Straw Bail House
We are a group of students, 3 from the University of Chicago, 1 soon to begin college in Germany. We obviously did not invent straw bail construction - in our research we simply discovered it was perfect to fulfill the goals of the 300 house challenge.

Solution

The Design

The walls of the straw bailed house will, as the name suggests, be constructed from rectangular bails of straw that are laid like bricks. Once in place, the straw walls will be coated with clay, stucco, plaster,  or a similar material depending on local availability and costs. The shape of the house will either be square or rectangular depending on the preferences and constraints of the owner. A supporting frame for the walls will be constructed out of timber or rebar, whichever is locally available and cheaper.  The roof will be constructed using corrugated iron. The house will have a door and 1-4 windows, the location of which will depend on preferences of the owner/builder.  The house will be constructed on a raised platform supported by sturdy wood or cement posts.

The Winning House Must Be�

Low Cost: straw bails are an extremely low cost building material. Labor cost is assumed to be 0 due to the ease of construction of straw bailed houses (1). Corrugated iron is already an extremely common building material in slums indicating its low costs and availability for the $300 houses target market (2).  We found estimates of the cost per square foot for straw-bailed houses in the United States in the range of $5-$20 per square foot (3). Taking the low end of this range as a fair estimate for the costs of building a simple straw bail house in a developing country puts a 60 square foot house within the $300 budget.

Self-built or self-improvable: As mentioned above, construction is possible with only low and medium skilled workers meaning that, with some direction, people are able to build their own straw bail houses.  Similarly, there are few obstacles to prevent the construction of additional rooms as the owners needs and means change.  

Low-tech: Materials such as straw, corrugated iron, wood, and clay/plaster/stucco molding fulfill this requirement.

Local materials: As the design portion of our proposal makes clear, the exact choice of materials is flexible depending on local circumstances. Straw should be available wherever agriculture is present.

Build greener: straw is obviously a renewable resource making the straw-bail house environmentally friendly. Moreover, due to the excellent insulation provided by straw, energy expenditure required to heat and cool a straw house is lower compared with conventional housing (4).

Replicable. The ease of construction combined with the flexible and local of materials makes the straw bail house extremely replicable.

Constructed with durable material that will resist the elements for 50+ years: Some straw bail houses built in Nebraska 100 years ago are still standing today (5).

Resistant to fire, storms, earthquakes and other natural disasters: straw bail houses perform extremely well during earthquakes and for that reason is an ideal material in earthquake prone regions (7). Due to the fireproof nature of the plaster/stucco coating, straw bail houses are fire resistant (6)(8). The fact that straw bail houses constructed over 100 years ago in the Great Plains region of the United States are still standing attests not only to the houses durability but also to their ability to survive the occasional severe storms that occur in the plains region (8).

Access to light, electricity and water:  Windows are easily worked into straw bail construction offering light, and connection to the electricity grid is common with straw bail construction in the United States (9). The corrugated iron roof can be used in conjunction with a simple gutter or drainage system to collect rainwater.


Risk Analysis: Summary

While we are concerned about the risk that moisture could pose to the structural integrity of straw bail homes in high relative humidity climates, we believe that for the vast majority of the time (704 out of 720 months), the relative humidity might not be high enough to cause the moisture content to cause the straw bail to begin to degrade. Furthermore, even if there are a dozen non-consecutive months over the 50 year lifetime of the straw bail homes where the relative humidity is high enough to cause the moisture content to cause the straw bail to begin to degrade, we believe that our building recommendations will help insulate the straw bail from the intense moisture during those periods. Finally, when it comes to insulation, fire resistance, earthquake resistance, durability, sustainability, ease and safety of assembly, material availability, aesthetic appeal, and price, straw bail homes are likely the best option. 

The Effect of Moisture on the Structural Integrity of Straw Bail Homes:

If moisture content rises above 25% within straw bail, degradation of the straw bail will commence (10). If this degradation continues for an extended period of time, it can have a material adverse effect on the structural integrity of straw bail. Research suggests that moisture content is an exponential function of relative humidity (10). Furthermore, many of the locations where our straw bail homes will be built are subject to high relative humidity. Therefore, in order for our idea to be a viable option, we must first insure that the moisture content within our straw bail will not exceed 25%. To do this, we will first analyze the relative humidity and estimated moisture content in six sample countries where we imagine our straw homes might be built and where the climate is similar to other countries where we might also build. We will then make recommendations as to how our home builders can minimize moisture content in high risk places and during high risk periods. The high risk places are the places on the walls where water and moisture are most likely to seep in. These places are the bottoms of the walls, the tops of the walls, and the surfaces of the walls. The high risk periods are the periods during the life of the walls when water and moisture are most likely to seep in. This period is during construction.


