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Straw Bale Construction

Contents

Introduction to Building with Straw Bales

History

While use of grass-family plant fibres has long been a part of building methods worldwide, dating far back into prehistory, actual straw-bale construction was pioneered in Nebraska in the United States, in the late 19th/early 20th century, in response the then-new availability of baling machines and the lack of significant amounts of timber or buildable sod needed to build barns and housing in the Sandhills region.

Under the Homestead Act of 1862 and the Kinkaid Act of 1904, the "sod-busters" were required to develop and live on their new property for five years in order to maintain ownership; building housing was a legal requirement. The straw-bale house was first seen simply as a make-shift structure, to provide temporary lodging, until enough funds were available to pay for the shipping in of timbers, to build a "real" house.

However, these homes quickly proved to be comfortable, durable, and affordable, and so became regarded as permanent housing. Over the past century they have indeed outlived many neighbouring timber-frame buildings, and a number are in continuing use today and beginning their second century.

After World War II a scattering of U.S. veterans turned to straw-bale for shelter, but modern straw-bale construction experienced a re-emergence in the late 1970s, after the 1973 energy crisis helped bring issues of real sustainability to the forefront, with first examples built primarily in the southwestern United States. Now, they are being built the world around, from northern Canada, Mongolia and post-Chernobyl Russia, to Mexico, Australia and New Zealand.

Because it is based on an inexpensive and renewable so-called "agricultural waste product," with a technique relatively simple for beginners to implement, involving few synthetic chemicals and providing effective energy-conserving insulation, it continues to grow in popularity, especially with do-it-yourself-ers "owner-designer-builders" and other proponents of sustainability.

Current Perspective and Regulations

Building with Straw Bales is slowly but surely gaining ever wider acceptance across America, Europe and Australasia. With some charitable groups using it in poorer countries it is also beginning to appear in South America and Eastern Europe.

Government bodies are in general less hostile than you might at first expect. There are now many reports and studies you can use to help win them over (see the section [/Related_Resources/Technical_Studies Technical Studies, Reports and Tests]).

Materials

Straw-bales can be made from a range of plant fibres, not only grass-family species like wheat, rye, barley, blue-grass and rice, but also flax, hemp, etc. (Bales of recycled materials like paper, pasteboard, waxed cardboard, crushed plastics, whole tires and used carpeting have also all been used or are currently being explored for building.)

Basic straw-bales are produced on farms and referred to as "field-bales". These come in a range of sizes, from small "two-string" ones 18 in (460 mm) wide, by either 14 or 16 in (350 to 400 mm) high, and 32 to 48 in (0.8 to 1.2 m) long, to three-string "commercial bales" 21 in wide, by 16 in high, by 3 to 4 ft long. These sizes range from 40 to as much as 100 pounds (18 to 45 kg).

Even larger "bulk" bales are now becoming common, 3 by 3 ft (1 by 1 m), or 3 x 4 ft (1 m by 1.2 m) by 6 ft (2 m) long and even 4 x 4 x 8 ft (1.2 by 1.2 by 2.4 m) long, weighing up to a ton, plus rolled round bales 4 to 5 ft (1.2 to 1.5 m) in diameter. All of these "economy-size" units also offer unique potential for imaginative designers.

A newer trend is the use of high-density recompressed bales, sometimes called strawblocks, offering far higher compression strength. These bales, "remade" from field bales, in massive stationary presses producing up to 1 million pounds of force (4 MN), were originally developed for cargo-container transport to over-seas markets.

But innovators soon discovered that where a wall of "conventional field bales" is able to support a roof load of 600 pounds per foot (900 kg/m), the high-density bales can support up to 3,000 to 4,500 pounds per foot (4,500 to 7,000 kg/m). This makes them particularly suited to load-bearing multi-storey or "living-roofed" designs, and they may be faced with siding, gyp-board or paneling and have cabinetry hung directly from them with long sheet-rock screws.

They are available in a range of sizes from different companies' presses but 2' long by 2' high by 18" wide might be considered "typical"; because they are bound with horizontally ties or straps, at 3" or 4" intervals vertically, they may be recut with a chain-saw at a range of heights. And they usually used in "stacked bond", with the straws running vertically for greatest strength and tied with "re-mesh" both sides, before stuccoing.

