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CCAT greenhouse natural wall

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The Humboldt State University Campus Center for Appropriate Technology (CCAT) is a hub for student experimentation with Appropriate Technologies and Sustainable Living. The students who facilitate this space work to uphold the principles of permaculture and embrace the production of food as a key pillar. Here in Humboldt County, a greenhouse is crucial for the cultivation of tomatoes, peppers and hot-weather foods as well as for propagating starts to get a jump on the growing season. When CCAT was in its original location, it was blessed with an enormous greenhouse for the community to use. However, when the HSU Behavioral and Social Sciences building was created, CCAT was relocated to the south of the new building. During this move, most of the projects on the grounds were dismantled including the greenhouse. After years of fundraising, CCAT was finally able to purchase a new pre-fabricated greenhouse. The model that was chosen was a lean-to style in that the back (north facing) wall needed to be constructed independently.This wall gives CCAT, and the Appropriate Technology classes, another opportunity to practice natural building and design techniques. The first attempt was made during the Spring Semester of 2012. Due to complications the project was remade by an appropriate tech team during Spring Semester 2013. See CCAT beachgrass slip wall for the new project.

Attempt 1: Spring 2012[edit]

Joshua Casey, Max Waterstone, Cecilia Uribe, and Greg Pitch,

Problem statement[edit]

The objective of this project is to construct an aesthetically pleasing natural earthen wall, by clay and straw slip methods. This will be built at the CCAT house near Humboldt State's BSS building. Bottle bricks like those used for the green shed in 2008 are going to be added to the wall, as to allow ample light to enter. Plaster and lime will coat the exterior of the earthen wall.


This section includes all possible aspects thought up by the group members for this project as well as the directors of CCAT. These criteria were chosen to evaluate the project based on the educational capacity. These criteria will help us to determine how much time, money, and effort needs to be invested in each aspect of the greenhouse natural wall based on our client's (CCAT) expectations.

Criteria Constraints Our Weight (0-10)
Aesthetics Must be pleasing to the eye and look presentable 7
Budget Must stay within our $400 budget set by CCAT 8
Building Regulations Must meet HSU building codes 8
Sustainable Sustainable source of lumber, SFC certified 5
Timeline/Deadline We must be completed with the project by April, which gives us 8-10 weeks to complete the project 8
Design Desirable color scheme and pattern for the glass, they prefer green & brown bottles to be used 7
Light Permeability It should not block to much sunlight from the house 7
Tools must obtain correct tools at cost and we must be approved by CCAT prior to the purchase 9
Measurement We need to get the appropriate measures since it will be bearing weight and it will be at a slant 10

Literature Review[edit]

The Steady state air to air thermal transmittance for buildings in the UK should be under 0.35 W/m(^2)K The three building materials that were tested in this report were Straw bale, Clay and Straw mixture and claytec bricks. The Volumetric heat capacity for Straw bale is .7 x10^(-3) J/m^(3)K, while the clay and straw mixture was measured with having 50 x10^(-3) J/m^(3)K. Seven times as much heat holding capability by mixing the straw with clay. The unfired claytec bricks measured in with having 20 x10^(-3) J/m^(3)K.[1]

Some Common mistakes that are worth considering is the lack of incorporation of passive solar into these naturally built walls and buildings. Incorporating passive solar into a structure is beneficial for maximizing the efficiency of the structure. Taking into consideration the angle of the sun's rays and which way the structure will be oriented. Another common mistake is failure to construct a foundation between the straw-clay wall and the earth, better yet if the requirements for the structure wanted to retain heat then the foundation should be insulated. [2] Another common problem to avoid is lack of ventilation during the drying phase of the clay or to have it against non-breathable surfaces as this can lead to serious mold and/or water damage. [3]

Once the clay/straw walls are in place they are left to dry thoroughly before receiving earthen plasters inside and out. It is not unusual to see some superficial mold on the surface of the drying wall. In fact, in the damper climate of Germany, Robert observed mushrooms sprouting from walls as a regular and accepted occurrence! Since every fiber of straw in a wheat field has mold spores on it, it is natural that these spores will begin to grow in wet conditions. However, once the dried walls are plastered, scientific testing has shown the indoor air is free of any ambient mold from wall construction. [4]

One of the reasons why fungicides aren't commonly used in straw bale construction is often their chemical origin. The use of natural material like straw leads to a higher environmental sensibility. Once people choose straw bales for the construction of their home, it is quite easy to employ other natural materials (like timber, earth, etc.) in the creation of non-toxic indoor environments. In the eyes of the occupants of straw bale houses, fungicide could be a poison which could spoil the "healthy home" objective. Nevertheless fungicides like borax, boron, and lime are of natural origin. [5]

