Appropedia needs your support - Please Donate Today

CCAT greenshed west wall

From Appropedia
Jump to: navigation, search
Bottle wall.jpg


Appropriate building techniques try to do more good than harm by using local knowledge and resources, with lower embedded energy, to create a building that is the most beneficial for the inhabitants and the community. CCAT, our Campus Center for Appropriate Technology, is building a functional and educational tool shed that demonstrates different green building materials. Each wall will be constructed from different earth/green materials and over time will tell us which materials hold up best and are most suitable to our local Arcata environment. This is one in a series of three pages on the construction of the CCAT Green Shed. This page is devoted to documenting the process of building the west wall of CCAT's greenshed using clay-straw slip. Straw clay slip provides insulation for people to work comfortably in the tool shed and uses resources with relatively low embedded energy.


There was an existing frame built for the Greenshed by CCAT employees. There were three walls which needed to be filled in. For the south wall, cob was chosen, the north, cordwood and mortar, and our west with clay-straw slip.

The frequently wet, cold, and humid nature of our climate is an opportunity to work with breathable and non-water soluble materials. Some methods using organic matter present the issue of rotting. The tendency of organic matter to rot in our wet climate will require the walls to "breathe" well so as to not trap moisture inside.

So far we are thinking light straw clay or woodchip-clay with a lime plaster would be a good application, since there is an overhanging roof, a raised foundation, and the ability to have lime plaster as well as borax added to the clay slip to prevent mold growth. There is already a test wall of straw clay with a lime plaster that we will use as a resource for seeing what works best in our environment here.

One advantage of clay slip is that in just a three person team, the work can be achieved in just a semester. With other heavier building materials, like cob, you need to have a community come out to help raise the wall. Another advantage is that while the south wall will be built of cob that has the thermal mass to absorb the heat from the sun, our west wall will have more insulation to prevent the flow of heat out of the building. By inserting bottles in to the wall we can increase the amount of light entering and these bottles provide better insulation than windows.

Project Requirements[edit]


Criteria Weight Constraints
durable in climate 10 longevity under seasonal weather patterns
educational ability 9 at least demonstrate appropriate technology
structural integrity 10 meets safety standards by CCAT
level of material locality 7 at least within Humboldt
maintainability 5 less than 1 weeks worth of one persons work a year
hrs required for building structure 6 maximum 1 semester without maintenance
cost 5 $500 max
labor intensity 7 3 people's manpower
level of embedded energy of materials 8 uses the least amount of energy
aesthetics 8 meets CCATs demand

Delphi Decision Matrix[edit]

Criteria Weight Straw + Clay Planar Shavings + Clay Mix Shavings + Straw Papercrete
durable in climate 10 38 / 380 30 / 300 32 / 320 39 / 390
educational ability 9 38 / 342 40 / 370 40 / 370 34 / 306
structural integrity 10 40 / 400 30 / 300 33 / 330 40 / 400
level of material locality 7 37 / 259 40 / 270 37 / 259 30 / 210
maintainability 5 42 / 210 42 / 210 42 / 210 42 / 210
hrs required for building structure 6 40 / 240 34 / 204 35 / 210 30 / 180
cost 5 50 / 250 45 / 225 47 / 235 33 / 165
labor intensity 7 48 / 336 43 / 301 44 / 308 40 / 280
level of embeded energy of materials 8 42 / 336 40 / 320 41 / 328 25 / 200
aesthetics 8 45 / 360 45 / 360 45 / 360 45 / 360
Total 3,113 2,860 2,930 2,710

Calculating the Embedded Energy[edit]

“Embodied energy is the energy consumed by all of the processes associated with the production of a building, from the acquisition of natural resources to product delivery.” [1] The embodied energy calculations used below do not include transportation costs. If transportation were factored in wood chips and straw would be close, straw comes from within Humboldt County and the woodchips come from the backyard of CCAT or from the Blue Ox mill in Eureka. As a coastal community with a lot of forest there is an abundance of woodchips, but these are in high demand by the biodigesters in Eureka, thus more and more of the woodchip scraps have come out of the waste stream.

