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How To Construct A Straw Bale Structure

It would never occur to most people to look for construction materials in a farmer’s field. But when wheat, oats, barley, rye, or rice are harvested, the fibrous plant husks left behind are not just a useless waste product. When these leftovers, known as straw, are tightly bundled up into bales, these plant remnants can actually be used as a building material – and a durable and inexpensive one at that.

Straw bale construction has been growing by leaps and bounds, as more and more people have been discovering the versatility and utility of an old method of building that has suddenly become new again. In the nineteenth century, settlers on the Great Plains who did not have access to wood turned to squared bundles of straw as a replacement, and some of the homes they built are still standing to this day. Straw bale construction has been attracting the attention of off-the-gridders and intentional communitarians because straw bales are cheap, readily available, and provide the perfect raw materials for those with a desire to build something aesthetically unique.

Specifics of the Building Process

Straw bale structures are not made entirely from straw. The foundation and roof of such a building will be made of conventional materials, and it is only the walls that will be constructed from a series of straw bales piled on top of each other like oversized bricks. Bamboo pins or rebar are used to join the bales more firmly together, and inch to inch-and-a-half thick plaster is used over a supporting wire mesh to cover the inside and outside of the straw bale framework.

The most elemental type of straw bale buildings, which rely on what is called the load-bearing technique, use the walls to hold up a stiff, one-piece roof plate, distributing the weight of the top of the structure uniformly so that the downward pressure on each individual straw bale pile will be kept at a manageable level. An equally distributed load also prevents the strength of the straw bale framework from being compromised in localized areas where spaces have been carved out of the walls for windows, doors, and any other features that may need to be added. Load-bearing straw bale walls are only appropriate – and up to code – when used to construct one-story buildings with relatively modest ceiling heights, since too much vertical extension above the top of the first-floor bale piles can overburden them and lead to structural instability.

In-fill straw bale buildings put up a conventional framework of wood, concrete, or steel as a shell first, with notched and shaped straw bales used to fill in the empty space between the inner and outer walls. With this style, most of the major advantages of straw bales are preserved, while the strong surrounding shell provides the level of support required to construct multi-story structures with more complex designs and larger amounts of glazing.

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The Advantages of Straw Bale

Straw bales have a number of outstanding characteristics that make them an ideal wall-building material. These advantages include:

  • Strength – because the plant remnants in straw are packed so tightly, the density of a bale gives it an advanced ability to hold heavy loads without excessive compression, as well as the capacity to resist any serious lateral strain. Testing has shown that straw bale structures built using the latest techniques can withstand the force of even the strongest hurricanes or earthquakes.
  • Durability – most people just assume that organic materials will decay over time, especially if they come from something as apparently insubstantial as green plant stems. But oxygen, which drives the process of organic decay, cannot penetrate the thick interior of straw bales, and as long as they are kept dry as well as airtight, bales used in construction projects should be able to maintain their structural integrity indefinitely.
  • Energy efficiency – straw bale walls are excellent insulators. Up to three times as energy-efficient as conventional structures, homes and other buildings made from straw bales can reduce energy usage by as much as 50 to 75 percent.
  • Cost and availability – it may be possible to purchase straw from farmers for as little as $2 to $3 per bale. Every year, 200 million tons of waste straw are dumped or burned because no one knows what to do with it, so no one planning to build with straw bales should have to look very far to find farmers in their area looking to unload their excess bales.
  • Protection against fire – the thickness and impenetrability of straw bales makes them highly resistant to fire. In independent testing, walls made from standard materials were incinerated in just thirty minutes to an hour, while it took more than two full hours for fire to burn through walls made from straw bales.
  • Pest resistance – mice and insects cannot eat or burrow their way through straw bales, which leaves straw bale structures largely invulnerable to infestation.
  • Sound insulation – because of their width, density, and basic physical properties, straw bales have excellent soundproofing qualities.

The Drawbacks of Straw Bale

The biggest potential drawback with straw bales is that they are useless for construction if they become wet. Because they are so impenetrable to air, when bales become overly saturated with moisture they cannot dry and will gradually rot from the inside out, which could mean big trouble if the bales in question have been used in a building project.

At the time of purchase, straw bales should have a uniform, healthy golden appearance. If they are uneven and spotted with dark patches, this could be a sign that the bales are wet and that rot or mildew has already started to develop. Also, if straw bales are green instead of golden, this means that the crops they came from were probably harvested too soon, and generally speaking, the earlier the harvest, the higher the water content of the plants that are cut. Before a straw bale is actually bought and paid for, a hand-held moisture meter should be used to make a definitive determination; if a reading of greater than 14 percent moisture content is obtained, this means the straw bale is too wet and would not make a good choice for use in a construction project.

