How to Build a Straw Bale House
Green Extreme: The Straw Bale House
To many people, the thoughts of a lovely home in a "good" neighborhood with a swimming pool and celebrity neighbors is the only way to live. Then, there are others who are friends of the earth, and make every effort to both fit in with nature and build a sustainable home that is energy efficient and low-cost. They build their homes using readily available materials in the building process. What was once a fad is now becoming more and more mainstream every year. The focus of this article will be on straw bale homes - a little history and a lot of facts.
Why Straw Bales?
Straw is a byproduct of the agricultural process of combining wheat, rye, barley and oats. As it is not nutritious for cattle, a lot of farmers in large agricultural settings destroy it - sometimes by burning. Smaller operations bale the straw to use as bedding for cattle and pigs.
Building homes from baled straw is nothing new. As early as the 1800's, settlers in Nebraska used straw to build homes out of necessity. There was little wood to build with, so the settlers used what was available from their wheat, barley and rye harvests.
But Are They Safe?
Most definitely. Straw on its own burns quickly, and is subject to rot from rain. The reason it works so well as a building material is because the construction, after initial build, is then covered with plaster made of several different materials. Adobe, mud, and papercrete are used to cover the finished interior and exterior walls.
Still think they wouldn't be safe? Guess again. Tests were conducted by Tony Perry, who owns Straw Bale Construction Management Inc. in Santa Fe, New Mexico. The walls of a straw built home were subjected to a furnace test, where the wall was exposed to 1,000 degrees Fahrenheit, then 1,750 degrees after 60 minutes. The home would "pass" the test if the interior wall temperature remained below 250 degrees and no flames penetrated the walls. As the temperature of the hot sideof the wall topped out at 1,940 degrees, the cool side temperature rose only 12.7 degrees. No flame penetrated the interior. Compare this to a wooden or steel built home. Wood burns at 500 degrees, and at the temperature the straw bale wall was subjected to, steel would buckle.
A wind test was also conducted, in which the equivalent of a 75 mile per hour gale didn't move the wall even one inch. When the wind speed was increased to 100 mph, the wall moved a barely measurable 1/16".
How Are They Built?
Before you run right out and buy bales of straw, check with the building code of your state and county. Some states, such as California and New Mexico have banned the building of straw bale homes. In actual construction, there are two methods of build out. One is weight bearing (Nebraska style), the other non-load bearing (or infill). The weight bearing construction is accomplished by using bales only. Infill construction uses a stick frame, and spaces between studs and joists are filled with straw bales.
Construction is begun by digging a trench to below the frost line. The preferred method of constructing the foundation is by making a rubble filled trench. This is very stable and has been seen in construction for centuries. The trench should be 16 inches wide - wider if the soil doesn't have adequate bearing capacity. A layer of fine gravel is used at the bottom of the trench, and a standard 4-inch perforated drainpipe is laid. The trench is then filled with road grade gravel, and a steel-reinforced grade beam is put in place on top. A concrete slab tops it all off.
Construction proceeds like this. We use the infill process as an example:
- Framework is installed, including walls, floors and roof
- Interior walls are framed
- Roof is finished off, including shingles
- Straw bales are installed between structural elements
- Electrical wiring and plumbing are installed and inspected.
- Exterior plaster is installed - the final coat is installed later
- Interior plaster is installed, except the finish coat.
- Windows are installed and doors are hung
- Exterior and interior trim is applied
- Interior drywall is installed
- Exterior and interior are given the final plaster finish
Construction is straight forward. The finished home has walls 16" - 18" thick with an R factor of approximately 42 and the walls have a very high sound absorption quality. Care must be taken to provide a breathable finish for the walls, especially for the exterior. Cement finishes can crack and cause water damage to the bales. It is recommended that the straw bales be given a coat of natural earth or lime plaster.
Cost of construction is about half of a normal build out. Be sure to spend time with local building officials when planning to build a straw bale home. The results are well worth it.
Alden Smith is an award winning author and regular contributor to DoItYourself.com. He writes on a variety of subjects, and excels in research.