The sizes of these 2 & 3 string bales are: "two-string" ones 18 in (460 mm) wide, by either 14 or 16 in (350 to 400 mm) high, and 32 to 39 in (0.8 to 1.2 m) long, three-string "commercial bales" 23 in wide, by 14 to 16 in high, by 42 - 48" long

Characteristics

The thick walls (typically 21 to 26 inches (530 mm) when stuccoed/plastered), result in deeper window and door "reveals", similar to stone and adobe buildings. Since the bales are irregular and may be shaped easily, they are readily adaptable to curved designs, and when plastered, tend toward a relaxed, imperfect texture and shape. If flat, straight walls are desired, this can be achieved, as well, by the application of more plaster.

  1. Acoustics
  2. Insulation
  3. Thermal mass
  4. Passive solar
  5. Availability, types and cost
  6. Resistance to pests
  7. Resistance to fire
  8. Structural properties
  9. Design and construction challenges

Acoustics of straw bale structures

A report carried out in Denmark measured the sound insulation performance of a wall in an existing home. The measurements were carried out in a wall with both horizontal strawbales (where the straws were perpendicular to the plane of the wall) and on a wall with vertical strawbales (where the straws were parallel with the plane of the wall). In both instances there were approximately 40 mm of clay rendering on each side of the wall. In the first instance the sound insulation (expressed with the sound reduction R’w) was found to be R’w=52 dB and in the second instance to be R’w=46 dB.

The second result is affected considerably by bed-lofts in both rooms that were carried by a wooden framework in the wall. It is estimated that a construction focusing on reducing the transmission due to openness in the construction (flank transmission), would be able to obtain values of 53-54 dB, regardless of the direction of the straws.

For comparison it can be mentioned that the requirement of the Danish Building Regulations in 2004 for a wall that separates apartments in housing blocks is 52 dB, while the requirement for walls between non-detached houses built in accordance with the Danish Building Regulations for Small Houses is 55 dB.

It should be mentioned that walls that only just satisfy these code requirements are not always perceived as satisfactory by the residents. For most other applications the strawbale-walls will have satisfactory sound insulation performance. Within a dwelling the sound reduction is particularly satisfactory and the actual sound insulation will most likely be determined by the doors.

Insulation

A carefully constructed straw-bale building has excellent thermal performance because of their combination of the bales high insulative value and the thermal mass provided by the interiors thick plaster coating. (Read the section on thermal mass for more on the advantages of a high mass construction.)

A good starting point is a discussion of what R-value is, and what it is not. It is not an absolute measure of how energy efficient your building is. It is not even a perfect way of predicting the wall’s contribution to thermal comfort. It is one piece of information about the wall that, with other information, can enable you to estimate the heat loss and heat gain through the walls.

R-value is the inverse of U-factor (R = 1 / U). U-factor is a measure of thermal conductance, or how easily a material (or system) allows heat to pass through it. This is how U-factor is defined (in the U.S.): the number of British thermal units that pass through one square foot of a material (or system) per hour with a one degree Fahrenheit temperature difference between the two sides of the material. Mathematically:

U-Factor

Btu = British thermal units, a = area in square feet, F = temperature fahrenheit

In most other countries U-factor is defined in terms of Watts per square meter per degree Kelvin [W/(m2K)]. To convert metric (SI) U-factors to inch-pound (IP) U-factors divide by 5.678; to convert the other way, simply multiply by 5.678. To convert IP R-values to metric R-values, multiply by 0.1761.

When a labouratory tests a material (or system) to determine its thermal conductance or resistance, they calculate the heat flow from one side to the other on the basis of measured surface temperatures and heat energy required on the warm side of the wall to maintain a steady heat flow. This provides the U-factor, which is then converted to R-value for some purposes. (Nehemiah Stone, 2003)

The theoretical R-value (thermal resistivity) for a 16.5 inch (420 mm) straw bale was calculated by Joseph McCabe as 52 (RSI-9.2). This is compared with a theoretical R-value for 3.5 inch (90 mm) of fibreglass (the conventional insulation material used in home construction) of 13 (RSI-2.3). This means fibreglass has an R-value of about 3.7 per inch (RSI-0.26 per centimeter) and straw bales have about 3.2 per inch (RSI-0.22 per centimeter).