The R-value of a building material is the building materials ability to insulate. Tests done in North America and Canada show that the R-value of a 12 inch Clay and straw slip wall is R-19, this is with a density of 40 to 45 pounds per square foot. Meanwhile a 6 inch fiberglass wall about equals the R-value of the 12 inch clay and straw wall. This is for most of the country a sufficient R-value for an insulating wall to pass energy code. There is one more point I'd like to make and that is that the internal wooden structure of the wall is not adding to the insulation capabilities. Options include placing less internal wooden studs inside the wall would increase the walls thermal capabilities. Just have to make sure the building regulations are met and the strength of the wall is not jeopardized. [6]

The amount of clay you use in your straw walls heavily affects the insulative properties of the wall. The more clay you use the stronger the wall is, however it becomes less insulative because there is less area for air and thus a smaller air barrier. The same applies to the straw and to what level you pack it down. The more you pack it down the less room there is for air and the stronger it gets. [7]

A clay and straw slip wall is dense enough that the fire can not burn in an almost oxygen free environment. Tests were completed on a straw wall, a clay and straw slip wall, and a timber framed and cladded building. These mediums lasted for 30 minutes, 2 hours, and 8 minutes in the fire respectively. Straw bales can not contain a moisture content of more than 15%. This can be managed with the use of an appropriate foundation, overhangs, gutters, and splash rocks. Splash rocks are placed at the bottom of the wall covering the foundation, this helps protect the foundation and lower wall from getting wet.[8]

To build with straw bales requires a good understanding of the basic characteristics of straw and how it behaves as a building material. In general, straw and other natural fibers have low compressive strength but when twisted, interwoven, bundled, baled or combined with other materials like clay, their compressive strength improves dramatically and they can then be used for a variety of structural and non-structural building applications. However, it is important to remember that despite the improvement in strength, bales do compress under loads. The more compact the bale, the less it will compress. Bales you use for building should be solid, compact, and keep their shape when you handle them, especially if they will be asked to support the kind of bearing walls that we will ask them to.[9]

Another very important factor to the overall comfort experienced in a clay/straw building is the role of thermal mass. When walls containing thousands of pounds of clay are heated by the sun, they will very slowly transfer the heat from the outside in, and by nighttime they will have a warming, moderating effect on the interior. Likewise, at night these walls will slowly cool, transferring the "coolness" to the interior during the heat of the day. This delayed reaction that works to our comfort advantage is called the flywheel effect... In colder climates, where there is a long heating season, the thermal mass is also advantageous. When the space is heated the walls absorb and store the heat, creating a source of additional radiant heat and providing "surround comfort". [10]

As straw bales become increasingly popular as material for building homes, outbuildings, and commercial and industrial buildings, they also provide interesting new opportunities for agricultural engineers. This "all American" technology was born on the treeless plains of Nebraska but has spread around the world. Many people are straw-bale-building enthusiasts because the structures are very quiet, energy-efficient, strong, durable, attractive, and fire-resistant. Combining straw bales, insulation value (U.S. R-30 to 40), solar orientation, and climate-adapted design features can create very comfortable and extremely efficient buildings. In Mongolia, straw-bale buildings have reduced energy use by 80 percent. They are also "friendly" to build, as families and communities often work together to create their own homes. [11]

Thermal insulation of building walls has a significant effect on the reduction of thermal energy consumption in buildings that leads to the reduction of CO2 emissions. Making a thermal insulation of a building external wall can in terms of economic aspects be approached as an investment. In this investment the cost is related to the purchase, transport and laying the insulation, whereas the profits are linked to the reduction of thermal energy consumption necessary to heat a building. The author determines the optimum thickness of the insulating layer that gives the maximum net present value of thermal insulation investment. Several versions of thermal insulation are presented. The following criteria were taken into account: energy sources, wall constructional materials and insulating materials. Bearing the sustainable development paradigm in mind, the best possible thermal insulation versions were determined by means of a two-criteria optimization for the economic and environmental criterion. [12]

Straw bales are a healthy choice. They do not contain the paints, chemicals, glues and toxins Combined with clay and lime renders and natural paints or oxides to finish the structure, straw bale walls can breathe and provide a natural, fresh and healthy living environment. The thick walls seal out noise. [13]

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 [14]


Initially our deadline for this project was to be completed before spring break, but unfortunately our plans weren't OK'ed by the school administration until about five weeks from the end of the semester, which was the ultimate deadline. We got the frame up within a day because we had built it on the ground prior to standing it up and installing it. It then took us until the last weekend to completely fill the wall with straw and clay. The reason for this was because of our need for the straw and clay to dry in order to build more on top, which was hard because of Humboldt County's ever wet weather conditions. The windows and bottle bricks were also installed during this last weekend of work. Unfortunately our group did not get to the plastering of the wall because of this delay.


