Straw Bale Construction/Techniques/Walls

WallsEdit

Straw Bale InfillEdit

The original "Nebraska" straw-bale building technique was one in which walls of straw-bales actually provided the support for the roof-structure above, so these are now referred to as load-bearing, and straw-bale homes of this style continue to be built and permitted.

An alternative method of construction uses a post and beam framing system to carry roof, wind and seismic loads. Once that structure is in place, the walls are then infilled with straw bales for insulation. This type of structure is popular because it allows bale placement to be accomplished with the roof already in place, "in the dry", and can easily be demonstrated to conform to building codes, using conventional engineering techniques or a pre-engineered pole-structure design.

Some projects best lend themselves to a combination of both techniques, with load-bearing perimeter walls and pole or stick-frame support at the interior or ridge; this is termed a "hybrid" structural system.

The building code in the State of New Mexico (1994 ed.) required that all straw-bale homes there be built with rigid structural frames, while other state or regional building codes lack this restriction (see codes for California, Pima County Arizona, etc.) In other jurisdictions without specific "straw-bale codes", strawbale construction is often approved under the building code provisions for alternate methods and materials. Plans are commonly required to be stamped by a licenced structural engineer.

Field bales are often laid in stretcher bond like bricks. They are easily retied to make half or custom sized bales. They may also be easily "pinned" internally or on both surfaces (with bamboo, reed, rebar or wood).

Bale stacking is often done in community "bale raisings", where family and friends pitch in together to raise the walls in a weekend or two. Novice owner/builders and their friends can continue the work through lathing and plastering of the bales, giving the house their own special imprint, and achieving savings in construction costs, as well.

Load Bearing WallsEdit

As in the original Nebraska straw bale homes, bales are so compact that they can succesfully be used as the structure of the building itself. Strictly speaking it is the outer surface of the bales which provides most of the structure. This matrix of straw fibres on the surface of bales is locked together by the stucco of whatever plaster is being used. Much like the reinforcing bars set into concrete, but over the whole surface and pointing in all directions.

Wooden stakes are pounded vertically into the straw bales. They connect the layers of bales, adding stability to the wall. Popular choices are hazel and bamboo. Sometimes, metal rebar stakes are also used. Metal rebar should be avoided because temperature differences between the metal and the bales cause condensation inside the bales.

Vertical compression of the bales into one surface is just as important for stability. The most popular way to do this is to run plastic straps vertically around the bales, compressing them between the top plate and the bottom plate. This compression not only strengthens the structure. It also helps the load bearing wall settle much more quickly, enabling a quicker installation of windows for example.

The last and least effective option to reinforce (citation needed) is to "cage" the bales on one or both faces with pre-welded or woven mesh, to increase pre-stuccoed wall stability. Do avoid using metal mesh, as it can crack the surface render. This is both because of rust and temperature differences between the organic materials and the metal. These temperature differences also causes condensation inside the bales.

The finish you use also has a great effect on the structural integrity of the finished wall. See the section on finishes.

Curved WallsEdit

An example of a tightly curved wall with flat bales (project: John Swearingen)

"Curved walls are fun, pleasing to the eye, and create glorious light patterns. But they are deceptively time consuming! I can build three flat walls for the price of one curved wall. And it has all to do with the foundation, curbs, window bucks, window flashing, roof details." (Straw Bale contractor Frank Tettemer of Living Sol)

As the above quote points to, time, and details, are an important consideration when deciding if your building will have any curved walls. How will you put the gutter on, what about the roof structure, the foundation? Some people also find any aesthetic advantages outweighed by the problems of using the rounded shapes on the inside. So, what needs to be considered?

For gentle curves the bales can be laid against a wall and kicked, as you would if you were breaking a small branch. This can be done with bales laid flat or on edge. Of course it's best if the bales on all walls are lying the same way, but it's not a strict necessity. For larger walls flat bales would be more prudent, especially if the wall is bearing some weight.

Bales placed on edge (largest face outwards) can be shaped well before placing into the wall, and hold their shape well. (The insulation value is almost the same as for bales laid flat.) If the curve is very tight the exposed strings could be a problem. Any such problems are solved if you use some form of surface mesh on both sides of the wall (plastic or metal) which you tie to each other through the wall.

The round bale layout results in pie-shaped gaps between the bales. These are best filled with a mixture of clay and straw, the clay serving to hold the straw together. Mesh on the outside of the wall will add additional restraint to the tendency of the bales to "explode" outwards. (for discussion see John Swearingen).

An additional way to increase the strength of a curved wall is to add large horizontal straps to each row of bales on the outer face, fixing these to something stable. Curved walls are, by their geometry, inherently less prone to overturning than straight walls.

The composite of mesh (tension) and plaster (compression), along with the geometry of the wall, can result in a very stable and strong building, if the continuity of the bale wall isn't broken by large openings.


References

Example of a round building: [1]

Structural Capabilities of Bale WallsEdit

The bale assembly can do a number of things, depending upon the structural design of the building:

  • Hold itself up, be self-supporting and resist tipping.
  • Keep out the wind; inhibiting air/moisture infiltration.
  • Resist heat transfer (insulation)
  • Reduce water intrusion and migration, store and transfer moisture within the wall.
  • Keep the assembly from buckling, under a compressive load.
  • Keep the assembly from deflecting in a strong wind, when pushed from the sides or end.
  • Keep the assembly from bursting apart in an earthquake, when pushed and pulled from all directions.
  • Hold the plaster at least while it’s curing.
  • Keep the plaster from cracking after it’s cured, from shrinkage or movement.
  • Support the plaster skins from buckling.
  • Transfer and absorb loads to and from the plaster.
  • Support the roof load (compression).
  • Reduce damage or failure from high winds (ductility).
  • Reduce damage or failure from earthquakes (ductility).
  • Stop bullets and/or flying debris.
Last modified on 30 September 2013, at 20:47