Some lab tests of straw-bale assemblies have found significantly lower R-values in practice. However, the more conservative of these results still suggests an R-value of 28, which is a significant improvement over the R-14 of an energy-efficient insulated 2x6 wall. Straw-bale experts suggest that it is possible to approach theoretical R-values by giving more attention to detailing.

Tests have shown a range of values from R-17 (for an 18” bale wall) to R-65 (for a 23” bale). Analysis at Oak Ridge National Lab, among other places, has shown that R-values for insulation materials used in “standard” walls are generally much higher than the R-value for the wall as an assembly of disparate materials. Joe McCabe recently postulated that the same phenomenon could account for the difference between the high values from his testing of bales and the lower values obtained in the 1998 Oak Ridge test of a straw bale wall system.

While it is possible that the relatively low densities where bales abut each other might contribute to greater heat loss than would be measured through an individual bale, it is unlikely that this would account for the entire difference. This difference between bales and bale walls is nothing like the difference between standard insulation and what is found in stud framed walls (insulation voids, thermal bridges, uninsulated headers, and other faults).

It is noteworthy that all tests of straw bale wall systems prior to the Oak Ridge test in 1998 had potentially significant shortcomings and should not be considered particularly reliable. The last Oak Ridge test had no identified deficiencies and is considered by most to be an accurate determination of the thermal resistance of straw bale walls. ORNL determined the R-value to be R-27.5 (or R-1.45/inch), or R-33 for three string (23”) bale wall systems. Shaving a bit off the top just for conservatism's sake, the California Energy Commision officially regards a plastered straw bale wall to have an R-value of 30.

A final note is a reiteration of a point made earlier: it matters little whether the final truth about the R-value of straw bales walls is R-33 or R-43 or even R-53. Above R-30, the differences are minor and will usually be overshadowed by windows, floors, doors and ceiling/roof details. Whatever the value, it is at least three times better than the average “R-19” wood studwall system. (Nehemiah Stone, 2003)

Thermal mass

The interior plaster on a straw bale wall works as an excellent thermal mass on a diurnal cycle.

Thermal mass reduces temperature swings due to daytime warming and night time cooling, by absorbing and then gradually releasing heat. This can result in a direct reduction in the need for fuel or electricity to regulate temperature, and indirectly in savings through lifestyle adjustments: occupants of a moderate environment, with only gradual temperature swings, are less likely to use artificial heating and cooling. This is most easily achieved at high desert altitudes where a clear sky contributes to both warm days (solar gain) and cool nights (nighttime cooling), but the principle still works in other climates as well.

Passive Solar

Passive temperature control refers to buildings designed to maximise the heating and cooling effects of the environment around them. They are called passive because there are none (or few) parts of the design that require energy to operate. The most common technique for passively taking advantage of the environment is maximising solar gain by exposing interior surfaces to the suns warmth and then designing the building to best contain that warmth. At the other end of the scale, where climates are hot and passive cooling is what's needed, one technique is using rising warm air to draw basement cooled air up through a building.

Any building taking advantage of passive solar gain must have well insulated interior surfaces which are exposed to sunlight and have enough mass to store daytime heat and release it at night. How suited a straw bale house is to taking advantage of solar gain depends of the mass (think of thickness) of the inside plaster coating, though some maintain that straw bale constructions are inherently unsuitable for passive solar gain (although the article seems to neglect the surface paster). It should be stressed that straw bale homes are not inherently good for passive solar gain, they need to designed to make use of it, it doesn't just happen. The same is true of any building material or system.

Following are the basic features that distinguish straw bale buildings designed to maximise passive (think of free and sustainable) heating and cooling:

  • Limited exterior wall surface with high insulation.
  • Equator-facing, East and West Roof overhangs correctly sized to block the summer sun (angle) and still allow the lower winter sun angle to provide heating of interior thermal mass.
  • Passive preheating/precooling of external air by drawing through cellers, porches, glass houses and heat exchangers.

Features specific to cold climates

  • Large (super insulated low-e) glass surfaces orientated for maximum sun exposure, especially to the buildings interior. In strawbale buildings the inside plastered surface of the bales is a great surface for collecting sunlights heat and rasdiating it slowly back to the inside space.
  • Superinsulated doors, windows and frames. Glazing with low-emissivity glass coatings facing outwards
  • Position doors for minimum wind exposure, preferably with an enclosed porch.
  • External postbox, not an in-door hole.
  • Building envelope air-tighthess (see below).
  • For extra winter heating the focus is on renewable fuels (plant oils/ charcoal and wood) or sun heated systems (solar collectors or heat pumps).