This is the cost table of what our group spent and what would be needed to complete our project. Unfortunately we weren't privy to some of the purchases made by CCAT staff used in our project and thus don't have an exact total.

Quantity Material Needed Source Cost Total Cost
3 Straw Bale CCAT $0 $0
1/2 Local Seagrass Bale Donated $0 $0
A lot High Content Clay Soil CCAT site $0 $0
2-4 Shovels CCAT $0 $0
Many Hammer CCAT $0 $0
A lot Nails Recycled from CCAT & Ace Hardware $40
A lot Screws CCAT & Ace Hardware $11
1 Power Drill CCAT $0 $0
22 for our specific design Recycled Glass Bottles to let in light Classmates, Group members, and CCAT $0 $0
30 12' 2x6 FSC Lumber Acquired through CCAT  ?  ?
1 Toothed drill bit for putting bottles in wall CCAT $0 $0
Sifter for Sifting Clay CCAT $0 $0
6 tubes Caulking Ace Hardware $3 $20
1 50ft roll 8" Aluminum Flashing Ace Hardware $21 $21
2 Glass Doors CCAT (recycled from HSU) $0 $0
Total $92


Frame- 9 ft x 24ft Slurry mixture- Straw Slip Test Bricks-

Test Bricks[edit]

After obtaining our design from the engineer that CCAT was working with we made our test bricks. We made four test bricks using scrap wood from CCAT. We measured the wood, cut it with the saw at CCAT, and screwed the sides. We left one side open on each in order to pack the slurry into the wooden box and also for proper drying purposes. Something that we learned during this process is that in order for the straw slip to dry properly and quickly we had to drill holes in the sides of the wooden frame before we packed the straw slip. The ratio that we thought worked the best for our project was 2 parts clay to 1 part straw. When we started making our test bricks we thought the ratio was clay to water but Professor Grafman corrected us by letting us know that the ratio was clay to straw and the water was just the magic that we added to our clay slurry in order to get the right texture.


CCAT was in charge of the foundation for this project.


Originally our group member Max Stonewater planned the frame in order to have a very minimal amount of waste. Unfortunately our lumber order was some how interpreted wrong therefore we did not acquire the lumber desired by our group. Due to this issue we had to build the frame by cutting and nailing two identical frames. Then we connected the frames when we placed the wall onto the foundation.

Preparation for Packing[edit]




Bottle Bricks[edit]

Foundation Problems & Dismantling[edit]

Unfortunately, due to an incorrect measurement, the concrete foundation that was poured by the CCAT Physical Site team was six inches (6") too small for the pre-fabricated greenhouse and had to be re-done. This required digging another trench on the north side of the straw slip wall, drilling into the existing foundation, inserting 3/4" rebar and pouring another cubic yard of concrete. Furthermore, the straw-slip back-wall had to be shifted on to this new foundation which led to some challenges. During the time it took to create the new foundation, the unsealed straw slip became exposed to the elements and began to degrade. Additionally, when we planned how to move the wall, a boom-fork-lift was our only option for the space and the straw and clay mixture (described above) added a huge amount of weight making it unfeasible to lift. Therefore, after much deliberation, the CCAT physical site team decided to knock out the straw-slip insulation and bottle bricks in order to lift the wall and place it in the correct location.

Attempt 2: Spring 2013[edit]

This project was remade in Spring 2013 by Elisabeth de Jong, Cheyenne Celada and Jacob Ferdman. See the writeup at CCAT beachgrass slip wall.


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  2. Mack, Peter, Magwood, Chris and Tina,Therrien.2005.More straw bale building: a complete guide to designing and building with straw.Canada:New Society Publishers.
  3. Snell, Clarke, and Tim Callahan.Building green: a complete how-to guide to alternative building methods. Asheville: Lark Books, 2005. Print.
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  5. Wihan, Jakub. "Humidity in straw bale walls and its effect on the decomposition of straw." University of East London School of Computing and Technology 1 (2007): 31.
  6. Baker-Laporte, Paula and Robert, Laporte.2005.EcoNest: Creating Sustainable Sanctuaries of Clay, Straw, and Timber.Utah:Gibbs Smith, Publisher.
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  8. Mohamed Salah Gharib Elsayed.2000.Straw Bale is Future House Building Material.Egypt.
  9. Steen, Athena Steen, Bill. Mother Earth News; Dec95/Jan96, Issue 153, p40, 9p, 17 Color Photographs, 2 Charts.
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  14. 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.