Recommendations as to How Straw Bail Home Builders Might Minimize Moisture Content in High Risk Places and During High Risk Periods:

    Our research shows that for the vast majority of the time, the moisture content of the straw bail will not exceed 25%, and therefore there is limited risk of straw bail degradation due to moisture content. Over the last ten years, there were at least 16 months in Haiti, India, Brazil, and Kenya when straw bail degradation could have begun due to moisture content�s being 25% or greater. Given this, we must recommend ways that home builders in Haiti, India, Brazil, and Kenya might minimize moisture content.
    Before we make our recommendations, we must first understand the places and periods where moisture is most likely to infiltrate the straw bail walls. The places are the bottoms of the walls,  the tops of the walls, and the sides of the walls. The period is during construction.
    In order to reduce the risk of moisture�s infiltrating the straw bail walls during construction, we recommend straw bail home builders test the moisture content of each straw bail brick before they build using a inexpensive, handheld moisture content reader. We also recommend straw bail home builders do not construct straw bail homes at times of the year when the moisture content could likely go above 25%. Handheld moisture content readers cost between $30 and $130 and can be shared among dozens of home builders. In 704 out of 720 possible months, the estimated average moisture contents in Haiti, India, Brazil, and Kenya remained well below 25%. Therefore, we believe both recommendations are highly practical.
    In order to reduce the risk of moisture�s infiltrating straw bail walls from the bottom of the wall, we recommend keeping the base of the straw bails at least 350 mm above finished ground level or 250mm above a permanently paved strip around the base of the walls that will keep moisture and plants away. We also recommend water proofing the top of the footing (11). We also recommend providing a damp-proofed toe up of at least 50mm above the floor to keep the base of the walls safe from any internal flooding caused by water leaks or sitting water if the roof is not built first. Due to the low cost and high availability of water proofing materials, we believe this recommendation is highly practical.
    In order to reduce the risk of moisture�s infiltrating straw bail walls from the top of the wall, we recommend building a humidity barrier around the top of the wall and covering it with lime and a water repellent (12). Due to the low cost and high availability of lime and various water repellents, we believe this recommendation is highly practical.

In order to reduce the risk of moisture�s infiltrating straw bail walls through the side of the walls, we recommend building the home walls with three coats of lime and then covering the walls with water repellent at least once a year (12). Due to the low cost and high availability of lime and various water repellents, we believe this recommendation is highly practical.

Analysis of Relative Humidity and Estimated Moisture Content in Six Sample Countries:

First and most importantly, we must estimate the moisture content of the straw bail based on the relative humidity in each of our six cities. To do this, we will use a function developed by researchers at the University of Bath in the United Kingdom. The function was published in the researchers� 2009 peer-reviewed paper, Determining Moisture Levels in Straw Bail Construction (10). The following is a graph of the function.


Relative Humidity vs. Moisture Content


The following are graphs of the average monthly relative humidity over the last ten years in Haiti, India, Brazil, Kenya, Mexico and Egypt. With each graph, there is an explanation of the estimated average monthly moisture content and the estimated maximum average monthly moisture content. All of the following relative humidity data was collected via Wolfram Alpha on May 25, 2011 (13).

It is important to note that the data we used was collected by measuring the relative humidity at one location in each of our six sample countries over the last ten years. For Haiti, that location was Maria Montez International Airport. For India, that location was Dr. Babasaheb Ambedkar International Airport. For Brazil, that location was Cachimbo AB General Airport. For Kenya, that location was Nairobi International Airport. For Mexico, that location was General Leobardo C. Ruiz International Airport. For Egypt, that location was Assiut Airport. It is possible that in other parts of the six countries the relative humidity varied materially from the relative humidity levels recorded in our data. Therefore, given our data, we cannot and do not claim that the estimated moisture content in the regions we plan to build will not regularly cause our straw bail homes to degrade. Given our data, we can only hypothesize that the estimated moisture content in the regions we plan to build might not regularly cause our straw bail homes to degrade.  Before building straw homes in any area, we would need to conduct a much more thorough analysis of the local weather and building conditions.

Average Monthly Relative Humidity vs. Time in Haiti

The average monthly relative humidity in Haiti corresponds to an estimated average monthly moisture content of 17%. The maximum estimated average monthly moisture content during the last ten years was 25%. There are many months during each year when the estimated average monthly moisture content is between 15% and 18% and straw bail homes could safely be built.

