Straw Bale House in the City, Part 3: Costs and Straw Bale Density

Reader Contribution by Adam D. Bearup and Hybrid Homes
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The Straw Bale in the City 3-part series walks readers through the codes, costs, and construction of a straw-bale build on a non-rural property. Part 1 covers issues with building codes and managing expectations of city inspectors. Part 2 looks at straw bale design through the lens of a test wall, while in Part 3, readers learn the density and moisture considerations that can plague straw-bale construction without proper planning while also covering costs of this project.

I had a somber tone as we drove back from the hay lady Laurie’s barn full of straw. I couldn’t believe that we had went to a completely different supplier and found exactly the same density straw bale. I used my phone to search for dense straw bale suppliers and found only one supplier out of New York. I wasn’t sure what to tell the developer, who so badly wanted us to build a straw bale house for her company. I hadn’t been able to locate a third party engineer to work with us during construction of the straw bale house, nor had I been able to find straw bales dense enough to meet code.

As we drove back to our Farm, Bob and I started discussing how a straw bale would have to be made in order for the straw bale to meet the building code for density. I reviewed the Michigan Residential Building Code Book’s section on straw bale construction and under the section labeled “Density”, it read:

Bales shall have a dry density of not less than 6.5 pounds per cubic foot. The dry density shall be calculated by subtracting the weight of moisture in pounds from the actual bale weight and dividing by the volume of the bale. Not less than 2 percent and not less than five bales to be used shall be randomly selected and tested on site. (2015 Michigan Residential Code, Page 840, Section AS103.5 Density)

At first glance, the way that the code is worded seems confusing; however, the formula is relatively simple once you understand how the formula works. Below is an actual entry from the notes that we gathered as we tested our straw bales during the mock up:

Straw Bale #3

Moisture Content of Bale: 13.8%

Weight of Straw Bale: 26 lbs

Dimension of Straw Bale: 35 inches by 19 inches by 13 inches

Volume of the Straw Bale: 35 inches by 19 inches by 13 inches = 8,645 cubic inches

Convert cubic inches to cubic feet: 8,645 / 1728 = 5.002 cubic Feet

Weight of moisture in pounds:  26 pounds – 13.8% = 22.412 pounds

 26 – 22.412 = 3.588 pounds of moisture

To get our pounds per cubic foot number, we follow the formula in the code:

Actual Bale Weight 26 pounds – Pounds of Moisture 3.588 = 22.412 pounds

22.412 / Volume of Bale 5.002 = 4.48

The code requires this number to be at least 6.5!

Have I lost you yet? Take a moment to look these numbers over again so that you can understand how the code instructs us to calculate the density of the straw bales. This formula can make or break your project, and it is very important for you to put testing density of the straw bales that you are going to use at the top of your list of priorities.

We will discuss finding dense enough straw bales shortly, for now, let’s see if it is possible to create a bale that is dense enough to meet code.

The hay lady Laurie kept referring to her straw bales as “Three Dollar Bales”. What does this mean? Laurie told us that the straw that goes through the auction has to compete with other straw from other farmers, so the straw bales will all be baled close to the same way and cost $3.00. The machines that bale hay and straw can be adjusted to make denser bales if the farmer chooses to.

Bob and I started to discuss what we would have to do to take a three dollar bale and compress it to where the bale would meet the code for density. We can manipulate any or all of the numbers in the formula that we used above to help us determine what a bale of straw would need to measure and weigh in order to meet the code for density.

After Bob did that math, he noted that we would have to compress our bales from 35 inches in length down to 21 inches in length in order to get our numbers to come out to the 6.5 pounds per cubic foot number.

I finally sent the developer an email stating that we could not find dense enough straw bales for her project. She responded with a message that said that she has been emailing back and forth with a professor from the University of Michigan who had built a straw bale house with his students. She said that she would ask him where he got their straw bales and maybe we could use his source and get dense enough straw bales through them. I eagerly awaited the professor’s response; maybe we could build this straw bale house after all!

The response that she got from the professor left me scratching my head. The professor responded to the developer’s email by saying, “I used pretty bad bales on our house actually – they came from farmer next door – I didn’t worry too much since I compressed my walls….”

I have not researched this professor nor have I researched his project but I can say that knowing that an average straw bale would need to be compressed from 35 inches down to 21 inches in length leaves me with a few questions about the professor’s project.

The compression would have to come from the ends of the bale, pushing the straw together (this is how the baler machine compresses straw into bales then ties it), as opposed to the top where the compression would be pushing down on the bales. The professor compressed the bales from the top side from what I was told and I question if he was able to create dense enough bales using this method. Maybe they could, but verifying the density of the bale, as required by code, would not be possible while the bale is compressed in the wall assembly.

In conclusion, I feel that it is important to mention again, that the very first thing that a person should do if they are getting serious about wanting to build a straw bale house, is to check local and regional sources for dense enough straw bales.

The best option would be for a person to locate a farmer that bales straw and talk to that farmer well before straw is baled to see if the farmer will make a specific density bale for a straw bale house.

If you can’t find a local farmer who will work with you, you may be able to find a company that will ship you dense enough straw bales from some other part of the country. In either case, you should figure $9.00-$15.00 per bale plus shipping when you are figuring out the cost of straw bales for your project. That is considerably more than the $3.00 per bale figure that we were told.

Best of luck to you if you decide to build a straw bale house!

Part 1 covers issues with building codes and managing expectations of city inspectors. Part 2 looks at straw bale design through the lens of a test wall.

Adam D. Bearup is a designer, green builder and farmer, who learned about biodynamic and regenerative farming for a project he built in Northern Michigan, The Earth Shelter Project Michigan. Adam has degrees in marketing and management and a Masters of Science in Green Building. Read all of his MOTHER EARTH NEWS posts here.


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