Straw bales have a higher R-value than other common natural building materials
Do you want to build your home out of natural materials? If so, you can build your walls with adobe, cob, cordwood, rammed earth, or wattle-and-daub. Although all of these walls have a long history, their thermal performance is poor. If you want a well-insulated wall, one natural material is the clear winner: straw bales.
A 23-inch-thick straw-bale wall has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of about R-33. Moreover, since virtually all straw-bale walls are plastered on both sides, these walls are relatively airtight.
If you are an owner/builder with lots of time on your hands, and you want to build your walls out of natural materials, straw-bale construction makes a lot of sense.
The first straw-bale houses were built in Nebraska in the late 19th century. (I'm proud of this Nebraska connection. Both of my maternal great-grandparents settled in Enders, Nebraska, in the 19th century, while my paternal grandmother grew up in a sod house in South Dakota.) According to Bruce King, the author of Design of Straw Bale Buildings, the oldest standing straw-bale building in the world is the 108-year-old Burke house in Alliance, Nebraska.
Many people confuse straw and hay. Hay is dried grass — the same stuff that your lawnmower spits out. Hay is used to feed cattle, horses, and other farm animals. Straw, on the other hand, consists of the dried stems of grain plants — for example, wheat, oats, barley, rye, or rice. Since hay is more vulnerable to rot and mold than straw, hay bales should not be used for wall construction.
Straw is often sold for use as stable bedding. However, in areas of the country where the supply of straw exceeds the market demand, straw is sometimes burned. The price of straw varies widely by region; in areas of the country where grain is commonly grown, a bale of straw is cheaper than a bale of hay. Elsewhere, however, straw often costs much more than hay.
Straw bales come in two sizes: a two-string bale measures about 15 in. x 18 in. x 36 in., while a three-string bale measures about 16 in. x 23 in. x 46 in. Either size can be used for wall construction. When used to build a wall, straw bales are usually stacked flat (not on edge), so a wall made from two-string bales will be about 18 in. thick, while a wall made from three-string bales will be about 23 in. thick. Of course, the thicker the wall, the higher the R-value.
All types of straw — including wheat straw, barley straw, oat straw, rye straw, and rice straw — are suitable for wall building. If it’s available locally, rice straw is often preferred, since it is said to be more resistant to decay than other types of straw.
Every straw-bale wall needs to be plastered on both sides. The plaster serves many functions: it is a structural element that increases the wall’s load-bearing capacity; it improves a wall’s fire resistance; it helps keep the straw bales dry; and it keeps out rodents. A wide variety of plasters are used on straw-bale homes, including lime-based plasters, cement-based plasters, and clay-based plasters.
Straw-bale walls are strong and durable. According to Bruce King, “Load-bearing straw-bale walls in Pensacola, Florida, easily survived a powerful hurricane — before they had been plastered. In short, the empirical evidence to date tells us that straw-bale walls of conventional dimensions are not appreciably affected by high winds.”’
Walls must be kept dry
The biggest enemy of a straw-bale wall (or, for that matter, of a wood-framed wall) is moisture. It’s vitally important to keep liquid water out of a straw-bale wall. To accomplish this goal:
- Straw bales must be dry when purchased.
- Straw bales must be kept dry during construction.
- The foundation must be raised above the surrounding grade.
- The first course of straw bales must sit higher than the floor.
- The roof must have generous overhangs on all sides.
- Decks and patios must be designed to minimize splashback.
- Windows and doors must be properly detailed and flashed to prevent water entry.
- Homeowners must avoid using sprinklers near exterior walls.
Hard experience has taught straw-bale builders the importance of impeccable moisture detailing. Building scientist John Straube told me one story of a straw-bale failure: “Years ago, I investigated an Arizona project with a moisture problem. It was a straw-bale house with a low-slope roof and no overhangs. There was a parapet with a rounded top and vigas — projecting beams that penetrate the walls. Water entered the wall at the top of the parapet, accumulated, and ran down the wall and found the vigas, where it leaked in. Most of one wall had to be rebuilt because of moldy straw bales.”
Like Straube, straw-bale expert Andrew Morrison emphasizes the importance of good detailing to prevent water damage. “Many builders can tell you horror stories about broken pipes and blown washing-machine hoses,” Morrison wrote. “Imagine the repairs if the leak flooded into a wall made of straw bales. The bales could wick all of the water right off the floor and up into the walls. It is possible that the walls would not be able to sufficiently dry out and would therefore be ruined and need replacing. That is a catastrophic repair and it is exactly why we do not place water pipes in the bale walls and why the bales never sit directly on the ground.”
To understand what can happen to a straw-bale wall without adequate detailing to prevent water entry, it’s worth checking out photos of a failed free-standing straw-bale wall in Tucson, Arizona — a city with a relatively dry climate.