Once straw bales have been transported onsite, a high-quality plastic tarp should be used to keep them completely covered until they are ready for installation. The foundational platform upon which the straw bales will rest should be elevated six inches or more above the earth, and the ground outside the location where the new structure will be placed should be landscaped with a downward slope to facilitate effective run-off during rainstorms or snow melts. As long as the layer of plaster applied to the bales is at least an inch-and-a-quarter thick, it should do a perfectly good job of preventing moisture infiltration from either the outside or inside air, even in a climate with high humidity levels, and a roof overhang that can protect the straw bale walls from precipitation is also a good idea.

While exposure to moisture can corrupt straw bales, the good news is that it is relatively easy when using them for construction to create a watertight framework that will protect the bales and keep them dry for as long as the rest of the building maintains its structural integrity. Some believe that straw bale construction is only suited for dry climates, but if proper techniques are used, these types of buildings can last for a very long time in even the most humid environmental conditions.

Getting Started

Instructional videos and websites dedicated to teaching straw bale building practices are now available in abundance, which means that anyone interested in experimenting with this style of construction should have no problem finding sage and expert guidance to help them get started. While building a straw bale home might be the ultimate achievement, many choose to construct something smaller and simpler the first time out, like a shed, a vacation cottage, a small building to house animals, a garage, or an art studio, just to name a few possibilities.

Straw bale construction is the perfect example of serendipity in action. No one would have ever dreamed that what is essentially a waste product occasionally used as bedding for farm animals would turn out to be one of the world’s best building materials. And yet, straw bundled up in bales has proven to be just that. God and nature do indeed provide, in ways that are often very surprising and unpredictable.

©2012 Off the Grid News

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5 comments

  1. I am sold on straw bale as a building material. Having participated in a straw bale workshop where my husband and I helped build a straw bale home, and the following year visited the very contented owners of that home, we are opting to build with straw ourselves.
    We want to build a very green home.
    In our research we find there are two types of green:
    shiny green and earthy green, and we hope to incorporate the best of both as defined by the particulars of our project.
    We are working with Whole Tree Architecture and Construction, who has designed and are building us a home with an unmilled, branching timbers, timber frame house. The straw bales will be covered with plaster clay inside and lime stucco outside. It will have a sod roof. It will also have a cutting edge solar hot water infloor heating system.
    It’s quite a journey. I’m blogging about it at http://digginginthedriftless.wordpress.com, if you want to follow our progress. We are feeling our way along.
    But back to the original point. Straw bales are an amazing material. They have all the characteristics you listed so succinctly. I’d like to emphasize the point about local availability. That’s something we are aiming for as much as possible. Supporting the local community and especially the farmers. This is a big part of a truly sustainable project.
    Thanks!

  2. Hi Andrew,

    Yours is one of the best intros to straw-bale building I’ve read. Been studying it for 15 years.

    One point to ask about tho. Is it that vermin and bugs >can’t< burrow into the straw or is it that they have no interest as the straw offers no food value for them?

    I have been interested in learning more about sheltering the building project from the elements during construction. If you had any info to share about that, I'd be appreciative.

    Best,
    Tom

  3. I think the idea of a straw bale house is fine, however, I chose a different “green” method. After considerable research and a school in earthbuilding I chose compressed earth blocks. The outside walls of my home including stucco are 2 ft. thick and the cross walls are 15 in thick. Engineers tested this method and found that the walls have 5X the compressive strength of a brick wall and have 3 1/2 sideways shear strength of a brick wall. There is NO air filtration, no vermin or insect problem at all. There is no fire danger as electrical pvc conduit is laid down the center or the wall at the appropriate height and wiring pulled through after construction. The R value is probably about equal to straw bales BUT the effective R value is much higher due to the U value or mass of the walls. I applaud the straw bale builder but would suggest anyone interested take a good look at earthbuilding methods. I know that when I say “earthbuilding” most people immediately think of a house built back into the earth which is understandable but in error. Anyway, there is a lot more to be said about the advantages of good earthbuilt structures. Just thought I would add my comment to perhaps open thought processes a bit more.

  4. Hi,
    very good article.
    Was wondering if you know if a straw bale house could be built on a wooden platform?
    We are allowed to build anything which does not have a concrete base.
    Could see problems with moisture but would appreciate any tips you might be able to give me.

    Many thanks

    Terry

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