposure/low maintenance projects generally receive rainscreen
cladding).<br>
<br>
We've since embraced the benefit of a lime-stabilized clay base to
support a durable and long-lived transition to a lime finish coat.
First and foremost, a really good mechanical key needs to be in
place - we are very disciplined about keeping a scratch tool close
to hand while plastering, and getting a good accurate scratch (not
too shallow, not too deep, covering all surfaces) when the timing on
the base plaster is right - not too soon (overscratch) nor to late
(underscratch). Having a chemical similarity always made sense, and
empirically the stabilized base plaster seems more compatible in its
physical properties. But in chemical theory, this was further
supported by a great thread on the old CREST (or maybe SB-R-US?)
listserv asking the same question. My recollection (and I'm taking
big paraphrasing liberties here) is that Harry Francis, the lime
guru, explained that if too small an amount of lime is added to a
clay soil, it essentially turns the clay to silt, and a significant
amount of lime needs to be added to induce enough of a chemical
change for the lime to enhance the viability of the mix (this is
easily felt empirically while mixing, as suddenly the mix becomes
much stiffer and stickier as the lime is added, depending on your
mix methodology). This balance varies mostly on the type of clay
used (assuming a standard Type S hydrated lime), and is driven, at
least in large part, by pH (needs to be high enough), if I remember
correctly.<br>
<br>
On the wall, one hypothesis about why lime over clay can delaminate
(and I believe this was more theory than proven fact) is that the
application of lime turns that outer surface of clay plaster to silt
- enough to flocculate the clay molecules, but not enough to fully
stabilize it - and sufficiently weakens that outer surface as to
contribute to premature delamination, especially where solid
mechanical key isn't provided. I've heard tell of colleagues, and
had success myself, applying a coat or two of limewash between clay
and lime, and having good success there. This would serve to work
quite a lot of pure lime deeper into the pores of the clay plaster
and stabilize that outer layer (again, working off theory here), as
opposed to just smearing a lime plaster over the surface.
Stabilizing the base coat goes a big step further, ensuring the
entire base coat is compatible, and saving a step between coats.<br>
<br>
Does anyone else have recollection of that thread? I don't have time
right now to search my archives but will try to do so later.<br>
<br>
Cheers,<br>
Jacob<div class=""><br>
<br>
<div>On 4/22/14, 7:16 AM, Rikki Nitzkin
wrote:<br>
</div>
<blockquote type="cite">Hi all,
<div><br>
</div>
<div>I have only skimmed over these emails because I am quite
busy, but I don`t see anyone making comments about applying Lime
Plaster over Clay plaster. If I repeat in my comments, forgive
me.</div>
<div><br>
</div>
<div>I am concerned, because I have found that applying Lime
plaster over clay plaster has given very "mixed" results here in
spain. Half the time the clay plaster falls off the walls after
a year or two. </div>
<div><br>
</div>
<div>I have heard several theories on why:</div>
<div>
<ul>
<li>badly applied (improper bonding)</li>
<li>wrong kind of clay or lime</li>
<li>in humid areas, the water penetrates the lime, reaches the
clay and the clay expands- pushing off the lime...</li>
</ul>
<div><br>
</div>
<div>The last theory may have more validity, because we have
noticed that the lime stays on the clay longer in dry areas.</div>
<div><br>
</div>
</div>
<div>If anyone has done any research on this issue, or has
experiences to share, i am very interested.</div>
<div><br>
</div>
<div>Here in Spain we have been experimenting with using "bridge"
layers of a clay-lime plaster (1/8-1/2 part lime for each part
of clay) to stabilize the mix and make the final lime plaster
stick better. At the moment it has worked quite well, although
it is too soon to give conclusive results. In many cases the
"pure lime" plaster finish has even been unneccesary because the
clay-lime plaster is quite strong and water resistant.</div>
<div><br>
</div>
<div>Has anyone else tried this?</div>
<div><br>
</div>
<div>take care, </div>
<div>Rikki</div>
<div><br>
</div>
<br>
</blockquote>
<br>
</div><span class="HOEnZb"><font color="#888888"><div>-- <br>
Jacob Deva Racusin <br>
Co-Owner<br>
New Frameworks Natural Design/Build<br>
<br>
Author, The Natural Building Companion<br>
Chelsea Green Press, 2012<br>
<br>
<a href="tel:%28802%29%20782-7783" value="+18027827783" target="_blank">(802) 782-7783</a><br>
<a href="mailto:jacob@newframeworks.com" target="_blank">jacob@newframeworks.com</a><br>
<a href="http://www.newframeworks.com" target="_blank">http://www.newframeworks.com</a></div>
</font></span></div>
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