Papercrete Embedded Energy: cement uses 1,574,000 BTUs/cubic yard of energy in production and use, and sand takes 29,000 BTUs/cubic yard, [2] while the recycled paper would not be using more energy for its use, but instead getting more use back from a product that all ready had energy invested into it for other applications. Given : a 3:2:1 paper to cement to sand mix Our wall is 7ft * 9 ft *6 inches = 31.5 cubic ft 31.5 cubic ft * 3(1 yard/3ft) = 1.67 cubic yards 1.67 cubic yards for the whole wall * 2/5ths cement = .668 cubic yards of cement for the whole wall .668 cubic yards * 1,574,000 BTU/cubic yard= 1,051,432 BTUs for the cement in our wall if we were to use papercrete 1.67 cubic yards * 1/5ths sand = .334 cubic yards of sand .334 cubic yards of sand * 29,000 BTU/ cubic yard = 9,686 BTUs for the sand if our wall were to be made of papercrete Papercrete total= 1,051,432 BTU for the cement + 9,686 BTUs for the sand = 1,061,118 BTUs for a papercrete wall

Straw Clay-Slip Embedded Energy: a straw bale uses .24 MJ/kg and the water and clay dug from on site took the earth thousands of years to break down and create, but this energy is not spent in the form of fossil fuels or coal or any other toxic pollutant. Given : 1.67 cubic yards for the whole wall 4:1 straw to clay mix .24 MJ/kg * 4/5 = .192 MJ/kg of straw for the wall convert MJ/kg (mass) into BTU/ cubic yard (volume) thus need to know density average density of straw bale is 7lbs /cubic ft [3] .24 MJ/ 1 kg * 1kg/ 2.2046 lbs * 7 lbs / 1 cubic ft * 27 cubic ft/ 1 cubic yard= 20.58 MJ/cubic yards 20.58 MJ/cubic yards * 1,000,000J/ 1 MJ * "9.488* 10-4" Btu/ 1 J = 19,526 BTUs for a straw clay-slip wall

Project Timeline[edit]

Date Activity
3/5/2010 Meet with Drea, Submit Project Proposal and Budget
3/6/2010 build test frames, gather materials
3/7/2010 make test mixes, testing
3/8/2010 weekly group meeting 4-5:00
Week of 3/22/2010 check test batches, check plaster
4/9/2010 begin wall construction (1st lift)
4/11/2010 complete 1st lift
4/25/2010 complete construction
Week of 5/1/2010 Begin plastering



Structure: We started with a traditional stick frame wall. We then placed 2*6 studs 16 inches apart by drilling stabilizers into the cement to secure the studs in place. We also placed a crossbeam diagonally through the studs to provide greater structural support. Additionally, we screwed 1*1in wooden keying to the interior of each stud. This acts as resistance to torque. The straw locks around the key and makes the infill less vulnerable to pressure on either side. We secured 2.5ft tall plywood lengths to either side of the wall with wood screws into the studs.


We obtained 3 bales of straw from a local feed store and began with clay dug from the CCAT property. Both the west and south walls used the clay in building, which quickly depleted the resources we had to work with on site. Luckily we were able to contact a local construction site which allowed us to use clay they had displaced and had no use for.


We broke up the clay and mixed it with water in 5gal buckets. Originally we used a ratio of 4clay to 1water but this changes depending on how wet the clay is to begin with. We then strain the mixture through a ¼inch screen to remove any large particles that didn’t disassociated in the water (ex. Rocks, grass, dirt clods, organic matter). We learned that its not good to leave out the unsifted clay suspended in water, since any grass or organic materials will start to decompose in your slurry and make a funky funky smell. So sift before you take a break from your project or mix it as you go.

The ideal slurry mixture will be the consistency of a thick milkshake after straining. Then on a tarp we spread out approx. 2 5gal buckets worth of straw and applied the slurry mixture incrementally to the straw. We mixed the straw and clay with our hands until the straw was evenly coated, with no golden straw left showing. One can test to see if the straw is coated properly by squeezing it. If it drips water, the slurry is too wet. If it holds together without dripping, the mixture is ready to pack.

