I have a bee in my bonnet about how to attach a substantial insulation layer of light straw clay (LSC) to the outside of a sml cob house.  My goal is to design a simple anchor one can build into a cob wall so that you can later attach form boards for backfilling with a LSC layer that will stay firmly adhered to the wall (retrofitting an existing cob building is another challenge altogether).  Of course, this may be problematic in some very wet climates, but perhaps if one had a deep roof overhang, good foundation and did a sturdy layer of lime plaster on the outside, it would work in moderately wet places.

The whole energy equation
When deciding how to build for each persons specific needs, I really think keeping the whole long-term energy equation in mind, ie. the embodied energy of building materials vs. the amount of energy to heat is important.  There are other factors to work in, such as the house size requirement and interior temperatures. I liked the point made recently about being willing to tolerate less than ideal inside temperatures, that, along with the willingness to live smaller are both factors in the whole equation also.  Living "smaller" with larger interior temperature fluctuations, isn't for everyone, but that's ok.  There are about 664,000 homeless people in the U.S. alone (not to mention the world), and what is smaller and cooler for us would be a step up for most of them.  There is value in working on a cob LSC hybrid system for that reason alone.


I have (on paper) been designing a minimal wooden anchor one could bury (multiples of) in a cob wall as you build.  These need to be small enough to not interfere with the integrity of the cob wall and sturdy enough to handle an outside anchor system which in turn needs to be strong enough to support a light vertical step by step framework that would receive and support your LSC material up til it dries.  Once it dries, hopefully the LSC wall (about a foot thick) would hold itself up and be aided by the interior anchors inside it and the cob wall. Intuitively I think "wrapping" a circular cob house is more stable than a squared off house, just as a serpentine garden wall is more stable than a straight one.

Interior cob anchor system
The small cobbed-in wall anchors would leave a small wooden "tab" (2-3 inches wide  by 1.5 inches deep?) sticking out of cob on the exterior of the wall.  (these anchors would be shaped like a triangle or wedge with one of the corners flattened to create the "tab" end that sticks out, and the outside or LSC anchors would be attached to these tabs by screws). 

I think I have figured out how to cut both sets of anchors (the inside and outside ones are different sizes and shapes) out of one regular board in "tessellation" fashion, so that the grain runs roughly perpendicular to the wall surface (otherwise plywood makes better sense purely for strength). You don't want torque force put on the tab to snap it off, nor do you want your outer anchors to split when screwing the form board to it, or when filling the form with LSC, so it seems wood grain direction is important.  

Exterior anchors and filling forms with LSC
Once the cob has dried, one could attach the larger wooden anchor (wider, deeper) to these tabs probably with screws, this would allow you to attach (temporarily, by screws) form boards to the cob wall thus creating a space for backfilling with LSC.  You would bury these flat anchors tilted on a 45 degree diagonal so that the exterior anchors would support your form boards in a stable manner. I think the diagonal support for the boards will prevent bulging better than vertical anchors would.  Also, each anchor would have two 2 inch holes cut in it for pinning through with pegs to help keep the anchor in the cob wall, and the LSC wall anchored to the cob wall.  One could use a hole cutter to cut all holes in the boards before cutting the anchors apart with a jig saw.

My son has built smaller indoor cob structures using twelve inch high, thin plywood form boards with lots of large holes, (2-3 inch diameter, to accelerate drying).  These worked well with heavy cob in a slow drying humid environment, so should work well with LSC.  Plywood despite it's environmental drawbacks, seems a must particularly if building in the round, and you wouldn't need that much of it as you use them over and over.  

Depth of LSC insulative layer
The thickness of your insulation layer is determined by how far out your anchors project.  As far as structural soundness/stability of the LSC, the thicker the better maybe?  I would think at least a foot thick minimally, for insulation purposes anyway, to make the effort worthwhile.  Attaching LSC insulation to the outside of a cob house = having the insulation where you need it, and thermal mass of cob inside where you need it for storing heat generated from interior sources, without using wood for stud or timber framing.  I'm thinking use wood for things we can't use anything else for, such as minimal heating needs.  After all, you can't burn clay.

Who would want to build this way?
It sounds like a labor intensive building process, but for someone who wants a small house of inexpensive, natural building materials, with lower embodied energy, I think this would be the way to go and worth the extra work. Living small is part of balancing the whole equation. 

I think cob and LSC isn't just for eco-friendly middle class folks. Perhaps more importantly there are hundreds to thousands of homeless people in some U.S. cities now (Portland is a prime example, in 2008 before economy worsened it lead the nation with 0.54% of pop. being homeless), and even more in other countries.  A large portion of these folks would probably love to live inside a 100-200 square foot, moderately heated building that they built. These folks have lots of time, but little in the way of money.  We've got to think beyond ourselves, this is another factor in the "whole equation" mentioned above.

More on affordable housing for homeless folks
For an interesting read, look up Destiny Village in Portland Oregon, where the city gave some homeless folks permission to build small (100 sq ft) homes (the city waived building codes).  Some use stick building method and pitch a tent inside, but some have used straw bale.  Destiny Village, by the way, is on macadam, so they don't need to dig foundations.  It might be easier to spec one of these round hybrids there on a urbanite base. 

I would like to try this out in Utah, anyone game?
My design needs refining and experimentation, and I'm not sure I want to stay in Utah long enough to invest time in building something for myself if I'm going to leave.  But I would be willing to travel this spring/summer for long weekends, up to half a days drive away, (southern Idaho to southern Utah).  Of course, this area is all in an earthquake zone along the Wasatch fault, so if you are doing a small project here it probably should only be a studio, workshop, or guest cottage.  I think on a small scale this method would hold up well along the Wasatch front during a quake, particularily if done in the round. Better than just cob alone, I bet.  

Bugaboos
I originally designed the anchors using plywood but the higher bodied energy to produce and the glues are an issue (also, is plywood be less stable in all that moisture?).  Now I'm thinking to cut them from 1 by 12 boards (recycled lumber would be a plus).  I want to come up with an anchor designed using "found" wood (small tree limbs) but am drawing a blank, anyone have any ideas there?  The anchor pins can be made from "found" materials though.

Some additional questions I have:
1. How far apart horizontally to space the anchors (as for vertically, probably one anchor every vertical foot, or as wide as the form board needs to be)
2. Would a light clay straw wrapped circular cob house be structurally more sound in an earthquake zone than a plain cob house?
3. Would screwing the outside anchors to the embedded tabs be problematic (rust) due to all the water present? Would it be better to peg them together instead?