Features specific to hot climates

  • Glass openings (and leisure areas) need to be protected from radiated heat from surrounding object like sun baked sand or earth, outside planting can greatly reduce radiated ground heat.
  • Shading and orientation to avoid sun exposure, especially to the buildings interior.
  • Position windows where they can make the most efficient use of prevailing wind for cooling and ventilation.

One common source of confusion when talking about 'passive' construction is the term 'breath' which is more accurately known as “vapor permeability”. People talk about straw bale walls breathing, but this has nothing to do with air moving through the wall, it's about moisture moving through the wall. Really it is better to refer to it as moisture permiability. In this way walls that can transport odour filled moisture to the outside contribute to a high air quality, without air moving through the wall.

Availability, types and cost

Availability

Straw is an agricultural waste product, a by-product of grain harvesting. Many different kinds of straw are baled and can be used for construction. Straw is widely available, and is generally an abundant, renewable resource. Relatively little energy is consumed in harvesting, baling and transporting bales to a building site. In bulk, straw bales are generally sold for close to the cost of baling and transport. Farmers will sometimes 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. Straw production and demand is relatively constant, however high demand for bales used for erosion control following forest fires can create a temporary shortage of bales.

Types

Bales are rectangular compressed blocks of straw, bound by strings or wires. Straw bales come in all shapes and sizes. Rectangular bales are the only bales suitable for building. The round bales that are now becoming popular require re-bailing before use but this is not recommended. Three string bales (585x405x1070 mm) common in western USA have an average weight of 29 kg. The two string bales (460x350-450x960 mm) which are common in the rest of USA and most of the world are easier to handle and have a weight ranging from 15 to 19 kg.

Besides these traditionally sized bales, big jumbo bales are also becoming popular. There are basically two sizes in use. The real jumbo is 1200x760x2400 mm and the mini-jumbo is 800mm wide and available in various lengths and heights depending on the bailing machine used. The jumbo bales are appropriate for bigger industrial buildings where they show definite advantages due to their high load carrying capacity of up to 3 t/bale for the 1200mm wide variety. Only machine handling is possible due to their weight. Greater stability and the bigger size of Jumbo bales compared to the conventional bales favours rapid and easy construction.

Cost

Small bales range in price from 1.50 USD per bale to 6USD per bale. Prices go up rapidly when you take into account transportation. Jumbo bales, including transportation, range in cost from 15USD to 30USD per bale. This depends upon when you purchase the bales, how far they need to be transported, and type of bale - whether it's wheat straw, flax straw, or rice straw. Different "waste" products have different values for farmers and some are less usable than others for agricultural purposes.

Resistance to pests

Straw bales are thick and dense enough to keep out many kinds of pests. As well, the outer layer of plaster makes them unattractive or impenetrable to animals and insects. Finally, because straw contains little nutrient value to most animals and insects, it does not attract pests.

Termites like moist damp conditions. While a wall is kept dry, there is little danger termites would have any interest. When termites do manage to enter a wall, they tend to bypass the straw and attack any wooden studs.

In North America, termites attacked straw bale houses only very rarely.

Resistance to fire

Although loose straw is quite flammable, once packed into a bale it is too dense to allow enough air for combustion. By analogy, it is easy to light a single piece of paper on fire, but difficult and time consuming to burn an entire phone book. In construction it is critical to have, at a minimum, a parge coat of plaster on all surfaces of the wall. Parge coating the wall involves troweling on a thin coating of mortar and brushing it smooth.

Typical failure of straw-bale homes involves frame walls set against straw-bale walls without a parge coat. A spark from an electrical short or an error by a plumber ignites the hair-like fuzz on the exposed bale. The flame spreads upward and sets the wood framing on fire causing the wood framing to burn. The typical fire results in little fire damage to bales, but extensive water damage due to the fire suppression activities.

The ASTM E-119 fire resistance test for plastered straw-bale wall assemblies in 1993 passed for a 2 hour fire-wall assembly. In this test a gas flame blows on one side of the wall at approximately 2000 degree Fahrenheit (1100 degrees Celsius) while the temperature of the other side of the wall is continuously measured. The results of this test had no burn-through and a maximum temperature rise of 60 degrees Fahrenheit (33.3 degrees Celsius).