The next step in the process is filling in the plywood forms. Handfulls of clay-coated straw are pressed into the forms. Several times per lift the straw clay is tamped down using a heavy piece of wood, or compressed using hands. If the wall is not tamped enough, the wall will not be structurally sound. A good way to test the wall’s strength is by pushing on it from either side. If there is movement, this means it was not tamped enough. If you tamp too hard then there will not be enough air between the straw to give you good insulation, and the lack of air flow might cause molding. Each lift is allowed to dry for a minimum of 12 hours within the form.

The form is then removed and placed above the previous lift, with a few inches of overlap. The next lift is then ready to be packed. This process is repeated until the top of the wall is reached. In our case, we could no longer tamp from above due to the top beam of our frame being in the way. We instead only used one piece of plywood and packed the last lift from the side, securing the final piece of plywood at the end. Altogether it took 5 lifts to complete our wall.

Upon completion, we noticed that the infill for the top lift was not stable enough in two sections (between studs). We removed these sections, which separated readily and re-packed with more tamping. Around our diagonal cross-beam we commonly found gaps between the straw and the beam. This was remedied by mixing a straw clay batch and patching the holes. The wet and dry clay adhered and a flush surface to the wall was achieved.

Fig 2: Planar shavings-clay
Fig 2: planarshavings-straw-clay
Fig 2: straw-clay with green sprouts and mold

Clay Tests:[edit]

  • Finger Print Test, roll the clay into a ball and press your finger against the clay, if your finger print shows on the clay you have enough clay in your soil.
  • Snake Test, roll the soil in between your hands, if it stays together forming a worm/snake then you have enough clay, if it separates and does not bid then you may not have enough clay.

Our clay from CCAT passed both tests

Wall Testing:[edit]

evaluated for

  1. adhesion, is there enough clay or binder, do they crumble when dry
  2. durability in environment, does it mold or start to deteriorate when outside
  3. ability to resist compression, raise on 2 by 4's off the ground and add 5 pound bricks until it breaks or the bricks fall and look at damage.
  4. shake test, Will it fall apart when it is torqued? I think a great way to do this is to put the tests in the back of a truck and tie them down then go off-roading. We didn't get a chance to do this, instead we shook them back and forth by rocking them on the heel of their frame and seeing how well they hold to the frame and how well they hold up.
  5. length of drying time, since we have a time limit of a semester

Test Bricks We made 7 test bricks of different ratio's of clay and fiber and sand. Those without sand did the best indicating that our soil has enough sand in it all ready. The bricks with straw as the main fiber upheld the best to the elements and upheld the best when pressure was applied. Test Frames We built several 2x6x2 ft. test frames in which we will test various mixes of natural building materials. Testing our materials in small 2 sq. ft. batches will not only give us an idea of how well the material holds up to the elements or which of the different ratios is needed for each mix,it should also tell us how much of each material will be needed for the entire wall, and familiarize us with the process involved in making the mix and tamping it into its form. We tested mixes of straw coated with a clay slurry, wood chips coated in a clay slurry and a mix of straw, wood chips and clay.

Temporary plywood forms were used to contain the mixture within the frame and hold the form of our materials as we tamp them down. In the framing of the actual wall, we attached two 1x1 strips vertically along the inside of the studs. This wood will act as keying, locking the straw- clay infill into place, making the wall less vulnerable to forces pushing on it. Once the forms have been packed and dry enough to hold their form, the plywood will be removed from both sides of the frame and the mixes allowed to dry fully. Once the test mixes have completely dried, they will be evaluated based on their strength and length of drying time.

We are testing three different materials, clay+ straw, clay + planar shavings (a wood carving waste material), and clay + straw + planar shavings.

(We have considered developing some kind of mixer to enable us to coat large batches of straw with the slurry at once.)

