Strawbale Homes

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Strawbale Homes

There is something downright joyful about strawbale homes, with a humble beauty that is beyond skin deep.

LA Times

With the introduction of the Homestead Act of 1862, "sod-busters" were obligated to develop and reside on their new property for five years in order to ensure ownership. This created a demand for a fast and affordable means of house construction. Initially strawbale houses were seen as make-shift structures to provide temporary lodging until funds became available to pay for the construction of a "real house". However, strawbale houses quickly proved to be comfortable, durable, as well as affordable, and so came to be regarded as permanent housing over time. Indeed, over the past century they have often outlived neighboring timber-frame houses and a number today are beginning their second century.

After the second World War ended, some veterans turned to strawbale construction as an economical means of building their starter homes. However, modern strawbale construction didn't emerge until the late 1970s when the energy crisis created by the oil embargo helped bring issues of sustainability to the public consciousness. Now, strawbale homes are being built around the world, including northern Canada, Mongolia, Russia, Mexico, Australia and New Zealand.

Strawbale construction's growing popularity stems from a number of reasons:

it utilizes an inexpensive and renewable "agricultural waste product"
it requires a relatively simple construction technique which is easy for beginners to learn,
it involves few synthetic chemicals
it provides effective energy-conserving insulation

For these reasons it's growing in popularity, especially with do-it-yourselfers, "owner-designer-builders", and other proponents of sustainability.

Materials

Strawbale buildings can be constructed from many different plant fibers, not just grass-family species such as wheat, rye, barley, blue-grass and rice, but also flax, hemp, etc. Recycled materials including paper, crushed plastics, whole tires, pasteboard, waxed cardboard, crushed plastics, whole tires, and used carpeting have also all been used for building.

A recent innovation is the use of high-density recompressed bales or "strawblocks. This method provides higher compression strength.

Techniques

Foundation

Standard concrete footing/foundations or thickened edge-slab-on-grade foundations have been typical. Bales can also be stacked over stem walls with joisted floors. With load-bearing strawbale homes rubble trench foundations or Earthbag construction foundations are increasingly used, as an alternative to conventional footings. Some pioneer designers are even using rock-filled gabions or earth-filled "bastions" in lieu of concrete. Strawbales have been used to insulate the floor from the slab, or to provide subgrade perimeter insulation, but this must be done with care, due to the importance of isolating the bales from undue moisture. (Moisture levels higher than 18% support mold growth in both straw and wood.)

Walls

The original "Nebraska" strawbale building method utilizes the strawbale walls as the support for the roof-structure.

A newer method of construction utilizes 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 strawbales for insulation. This type of structure is popular today because it allows bale placement to be accomplished with the roof already in place, or "in the dry", and can more readily be demonstrated to conform to building codes.

In some cases, both methods can be used, with load-bearing perimeter walls and pole or stick-frame support at the interior or ridge.

Roofing

Strawbale buildings can employ many different types of roofing designs. Sometimes a barrel-vaulted roof is used, but most often a conventional roof structure is attached to a load-distributing plate or beam at the top of the straw walls. These conventional roof structures are typically insulated with strawbales which then provide high insulative and acoustic benefits. Additional insulation options include soy-based foam, rice-hulls, wool or cotton batts, and even recycled cellulose.

Exterior and Interior Finishes

Typically strawbale walls have stucco exteriors and plaster interiors. The combination of straw-bale/stucco behaves like a sandwich panel, with the rigid stucco skins bearing much of the load in addition to adding considerable overall wall strength.

Stucco for straw-bale walls is most commonly a mix of cement and sand. However, mixtures containing earth or clay and/or a high percentage of lime in place of the cement are increasingly popular. This alternative increases water vapor permeability through the walls.

Characteristics

Strawbale walls are typically 21 to 26 inches wide when stuccoed and plastered. This thicker than average wall results in deeper window and door "reveals", similar to those found in stone and adobe buildings. Since bales can be shaped easily, they can be used to produce curved designs. When used in combination with plaster, they can create an imperfect texture and shape which some people find appealing. Flat straight walls can be achieved by the application of more plaster.

Advantages

Insulation

Straw-bale buildings offer high insulation value.

Thermal mass

The typical interior finish of a straw-bale wall is either cement or gypsum plaster, or a combination. This results in excellent thermal mass on a diurnal cycle.

A strawbale home's thermal mass minimizes the thermal swings due to daytime warming and night time cooling, by first absorbing and then gradually releasing heat. This can reduce the need for fuel or electricity to regulate temperature.

Simplicity

Strawbale building utilizes locally available materials, and basic construction techniques that require little specialized or proprietary materials and equipment. It has often been successfully used by inexperienced builders working on their own homes.

Straw's availability and cost

Since straw is an abundant agricultural waste product it is, therefore, relatively cheap for a building construction material. In bulk, straw bales are usually sold for approximately the cost of baling and delivery. Farmers will occasionally sell bales for under cost in order to clear storage sheds prior to a harvest.

In most regions, straw is baled only once each year, and so must kept dry and stored for use at other times of the year.

Resistance to pests

Strawbale walls are thick and dense enough to keep out most pests. In addition, the plaster exterior makes them unattractive and impenetrable to animals and insects. Finally, because straw contains minimal nutritional value, it does not attract pests.

Resistance to fire

While loose straw is highly flammable, after being packed into a bale it becomes too dense to allow sufficient air in for combustion. Think of it this way, it is far easier to light a single sheet of paper on fire, but next to impossible to burn a closed telephone book. In construction it is critical to have, at a minimum, a parge coat of plaster on all surfaces of the wall. Application involves troweling on a thin coating of mortar and then brushing it smooth.

Failure to add the parge coating can create a fire hazard. A spark from an electrical short can ignite the hair-like fuzz of an exposed bale. The flame spreads upward and sets the wood framing on fire causing it to burn. The typical fire will result in little fire damage to bales, but extensive water damage due to the fire suppression efforts.

Disadvantages

There are limits to strawbale's structural strength. Load-bearing strawbale walls are typically used only in single-storey or occasionally double-storey structures. A basement foundation is rare.

Strawbale design and construction challenges:

Straw-bale construction is still considered an experimental construction method in many jurisdictions. This means that:

local building codes may not cover it,

local authorities may not recognize it, and

most contractors will not be experienced in its use.

Additional drawbacks:

It is important that strawbale buildings be designed to eliminate the possibility of moisture entering the walls, especially from above. For this reason, designs usually incorporate roof shapes and overhangs which are wider than normal to minimize the risk of water entering the walls.

Finally, because strawbale walls are significantly thicker than normal walls, the amount of actual living space within the walls can be significantly smaller than the footprint of the building.

Additional resources on strawbale construction.

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