Limits to structural strength

Load-bearing straw-bale walls are typically used only in single-storey or occasionally double-storey structures. A dug foundation (basement) is uncommon.

An all-straw vaulted building was designed and built in Joshua Tree, California, and greatly exceeded the structural requirements for this highly active seismic zone.

Post and beam straw-bale structures have been used for buildings as large as 14,000 square feet (1,300 m²) and even for a United States Post Office, in Corrales, NM.

Design and construction challenges

Straw-bale construction is still considered experimental in many jurisdictions. Building codes may not include it, local authorities may not recognise it, and most contractors will probably not be experienced in its use.

Straw-bale buildings must be carefully designed to eliminate the possibility of moisture entering the walls, especially from above. Successful designs often incorporate roof overhangs that are wider than normal and roof shapes and detailing that minimise the risk of water splashing against walls.

Because straw-bale walls are much thicker than normal walls, there is sometimes a compromise between the size of the building's footprint and the amount of living space.

  1. Foundations frost, soil types, insulation
  2. Walls load bearing, non-load bearing, curved
  3. Finishes clay plaster, cement render, lime based plaster, mechanical application
  4. Openings water proofing, tighthess, design considerations, location, natural lighting
  5. Roofing green roofs, straw insulated, seashell insulated
  6. Non-residential Buildings
  7. Pushing the Limit arches, domes, stringless bales
  8. Building Services electrical cables, plumbing, heating and cooling

Foundations

There are several options for making footings under Straw bale walls. Standard concrete footing/foundations or thickened-edge-slab-on-grade foundations have been typical though do not fit with some environmental considerations as cement uses large amounts of energy in its production. Bales can also be stacked over stem walls with joisted floors. With load-bearing straw-bale 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. Straw bales 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 mould growth in both straw and wood.)

In the same way as a rubble bed, a bed of shells has been used with much success in Denmark. At a thickness of between 119.4 and 124.9mm conductivity is between 0.120 and 0.112 W/mK. Compared to industrial products (such as expanded ceramic or spun glass or rock) shells therefore provide good insulation as a nearly carbon neutral industrial waste product.

While thinking about the design of your foundations, or more specifically the foundation pad, this is the time to think about heating options. One of the options gaining popularity is in-floor radiant heating. You can read more about this in the section on building services under heating and cooling.

Further reading

Walls

Straw Bale Infill

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 running 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 Walls

Here is some information specific to load-bearing straw bale walls.

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.

If you want to further increase the load bearing ability of the wall you can "cage" them on one or both faces with pre-welded or woven mesh, to increase pre-stuccoed wall stability. But whether or not you use metal mesh should depend on the moisture content you expect for your outside wall surface, as rusted metal can crack the surface render. If you use cement stucco then there should be 5cm of plaster before you reach any metal cage, othewise rusting will occur. Thickness would increase for more moisture permeable materials like clay.

Curved Walls

"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.

Structural Capabilities of Bale Walls

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.

Finishes

Straw-bale walls are most typically plastered on the outside with lime, clay, or a cement and lime mix. Inside surfaces are typically lime, clay, plaster baord (gypsum) or Structolite product. Structural analysis has shown that the straw-bale/stucco assembly behaves much like a sandwich panel, with the rigid stucco skins initially bearing most of the load and adding considerable strength to the wall.

An important consideration when choosing a finish is that the outside surface of the walls must be more permeable to moisture than the inside surface. Failing to follow this rule will result in moisture accumulating in the wall, which will eventually rot the bales, just as it would rot anything untreated. As two extreme examples, if you chose to finish the inside surface with cement plaster and seal it with acrylic or latex paint, then any moisture in the wall can effectively only move outwards (assuming that's not also painted). If you did the opposite and used natural finishes on the inside but painted the outside with plastic paint then you are trapping moisture into the walls and rotting is likely.

Cement/ sand stucco

Stucco for straw-bale walls can be cement/sand-based, although mixes containing earth or clay and/or with a high percentage of lime, replacing part or all of the cement are increasingly popular trends. (Advocates of sustainable construction are becoming increasingly concerned with the fact that for every ton of cement manufactured and used, another ton of climate-changing fossil CO2 is released into the atmosphere.)

