[GSBN] lime over hessien
misha.rauchwerger at gmail.com
Tue Apr 22 11:01:45 CDT 2014
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.
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.
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.
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.
On Tue, Apr 22, 2014 at 7:45 AM, Jacob Deva Racusin <
buildnatural at googlemail.com> wrote:
> "Lime over clay" is a topic just about as heavily-debated as "drainage
> under bale walls", it seems...
> 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).
> 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.
> 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.
> 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.
> On 4/22/14, 7:16 AM, Rikki Nitzkin wrote:
> Hi all,
> 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.
> 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.
> I have heard several theories on why:
> - badly applied (improper bonding)
> - wrong kind of clay or lime
> - in humid areas, the water penetrates the lime, reaches the clay and
> the clay expands- pushing off the lime...
> The last theory may have more validity, because we have noticed that the
> lime stays on the clay longer in dry areas.
> If anyone has done any research on this issue, or has experiences to
> share, i am very interested.
> 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.
> Has anyone else tried this?
> take care,
> Jacob Deva Racusin
> New Frameworks Natural Design/Build
> Author, The Natural Building Companion
> Chelsea Green Press, 2012
> (802) 782-7783
> jacob at newframeworks.com
> GSBN mailing list
> GSBN at sustainablesources.com
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