  • For Straw-Clay, no ratio of clay to straw was needed, we just covered it in clay until it was completely coated, but not to the point where it dripped clay slurry. (
  • For the Planar shavings -Clay, we mixed 5 shavings to 1 clay using a coffee can that measures out 3/4ths gallon. We also used a clay slurry with a ratio of 1 clay to 3 water. 7 of the 5:1, or 26.25 gallons of shavings and 5.25 gallons of clay, were used to fill the 2X6X2 ft test frame. (This was the easiest job to mix.)
  • For the Planar Shavings-Straw-Clay, we mixed 5 coffee cans of shavings to 5 gallon buckets of straw to 2 coffee cans of clay slurry. 3 of of the 5:5:2 were used to fill the 2X6X2 test frames. (This was the hardest to mix since we were mixing to get the two materials to adhere together, which doesn't really happen until it dries.)

Wall Test Outcomes[edit]

  • Straw-Clay, BEST PERFORMANCE. This had the best resistance to compression. We raised the straw clay on a few 2 by 4s off the ground and added weight on top until either the frame broke or the bricks fell over, we got to 50 lbs worth of bricks before all the bricks fell over. This material required the least clay for the greatest adhesion making it better fit our criteria of finishing within a semester, since digging up clay takes a while. The material started to sprout a few green hay babies from seeds stuck in the hay and it started to mold after 12 days in a warm room, so we are hoping to use a bit of lime or borax in another test batch. The mold died when exposed to the outside elements of wind, sun, and cold. The next batch wit borax did much better and we found that you can take off the plywood within a day to prevent molding and still have it hold its form.
  • Planar shavings-Clay, WORST PERFORMANCE for our mix. It didn't bind sufficiently, we tried thicker and thicker clay slurry batches from 2:1 clay to water up to 5:1 clay to water, as well as just using more clay. If we had another binder like flour paste, lime, or rice hull ash this might have worked better.
  • Planar shavings-Straw-Clay, NOT GOOD PERFORMANCE. Again it wasn't binding sufficiently, suggesting not that the material of Planar shavings was problematic, but that we needed more clay or another binder.

The planar shavings mixes seemed to need a lot more clay then the straw mix. Though our Humboldt region has clay rich soils the types of clay change quickly over a fifteen foot span and if we use different sources of clay we will have to redo our tests for each new clay type before building. We are also sharing clay sources on the CCAT grounds with another group doing cob, so using less clay in our project will help prevent tapping the communal resource. Also since we are on a short time line of one semester's worth of work it seems using the method that requires the least clay is the most appropriate, since we wont run out of clay as often and have to redo tests as often.


Why Lime Plaster[4] for the outer wall:

  • Earth plasters made of clay (or silica "platelets") have an attraction to water, evaporation of water is what causes severe cracking, and lime helps prevent this. Lime prevents cracking by changing the bonds clay has for water, making it hydrophobic, so swelling and shrinking are greatly reduced. [5]
  • Lime plasters dry slowly gaining strength over time, bonding with clay to form a pozzolan, or natural cement. The longer the clay and lime are together the stronger the bond.
  • The average drying time between plaster coats is two weeks. This is an advantage if you want to continue to work with the plaster. Any mistakes or cracks can be fixed, since it can be reworked for days after it's put on wall, unlike cement which sets-up in hours and cannot be altered once its mixed.
  • Lime also has the ability to preserve houses for thousands of years. It's finish is hard, weather resistant, inhibits mold growth and repels insects. This is an essential element for our outer wall.[6]
  • Extra materials from lime building can be recycled for future use or put back in the earth without releasing poison's into your environment.
  • Lime also allows the wall to breathe, preventing liquid penetration, but allowing vapor permeability. This breath ability prevents moisture from being trapped in the wall and thus prevents the inside from rotting.
  • After the slacked lime is mixed and applied to a building the chemical reaction that occurs produces crystals of calcite (calcium carbonate). These crystals are have a dual refractive index so that light entering each crystal is reflected back in duplicate giving a glowy shiny look not found in other finishes. [7]

Essentially after you've applied the plaster it turns back into its original form as a dense rock, it's application is forgiving, it is less likely to crack, has a beautiful glowy finish, and has preserved houses for thousands of years.