Clay plaster

Clay plaster allow higher water vapour permeability through the walls than lime plaster, which in turn is much more than cement plasters. This means the right type of wall will dry quickly when wetted by rain and will effectively transfer any moisture which accumulates in the wall, whether from a leak or from normal day-to-day living (a significant amount).

Clay plasters are great regulators of the indoor climate, they 'breath', which means moisture is absorbed and released - it does not mean that air trickles through the wall. On the inside of a house this property makes it well suited to damp areas like kitchens and bathrooms, it will absorb periodic moisture and to some extent odour, and slowly release it again. Because clay plaster typically is quite thick it also serves to regulate temperature by warming and cooling quite slowly. On the outside of the house this effect can even mean that the clay will wick (pull) moisture out of the straw and release it to the exterior air.

Lime plaster

(This section needs improvement) Performs in a similar way to clay plaster.

Resources to assess: and Modern lime plasters Lime Plasters consist of Lime, aggregate and other additives. Lime Plasters are more resistant to weather, mould and impact than clay plasters, but are more time consuming and challenging to finish.

Tadelakt

Tadelakt is a bright, nearly waterproof lime plaster which can be used on the inside of buildings and on the outside. It is the traditional coating of the palaces, hammams and bathrooms of the riads in Morocco. Its traditional application includes being polished with a river stone and treated with a soft soap to acquire its final appearance and water resistance. Tadelakt has a luxurious, soft aspect with undulations due to the work of the artisans who finish it; in certain installations, it is suitable for making bathtubs, showers, and washbasins and confers great decorative capacities. Traditionally, tadelakt is produced with the lime of the area of Marrakech.

The restoration of the riads of Morocco has focussed attention on this ancient technique.

Tadelakt more generally refers to any lime plaster applied according to the principles and techniques of Moroccan tadelakt, but using the lime bodies available in places other than Marrakech.

The basic characteristics of a tadelakt plaster are these:

1) It is a lime plaster. It does not include Portland cement.

2) It may include fine marble or limestone sand, but not other aggregates.

3) It has been compressed when plastic, eliminating all voids.

4) It has been mechanically polished, using stones or abrasives that are harder than the plaster finish, to provide a smooth, sometimes shiny, finish.

5) It has been treated with a natural soap (often "black" or olive oil soap) to speed carbonation of the surface and render the surface more water-resistant.

Floor finishes

Magnesite or magnesium oxychloride cement, patented in 1800's as Sorel's cement.

Openings

(Please help us write this section) This section could take as its starting point the discussion about waterproofing of window openings archived here: http://finance.groups.yahoo.com/group/GSBN-Greenbuilder/message/608

Roofing

Building with straw bales does not dictate that you use a certain type of roofing system. Depending on your view, straw bale designs might suggest that certain roof types are more appropriate. If your reason for choosing to build with straw bales includes an element of environmental concern then some options quickly become more attractive than others. In this context the main concern (after you're sure that the roof will keep you dry) is the embodied energy of the roofing system and the potential for reuse of the materials at a later stage.

For example the production of new roofing tiles uses very large amounts of energy, which contributes to our burden on the environment. Clay/ teracota tiles require large ammounts of energy to bake and concrete tiles must take the burden of the energy used in the extration and heating of lime to make cement. On the other hand some types of roofing tiles can easily be removed and used again and again for several hundred years.

In many cases and depending on where you live, collecting rain water or minimising roof runoff can be important. If you are not collecting your roof water then a green or living roof can be an option. Who wouldn't like a roof garden?

Another direction for roofs in straw bale buildings is to make arches and vaults of straw bales so that they all press on each other giving a stable compressive structure just like the stone arches of ancient roman times. There is more about arches in the section on Straw Bale Construction/Pushing_the_Limit.

The most typical solution is a conventional roof structure attached to a load-distributing plate or beam running all the way along the top of the bale walls.

The Green Roof

One advantage of green roofs is the stability they add to the temperature of the roof. Because of their size and mass they are slow to warm up and cool down. In places where there is a large temperature change from day to night this can be a great advantage.

The actual insulation value of a green roof (and here we're talking about not more than 30cm thick) is unclear. The presense or absence of water and roots makes such a large difference that it is hard to generalise. Naturally if the layer of soil is thick enough it will provide more insulation and a considerable ammount of stability to the temperature.