Of course, water entry can do as much damage to a wood-framed wall as a straw-bale wall. Straube told me, “I’ve seen more regular houses rotting that straw-bale houses rotting. Of course, that’s because there are more regular houses. The thing that has kept straw-bale homes out of the ditches is that most straw-bale practitioners pay close attention to these moisture issues. You can manage these moisture risks with experience and prudence. Straw-bale builders are much better than the average yahoo builder who throws up a stick-built house.”
Compared to bulk water entry, other forms of moisture transport rarely cause problems in straw-bale walls. Of course, an air leakage path through a straw bale wall could lead to condensation, so standard attention to air-sealing details is important. However, vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. is very unlikely to cause any problems.
Load-bearing or infill?
There are two basic types of straw-bale buildings: buildings with load-bearing straw-bale walls (also known as “Nebraska-style” buildings), and post-and-beam buildings with straw-bale infill.
Before choosing to build a Nebraska-style building, you need to overcome two hurdles:
- Verify that your local building official will approve a Nebraska-style straw-bale building; and
- Develop a plan to keep all of your straw bales dry during construction.
“Keeping the bales dry during construction may not be a problem in Arizona,” says Straube. “In Vermont, it is.” If you have no code barriers and you live in a dry climate, a Nebraska-style building may make sense. Most such buildings are limited to one story.
Compressing the wall
Before you can plaster a Nebraska-style building, you have to consider the problem of bale settlement. After a straw-bale wall is built, the weight of the upper bales causes the wall to settle. If you wait a couple of months, you’ll find that the wall has settled by 1/2 inch to 4 inches.
If your construction schedule doesn’t allow you to wait a few months before plastering your walls, you can “pre-compress” your walls. There are several wall-compression techniques:
- According to Bruce King, “Builders have found … that [settling is] drastically reduced if bales are emphatically stomped into place both downward and against adjacent bales.”
- The bucket of a front-end loader can be used to compress the wall.
- The wall can be cinched down with polyester packing straps or fencing wire that passes through a curved plastic conduit that is positioned in the concrete footing under the wall. The packing straps or wire pass over the wall top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate., and a ratchet device is used to cinch the strapping.
Advantages of the post-and-beam approach
Builders in damp climates usually choose to build a post-and-beam building with straw-bale infill. The main advantage of this approach is that it is possible to install the roof before the straw bales are delivered to the job site, greatly simplifying efforts to keep the bales dry.
If you live in an area of the country where straw-bale homes are still rare, you may find it easier to get approval from your local building official for a post-and-beam building than a Nebraska-style building.
More design issues
Once you’ve decided whether you’re building a post-and-beam building or a Nebraska-style building, the design process can begin. To ensure good roof overhangs on all four sides, a hip roof makes more sense than a gable roof. Straw-bale walls should never include a vapor barrier on either side of the wall.
Because straw-bale walls are quite wide, the easiest foundation option is the thickened-edge concrete slab on grade. It’s a good idea to leave rebar pins sticking out at the perimeter of the foundation; you want to have at least two rebar pins per bale. (Instead of rebar pins, some builders install projecting spikes in the parallel 2x4s used to create a “toe-up” under the first course of bales.)
Builders use a variety of techniques to raise the first course of bales above the level of the slab. According to straw-bale expert Mark Piepkorn, “In slab-on-grade applications, a [concrete] bale-wide curb can be poured integrally to raise the bales above floor-level. Similarly, a ‘toe-up’ between the floor and the first course of bales is becoming increasingly common. The toe-up is a set of parallel rails at bale-width, generally wood ..., set with anchor bolts to the slab, or ram set, or screwed to the decking. The area between these rails can be filled with anything providing a capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break, such as crushed gravel. … Besides lifting the bales off the floor, this technique provides a convenient nailing strip on the exterior for the stucco reinforcement and an aluminum drip edge. It’s also been suggested that the interior rail can be inset from the true edge of the wall to form a wire chase behind baseboards.”
The second course of bales is offset from the lower course by half a bale length to create a “running bond” pattern. To cut a bale in half, each half bale is tied together tightly with baling twine before the bale's strings are cut. If necessary, a large wooden mallet — a “persuader” — can be used to nudge bales that are out of plumb back into position.
Bales are often anchored to lower courses with wood stakes (usually 1x2s about 36 in. long), bamboo stakes, or rebar. However, according to Laura Bartels, a builder and green-building consultant in Carbondale, Colorado, many straw-bale builders have concluded that staking bales is unnecessary.
Plywood or lumber window bucks are used to establish window rough openings. Straw-bale builders should research window-buck methods used by other builders before designing their own window bucks.
Straube advises builders to keep a fire extinguisher handy. “You need to beware of fire during construction, since there will probably be a lot of loose chaff and cut-up straw on the job site,” he says. Once the walls are plastered, however, the fire risk is minimal.