Using lime:

  • Lime (or calcium hydroxide) when mixed with water, is called slacking (or hydrating) your lime. Slacking can be done a month before building and this time helps turn the calcium oxide into calcium hydroxide,W which sets slowly by absorbing carbon dioxide from the air. In slacking, as the water to lime ratio is decreased it's generally thought that your product increases, we tried a water to lime ratio of 1.0 lbs of lime with 0.32 lbs of water. [8]
  • Usually the inside plaster would be different from the outside plaster, because the outsides protective coating such as lime is unnecessary inside a home. Since lime does require an energy intensive process including temperatures up to 2000 degrees Fahrenheit to break it down, we don't want to use it unless we need to. The outside wall which will have to holdup against the rains could definitely use the help, but most internal walls will not be exposed to much weathering processes. Since this is a tool shed, in our case equipment is likely to get banged around a bit. Adding lime to the inside would be appropriate to protect the internal walls from the heavy ware and tare of tool shed users.

We got our plaster mix from the Cob bench group at CCAT.

Make sure the prior coat is dry before adding the next layer,then wet surface, then apply with a trowel.

  • Though lime is a product of nature when working with lime its important to not inhale or ingest it. Inhalation can lead to respiratory tract irritation, Coughing, shortness of breath, and chemical bronchitis, while ingestion can lead to internal bleeding, possible perforation of esophagus, severe pain, vomiting, diarrhea, and collapse, among other problems. To take precautions we will wear goggles, gloves, and bandannas for covering our nose and mouth.
  • Lime plaster tends to crack around corners so we will apply our plaster only up to the edges of the wall were the 2 by 6 is our stopper and corner. [9]


Our testing consisted of building several 2x2ft test frames out of 2x6's. We tested three different mixtures. They were straw and clay, wooden planar shavings and clay and mixture of all three. We attached plywood to either side of the test frames to create an infill space.

After filling the forms and waiting approximately two days we removed the plywood. We gave the test walls about a week of drying time.

Planar Shavings and Clay:
We found this mixture to be ineffective at keeping its shape. It never dried properly and came apart easily when disturbed.

Planar Shavings, Straw and Clay:
We found the mixture of all three to be more structurally sound, however it also dried very slowly.

Straw and Clay:
The most stable of all of our test walls turned out to be our simple straw clay mixture. It dried the quickest and held its shape under several different stress tests. Based on these findings we decided to go ahead with the straw-clay slip.

Challenges of Earthen Plaster on Wood[edit]

We have 2 by 4 by 6 studs along our wall and wood likes to soak up the water and dry the plaster out faster then other plastered areas, which can lead to cracking. [10] Since earth plasters do not like to stick well to exposed wood or other smooth surfaces, we tried using burlap soaked in clay slip and stapled to the wood beams.

Using Glass Bottles[edit]

For our wall, we decided to incorporate recycled glass bottles. They will serve as a way to let light into the shed and as an aesthetic element of our design.

Collect: We collected blue and green bottles from local bars, Sushi restaurants, and other restaurants in town.

  • keep in mind: you will need twice the amount of bottles as places for them in your wall. This is because we put the cut edges together within the wall so that the parts that show outside are both the bottom of the bottle. We hold them together with aluminum flashing and duct tape.

De-label: To remove the labels from the bottles, we got them wet and rubbed the labels off with wire brushes. You can also use steel wool, or soak the bottles for easier removal.

Cut: We used a tile saw to cut bottles in two. Tile saws work so well at cutting glass because they get the glass wet as your cutting, this helps reduce the friction and because the water has a high heat capacity the water absorbs excess heat. Our wall is six inches thick, and it is advisable to leave at least half an inch on either side to allow for plastering. This means the bottle pieces should be 7 inches long. Therefore, we set the saw to cut our bottles 3½ inches from the bottom.

  • Be sure to read all instructions for the tile saw, don't forget to fill it with water, wear goggles, wear a mask to prevent the glass from getting in your lungs.