Roof and Ceiling Insulation

One of the easiest and most effective places to add extra insulation is in the space between your ceiling and the roof. So don't overlook this important part of the overall design.

Conventional roof structures may be insulated with straw bales, taking advantage of their high insulation values and good acoustic properties. Other alternative insulation includes rice-hulls, cotton or wool batts, soy-based foam and recycled cellulose. According to comments on an unrecorded test by Tim Owen-Kennedy of Vital Systems [8] rice hulls perform just as flame retardent as borate treated cellulose or better, without being treated. According to The Rice Hull House ([[../../Bibliography|Paul A. Olivier]]) from around 2004;

...rice hulls are unique within nature. They contain approximately 20% opaline silica in combination with a large amount of the phenyl propanoid structural polymer called lignin. This abundant agricultural waste has all of the properties one could ever expect of some of the best insulating materials. Recent ASTM testing conducted R&D Services of Cookville, Tennessee, reveals that rice hulls do not flame or smoulder very easily, they are highly resistant to moisture penetration and fungal decomposition, they do not transfer heat very well, they do not smell or emit gases, and they are not corrosive with respect to aluminum, copper or steel.

(The following quote from a referance in the same article needs to be followed up)

"Rice hull has a thermal conductivity of about 0.0359 W/(m.°C); the values compare well with the thermal conductivity of excellent insulating materials (Houston, 1972).” Juliano (1985), p, 696. The thermal conductivity of rice hull ash is reported to be 0.062 W.m-1.K-1. See UNIDO, p. 21. A more recent test done by R&D services of Cookville, Tennessee, indicates a 3.024 R-per-inch."

Many of these natural products have a very low impact on the environment and perform excellently, sometimes better than synthetic insulation like rock wool or fibre glass insulation.

If straw bales are used in the roof, their weight needs to be considered. Moisture is another consideration, and there is a fire risk if any loose straw is left exposed. Weight considerations are overcome by the fact that web-beams built to the height of the bales can easily bear their weight. To avoid moisture problems, it is important that the bales be treated just as walls are. They need to have good ventilation on the outer surface (a ventilation space) and should be coated with some plaster (typically clay or lime plaster) that can absorb, redistribute and release to the air any moisture. It cannot be overemphasised that no straw should be left exposed, plastering should be done in such a way that it acts as a suitable fire retardant.

Glossary of Terms

Bale Needle

A pointed metal rod or plate with a handle at one end and a hole at the other used to push twine through the bales and stitch them from one side to the other, holding mesh tightly to each surface.

BTU or British Thermal Unit

This is a unit for measuring energy which is now mostly replaced by the joule. One BTU is the amount of heat required to raise the temperature of one pound avoirdupois of water by one degree Fahrenheit. One BTU is approximately 1054–1060 joules.

Cold bridge

If a structure is made a various materials some of which insulate more than others, any part of the structure which is a potential path warmth can use to escape is a cold bridge. A common example is a well insulated house with solid aluminium windows which transfer large amounts of heat throght the structure.

Ecological footprint

The land, air and water that a city or nation needs to produce all of its resources and to dispose of all its waste. It is a way to determine if the lifestyle of a community is sustainable. It shows if a city or nation is utilizing more or less than its fair sustainable share of the world’s resources.

Embodied energy

The total energy used to bring a product or material to its present phase in its life cycle. It includes the energy required to extract or produce raw materials, their transport to the place of production, and the energy used for manufacturing. It can also include the energy used in the distribution and retail chain, for maintenance processes, for repair, etc. It is measured in MJ per kg or GJ per tonne.

End-of-Life (EoL)

The moment when a product ceases to fulfil the tasks it was designed for. The end-of-life of a product is not the end of its life cycle, since its environmental impact has not yet come to an end; the disassembly, recycling, incineration, and/or disposal phases still remain.

Gabion

A metal cage full of some hard material, typically stones. Often used for retaining wall especially on river sides. Can be succesfully used as part of a building foundation.

HVAC

Heating, Ventilation and Air Conditioning.

Infill

Straw bales used between the vertical elements of a structure to form non-bearing walls and act as insulation. Often an option where building regulations are otherwise too restrictive.

Life Cycle Analysis or Life Cycle Assessment (LCA)

A calculation of the environmental impact of a product over its complete life cycle. It starts with an inventory of the ’input’ (all resources and energy consumption) and ’output’ (emissions, solid waste, waste water). The elements in this inventory are grouped into environmental categories, which are quantified according to their environmental impact. The goal is to compare different design strategies within a category.

