Straw Bale Design- Choosing the Right Size Straw Bales
By Chris Keefe
Well the first question is what kind of bales: two or three stringers?
There are many ways to answer this question, and just like the rest of the straw bale design field, you have to see what you have in front of you. How much space can be afforded to the bale wall? What kinds of bales are available to your client locally? Is the climate hot, moderate or cold?, and What kind of man, woman, or machine power will be doing the construction?
These questions could be answered separately but the plain fact is that they must all be addressed as a whole before proceeding at least past the schematic design phase. To begin with, the three stringers tend to be roughly 24″ wide X 16″ high X 48″ long and the two stringers are roughly 18″ wide X 14″ high X 36″ long. [see figure 1]
If you are designing a smaller building, the two stringers may be the better choice to allow for more interior floor space. If it is a larger building the three stringers may be the better choice for reasons of proportion. It is hard to tell, and the answer may lie simply in the availability of the straw bales.
Finding the bales locally will probably save money in delivery costs for the client, and will help support a local farmer. Next, if the local climate is really cold, the three stringers, being thicker, provide a higher R-value, but they are very heavy, weighing as much as 80 lbs. This could become overwhelming for even the strongest of backs. Two stringers, not being as large are not as thick, so they won’t give you quite the level of R-value as three stringers but it will be much higher if compared to conventional construction values. Plus, with a two stringer weighing around 45 lbs., moving them from here to there is much easier on the back muscles.
Back to the Drawing Board
So, once the appropriate bales have been chosen and found, the next step is to apply the actual dimensions of the bales to determine door and window locations, top plate height, and overall building dimensions. This practice will cut down the amount of bale resizing and stuffing, will save a lot of time and money in labor costs, and additionally it will insure a more solid and tight building.
As an example to convey this idea, we will use a post and beam structure, place windows, doors, and top plate on the exterior plane of the building upon a 3-1/2″ 4X4 sill plate, and use a two string bale size of 14″ Height X 18″ Width X 36″ Length. The bales will be stacked in a running bond, or the way that bricks are stacked. Beginning from the corner, door and window openings should never be less than one bale length (one and a half for load bearing) from the exterior corner of the wall, or in our case 36″. This creates a solid corner to help stabilize the wall system. It is also a good idea to have at least one bale length between openings which again reinforces the integrity of the wall system.
The window sill height should correspond with the dimensions of the bale heights. For example, our bales are 14″ in height, so including 3-1/2″ for the sill plate, a sill height of 31-1/2″ (two courses) [see figure 2], or 45-1/2″ (three courses) would be ideal because although you can resize the “length” of a bale relatively easy, resizing the “height” is a bit more tricky. Other options include placing a box beam over the bale course within the window frame to achieve the desired height but as I said, this adds labor and time.
For the top plate height, again it would be smart to base it on the bale’s height. It would be ideal to have all of the bale courses fit under the top plate. For our example, the wall will have seven courses of bales plus the 3-1/2″ sill plate which comes to 8′ 5-1/2″. Once this wall is raised, it must be compressed to give it more rigidity.
Keep in mind that an eight foot bale wall when compressed will probably shrink about an inch. It is a good idea to use the top plate to compress this wall either by notching it into the top bale, or by having the top course wedged snuggly under the bottom of the plate.
If the plate is notched into the bale wall, than the top of the plate can remain at 8′ 5-1/2″. [see figure 3] If the bales are wedged under the plate, than a higher wall is achieved and a box beam could actually be sistered on to the plate beam. And as a reminder, when dimensioning the framing, don’t forget to account for the lost inch in the bale wall after compression.
Finally, when it comes to determining overall dimensions of the building size, knowing the size of the bales can help greatly. Our example bale is 36″ or 3′ long, so sizing our building proportionally with that length (i.e. 24″ X 36″) [see figure 4], we can maximize the use of full bales and minimize the need to resize many of them to complete the wall.
A Final Thought
With all of this in mind I would like to heed caution to designers as they embark on this journey of straw bale design. These ideas are not absolute. Taken very literally they could act as constraints that result in a very predictable and boring design. It is the intention of the author to have them received as an awareness tool for creating a building that is efficient, beautiful, and well built, all at a reasonable cost. At the present time, the cost of labor to build a straw bale house is what drives the overall initial cost of new straw bale homes 20% higher than conventionally built homes. Smart designs which include the some of the ideas stated above, can bring down those labor costs by simplifying the process of construction, which overall will elevate the field of straw bale design and construction to the next level.
ABOUT THE AUTHOR
Chris Keefe, from an early age, discovered his creative spirit in art. In 1996, he received a B.A. in Liberal Arts focusing on drawing, Music and Philosophy from San Francisco State University. In tandem, he was actively involved in a grassroots environmental project for five years at the University of California in Berkeley. He became interested in the field of straw bale as he began his graduate studies in 1999 at the San Francisco Institute of Architecture. Focusing to integrate his work in the environmental and sustainability field with his creative imagination, he received a Master of Architecture and Ecological Design in 2001. Soon thereafter, he founded a company called Organicforms Design which offers ecologically and artistically based design utilizing natural and sustainable materials. Since then, he has worked and completed several exciting design/build projects in Southern Oregon. In 2002, he began to focus primarily on straw bale research and design as the lead designer on the innovative project, The Straw Bale Village in Jacksonville, OR. Please visit his website at www.OrganicFormsDesign.com
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