Average Monthly Relative Humidity vs. Time in India:

The average monthly relative humidity in India corresponds to an estimated average monthly moisture content of 9%. The maximum estimated average monthly moisture content during the last ten years was 25%. There are many months during each year when the estimated average monthly moisture content is less than 15% and straw bail homes could safely be built.

Average Monthly Relative Humidity vs. Time in Brazil:

The average monthly relative humidity in Brazil corresponds to an estimated average monthly moisture content of 13%. The maximum estimated average monthly moisture content during the last ten years was 25%. There are many months during each year when the estimated average monthly moisture content is less 15% and straw bail homes could safely be built.

Average Monthly Relative Humidity vs. Time in Kenya:

The average monthly relative humidity in Kenya corresponds to an estimated average monthly moisture content of 17%. The maximum estimated average monthly moisture content during the last ten years was 27%. There are many months during each year when the estimated average monthly moisture content is between 15% and 18% and straw bail homes could safely be built.


Average Monthly Relative Humidity vs. Time in Mexico:

The average monthly relative humidity in Mexico corresponds to an estimated average monthly moisture content of 8%. The maximum estimated average monthly moisture content during the last ten years was 18%. There are many months during each year when the estimated average monthly moisture content is less than 15% and straw bail homes could safely be built.

Average Monthly Relative Humidity vs. Time in Egypt

The average monthly relative humidity in Egypt corresponds to an estimated average monthly moisture content of 7%. The maximum estimated average monthly moisture content during the last ten years was 13%. There are many months during each year when the estimated average monthly moisture content is less than 10% and straw bail homes could safely be built.

In 704 out of 720 Months, the Estimated Average Moisture Content of Our Six Sample Countries Did Not Exceed 25%.

The average estimated moisture content of our six sample countries over the last ten years was 11.83% with a range of [7%, 17%]. That is, all of the average estimated moisture content values were well below the 25% threshold. In 120 months, there were two non-consecutive months where the estimated average monthly moisture content in Haiti reached 25%. In 120 months, there were two non-consecutive months where the estimated average monthly moisture content in India reached 25%. In 120 months, there were six non-consecutive months where the estimated average monthly moisture content in Brazil reached 25%. In 120 months, there were six non-consecutive months where the estimated average monthly moisture content in Kenya reached 25%.

Haiti, India, and Brazil all had maximum estimated average monthly moisture contents of 25%. Kenya had a maximum estimated average monthly moisture content of 27%. Mexico and Egypt had maximum estimated average monthly moisture contents of 18% and 13%. This suggests that in each of the locations where the data was recorded in these two countries, over the last ten years, moisture content would likely not have caused straw bail to have begun to degrade.

Overall, our data suggests that in at least 16 out of 720 months, the estimated moisture content in our six sample countries could have caused our straw bail homes to degrade.


Notes:

1. Ease of construction is commonly considered one of the primary benefits of straw bail houses.

2.  Although this point leads little supporting evidence for anyone who has seen an image of a slum, see, for example, an image from Mathare slum in Kenya http://en.wikipedia.org/wiki/Mathare

3. http://www.oberlin.edu/news-info/98sep/strawbailhistory.html

4. Nehemiah Stone  �Thermal Performance of Straw bail Wall Systems.� Ecological Building Network. Oct. 2003.

5.  Mark Morgan. �Straw Bail Construction.�  Midwest Renewable Energy Association Fact Sheet.� 1999.

6. Minke, Gernot, and Friedemann Mahlke. Building with Straw : Design and Technology of a Sustainable Architecture. Basel ; Boston: Birkhäuser, 2005. Print.

7.  A video showing a straw bail house on a shake table that remains intact after an strong earthquake simulation. http://www.youtube.com/watch?v=x8Uz-2PonEk

8. http://strawbail.sustainablesources.com/

9. http://www.strawbail.com/strawbail-faqs#electrical

10. http://www.sciencedirect.com/science/article/pii/S0950061809000919#secx13

11. �Guidelines For Strawbale Building In New Zealand�. Build Right 98 BUILD. G.North, R.Walker, B.Gilkison, N.Crocker, A.Alcorn, T.Drupsteen BRANZ, 1998

12. <a href="http://www.homegrownhome.co.uk/pdfs/StraubeMoistureTests.pdf" rel="nofollow">http://www.homegrownhome.co.uk/pdfs/StraubeMoisture_Tests.pdf

13. Data was collected on May 25, 2011 via Wolfram Alpha (http://www.wolframalpha.com) with the query �Relative Humidity �Country Name��

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