At the top of the wall, builders of Nebraska-style homes usually install a wide wooden top plate or a concrete bond beam. The roof is framed conventionally, using rafters or roof trusses.
Window flashing and plaster
Because liquid water is the enemy of a straw-bale wall, windows should be installed as “outies,” not “innies.” Window sills must be carefully detailed. “The fundamental requirement [for window sills] is that there be an outer layer of claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. , with a drainage gap below and a waterproof layer below that,” says Straube. “You also need an opening to let the water out. You can use a Benjamin Obdyke product to create the drainage gap, or in a dry climate, you can use two layers of building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt.. You also need some type of flashing with a drip edge and an exit slot for the water.”
Once the walls are up (and, if necessary, compressed), it’s time to plaster. While some straw-bale builders apply plaster directly to the straw bales, Straube recommends attaching chicken wire or wire lath to the bales before plastering. “I want metal mesh in the plaster, even if it’s only light chicken-wire, for a bunch of reasons,” says Straube. “The main purpose of the mesh to keep the crack sizes small.” Chicken wire is attached to the bales using U-shaped pins poked into the bales or wires that go all the way through the bales; this is done with the help of a tool called a “bale needle.”
Laura Bartels advises against the use of chicken wire. “We usually avoid the use of metal meshes unless required structurally,” says Bartels. “Chicken wire or stronger is required in high seismic areas, but other than those cases, you will mostly find that plaster prep involves reinforcing at corners, material transitions and over framing or other materials.”
Traditional plastering techniques require three coats of plaster: a scratch coat, a brown coat, and a finish coat. The total thickness of the plaster is usually between 1 in. and 1 1/2 in.
If you don’t want a stucco exterior, you can install any type of siding you want. However, it’s essential that the exterior side of the straw-bale wall be plastered, even if you later install furring strips and wood or fiber-cement siding.
“It’s very easy to put furring strips on,” says Straube. “One way is to install a scratch coat of plaster, then install your furring strips screwed to the stucco, and then install another coat of plaster between the furring strips. A better way to do it is to install two coats of plaster and then install the furring strips, shimming as needed, with Tapcon screws. The plaster will hold the Tapcons.”
Mark Piepkorn likes the idea of a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap on straw-bale walls. “The general consensus of experienced and knowledgeable straw-bale builders indicates that external cladding [over a rainscreen gap] will perform better than stucco alone in nearly all circumstances, if properly implemented,” Piepkorn writes.
Some builders install 2x4s on the inside surface of straw-bale walls to facilitate attaching kitchen cabinets; these 2x4s can be through-bolted with 1/2-in. threaded rod. (You’ll need a large plywood washer on the exterior of the wall). Another approach (one mentioned by Bruce King) is to attach cabinets to wooden stakes that are pounded into the straw before plastering.
Wiring and plumbing
Any wiring embedded in a straw-bale wall is best installed in conduit. “Conduit can be let into grooves carved by chainsaw or ‘weed whacker’ into the straw surface,” King advises.
It’s best to avoid any plumbing pipes in a straw-bale wall; in a pinch, it’s always possible to build a chase or a faux framed wall for the plumbing.
A full survey of the status of straw-bale building codes is beyond the scope of this article. Code requirements change all the time, so it’s best to research the current situation in your local jurisdiction.
The first straw-bale code in the U.S. was adopted by Tucson, Arizona, and Pima County, Arizona, in 1996. California followed Pima County's lead a few months later with statewide guidelines for straw-bale structures.
In 1996, New Mexico adopted a straw-bale standard that (unlike the Arizona code or California guidelines) does not permit load-bearing “Nebraska-style” walls. The New Mexico code requires straw-bale buildings to include a post-and-beam frame.
Several cities with active green-building communities adopted straw-bale codes in the late 1990s; among them were Austin, Texas (1997) and Boulder, Colorado (1998).
This introductory article is an attempt at an overview of straw-bale construction methods.
It should not be considered a guide to construction, so if you are planning to build your first straw-bale house, be sure to research the topic thoroughly, using some of the many available books, Web sites, or instructional courses.
Does a straw-bale house make sense for you?
If you are an owner/builder with plenty of time and access to inexpensive straw, a straw-bale home can be an affordable method of construction.
However, straw-bale construction is relatively time-consuming; installing metal lath and plastering goes slowly. If you are hiring a contractor, you can expect a straw-bale house to cost more than a stick-built house.
Last week’s blog: “All About Larsen Trusses.”
- Peter Lenardon
- Laura Bartels
- Heron Construction and Straw & Timber Craftsmen Inc
- Sandra Cohen-Rose and Colin Rose
- Peter Leth
- Kelly Lerner, www.one-world-design.com
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