Prepare bottles: Bottles of the same size and color are put together at their recently cut edges. They are then wrapped together with aluminum flashing and duct taped around the outside. The flashing allows more light to enter the inside of the structure by refraction. If you do not put enough tape around the bottles and its a sunny day the metal flashing will expand and pop off, so be sure to wrap your tape all the way around to ensure that you have equal pressure on all sides of the bottle.

Set bottles: With a material like ours, it would have been problematic to set bottles during construction (using plywood framing wouldn’t have allowed us to leave a half inch on either side). Instead we decided to build the solid wall and bore out holes where we intend to place the bottles. We drilled the holes with a 3" and 4" toothed hole saw for the wine bottles. For the large Carla Rossy bottles we put in we made our own hole saw out of a coffee can that we ground teeth into with a bicycle powered grinder. We bolted a 2 by 4 wood piece to the toothed coffee can as a lever.

Glass Bottle Tests[edit]

glass bottles

Fig 7: Materials: trash can, 5 gallon buckets, tarp, clay, straw....


Quantity Material Needed source cost total cost
3 straw bale Eureka Farm Store 7.00 21.00
lots high content clay soil From the Earth around us 0 0
1 tarp CCAT 0 0
4 5 gallon buckets sand From Wes Green 0 0
1 big bucket for mixing plaster CCAT 0 0
3 bandannas for mixing plaster my own 0 0
3 gloves CCAT 0 0
2 shovels CCAT 0 0
1 hammer I have one 0 0
40 nails CCAT 0 0
1 power drill CCAT 0 0
60 screws CCAT 0 0
1 metal paddle for stirring clay CCAT 0 0
1 sifter for sifting clay slury CCAT 0 0
2 saws I have one and CCAT 0 0
3 yogurt container tops for applying plaster Recycling center 0 0
some recycled glass bottles to let in light Recycling Center 0 0
9 2" * 6" scrap wood for 4 2' by 2' test frames Do It Best 3 27
some plywood construction sight next to CCAT 0 0
1 2 * 6 * 12 wood crossbeam Do it Best, Arcata 5.50 5.50
10 1 * 1 wood blocks for grip scrap wood from CCAT 0 0
4 5 gallon buckets CCAT 0 0
1 trash can for storing straw CCAT 0 0
4 yards aluminum sheathing for glass bottle bricks CCAT and ACE 5 a roll 10
some Yellow Iron Oxide for plaster color CCAT 0 0
1 box borax CO-Op 3 3
1 4" toothed drill bit for putting bottles in wall ACE 34 34
1 jug wood sealer to protect the wood from rotting CCAT 0 0
2 burlap sacks CCAT 0 0
4 bags of sand for plaster Wes Green 10 40
1 bags of lime for plaster CCAT 0 0
total 140.50

Updates October 2013[edit]

Very little has been done to this wall as well as the Cob wall. It was decorated with Earthen Plaster but because of the teams research and preparation was so thorough, there has been little to no maintenance done on the wall. So advice from one of the Co-Directors was to take your time. Because it’s going to take a long time anyway, it’s better to do it right the first time then to have to spend more time having to re-do some aspect of the project.

WestWall photo 1.jpg

Contact details[edit]

Carrie Schaden, Phoebe Sager, Juliana Willsen

References Annotated[edit]

  • Chiras, Dan. "Building with Earth" Mother Earth News, 191(April/May 2002): 28-35.

The advantages and disadvantages of adobe, cob, rammed earth, tire homes, and earthbags.

  • Guelberth, Cedar, and Chiras, Dan. The Natural Plaster Book: earth, lime, and gypsum plaster for natural homes. Gabriola Island: New Society Publishers, 2003. [[1]]
  • Kennedy, Joseph, F. Building Without Borders: Suistainable Construction for the Global Village. Gabriola Island: New Society Publishers, 2004.[[2]]
  • Day, Christopher. Places of the Soul: Architecture and Environmental Design as a Healing Art 1993.
  • Minge, Jeanine Marie. "Cob Building: Movements and Moments of Survival." Ph.D. diss., University of South Florida, 2008.

This dissertation is not just a how to, but she also disuses how cob is an "arts-based research process, [which] creates movements and moments of survival". She discusses how "there are no monocultures in nature" and instead talks of building with "nature’s fundamental geometries" keeping in mind "nature’s conservation of resources".

  • Pullen, Quinn M. "Strength and Composition of Willamette Valley Cob:An Earthen Building Material." Honors Baccalaureate of Science in Civil Engineering thesis, Oregon State University, 2009.

He discusses the larger variation in properties of cob between builders than conventional building materials like concrete within the Pacific NorthWest.

  • The Cob Cottage Company; Evans, Ianto; Smith, Micheal; Smiley, Linda. Earth Building and the Cob Revival: a Reader. Cottage Grove: The Cob Cottage Company, 1996. [[3]]

A collection of 11 international essays on how to build cob and different projects areound the world.

  • Wojciechowska, Paulina, Building with Earth: A guide to flexible-form Earthbag Construction [4]

She introduces a variety of natural building materials including adobe,cobb,rammed earth, wattle and daub, and Earthbags. She goes into detail about how and why to use earthbags and introduces leaders in the field of earthbag construction. She also discusses the how and why of different architectural designs like arches and domes that are useful for any earth material construction.

ENGR 305 Literature Review CCAT Green Shed West Wall Juliana Willsen

Uses of Earth as a Building Material:

• formed into blocks • packed between forms • molded by hand

Wet Climates and Earth Building:

• mud mixes are often placed wet, directly by hand • should be elevated off ground to avoid sitting in moisture

Cob Walls:

• made of aggregate (various sizes of rock and sand) and straw bound together with clay • roughly 3:1 ratio of sand:clay needed • earth mixture strong against compressive forces, straw gives tensile strength • soil must be tested for clay content • small test mixes will be necessary

Pros- • construction simple, using few tools • no framework needed • hygroscopic, can take on and give off water vapor from air (regulates humidity in building and discourages mold growth) • fireproof • shelves can be built in

Cons- • labor intensive/time consuming • can be poor insulator, depending on amount of straw used • needs to dry for a while before being plastered

Rammed Earth:

• compressed soil, compacted into form • soil mix is placed in layers about 1' thick • plaster may be required depending on materials used • • Methods: • Earthbag: bags filled with earth, stacked in layers, tamped down to lock into place. Barbed wire can be laid between layers for tensile strength. • Block Press: used to make bricks • Superadobe • Clay Slip


• Cords bonded with mortar

Pros- • hygroscopic • no plaster required

Cons- • wood can shrink or expand causing gaps and cracks • wood can mold, rot, be destroyed by insects/animals

Straw Bale: • stacked straw bales, covered with a plaster

Pros- • excellent insulator

Cons- • very vulnerable to water, must be lifted off ground in moist climates • plastering can be difficult due to texture/non-uniformity of bales


• applied when wet using hawk and trowel • can be made of earth, lime, cement • clay used must be broken down and sifted finely • lime gets soaked in water to become calcium hydroxide • plaster must be tested • the thicker the coat is applied, the more likely it is to crack while drying • 3 coats is typical • 1st coat makes uniform surface and is scratched so that the 2nd coat will adhere • 2nd coat covers any cracks and smoothes rough spots • 3rd coat is thinnest

Works Cited:

King, Bruce (1996). "Buildings of Earth and Straw", Ecological Design Press, Sausalito, CA

Snell, Clarke & Callahan, Tim (2009). "Building Green: A Complete How -To Guide to Alternative Building", Larke Books, New York, NY

Chiras, Daniel (2004). "The New Ecological Home: A Complete Guide to Green Building Options", Chelsea Green Publishing Company, White River Junction, VT

Bee, Becky (1997) "The Cob Builders Handbook: You Can Hand-Sculpt Your Own Home", Groundworks, Murphy, OR

Goodhew S., Griffiths R.(2005). "Sustainable earth walls to meet the building regulations", Energy and Buildings, 37(5), p. 451-459.

This page was part of a project for Engr305 Appropriate Technology, which finished on May 15, 2010. It is now open edit.