Load-bearing

A load bearing wall is one where all or most of the weight of the building is taken by the straw bale walls. The walls are 'bearing' the 'load'.

Modified Post and Beam

See Post and Beam. Modified simply means that the dimensions are modified to suit the dimensions of your straw bales.

Post and Beam

A construction using vertical elements (posts) and horizontal elements (beams) to form a structural framework. The term often refers to using a smaller number of larger than normal timbers compared to conventional 'baloon' timber framing.

R value

Standard insulation value which measures the Resistance in a material to the passage of heat. An R-value is the inverse of a U-value which measures the conductance in a material of heat.

Rubble trench foundation

A trench dug into the ground and filled with a rigid material such as demolition rubble, gravel or sea shells. Important functions of such a wall are: it cannot be further compressed, and moisture will not rise through the rubble into the wall.

Precompression

When stress it put into your structure some elements will be compressed. Pre-compression adds this stress to the building before it is finished to stop the structure suddenly settling, or the plaster suddely cracking, once finished.

Stem walls

A stemwall is the part of the foundation between the floor level and ground level, and may rest on and be attached to a rubble trench or whatever else is in the ground. It can be made of concrete blocks or such or be concrete poured into forms. Think raised foundation.

Subgrade

The ground of the site and anything below the surface. Subgrade foundation insulations is therefore insulation below the finished ground level to insulate the foundation from the surrounding temperature.

Top Plate

plate (normally wood) used to precompress bale wall, use as a roof connection, and help distribute roof weight.

web-beam

A beam made up of a large number of small elements, typically in a criss-cross pattern, which together perform as one large beam.

Heating specific definitions

A.F.U.E.: Annual Fuel Utilization Efficiency represents the percentage of fuel that is converted into usable heating energy - the balance is vented through your chimney or other venting systems. It is an industry agreed upon standard. All furnaces and brands are tested the same way to provide "apples to apples" comparisons.

Air Conditioner - a device used to decrease the temperature and humidity of air, which moves through it. Typical air conditioners include central air conditioning which utilizes existing forced air ductwork and “Ductless Splits”.

Anode Rod - a sacrificial metal used to protect against corrosion in a hot water heater.

Baseboard Heating - heating elements located around the perimeter of a room, used to warm room air by transferring the heat from the hot water circulating through them.

Blower – an air handling device used with a furnace to circulate air through a network of ducts.

Boiler: A heating appliance that heats water to a pre-set temperature and feeds it to a circulator, which transfers the water to radiant heating units including some or all of cast iron radiators, slim baseboard radiators, under floor tubing or wall panels. Some boilers produce steam for heating purposes.

B.T.U.: British Thermal Units are the standard efficiency comparison between heating fuels. One BTU is the amount of heating energy that will raise one pound of water one degree Fahrenheit.

B.T.U./Hr.: British Thermal Units Per Hour. Used to express capacities of furnaces and boilers.

Burner - a device which supplies a mixture of air and fuel to the combustion area.

Cast Iron - a durable metal with an exceptional capability to hold and transfer heat.

Chimney Liner: A clay-tile or metal liner that is inserted into a chimney.

Chimney Venting - a vertical vent used to transfer products of combustion from a furnace or boiler to the outdoors.

Combustion - the process of converting fuel into heat. This requires oxygen.

Combustion Air: An air supply brought into the furnace's combustion chamber - supplied from within the basement, or from outdoors. Combustion air is necessary to burn fuel.

Controls: Devices such as a thermostat that regulate a heating or cooling system.

Convection: The transfer of heat through a moving gas (air) and a surface, or the transfer of heat from one point to another within a gas. In hydronic heating, cool air falls to the floor where it is heated by metal fins in a baseboard radiator and then rises to transfer heat to the environment through natural convection.

Convective Heat: the natural circulation of air across a heat source to heat the air.

Degree Days: A system by which heating oil dealers measure and record the daily temperature. This information is compared to what they know about your heating system to ensure automatic delivery before your system uses all of the oil in the storage tank.

Direct Venting: A process in which the products of combustion are vented to the outdoors via sidewall venting, (without the use of a chimney).

Direct Vent - a furnace or boiler design where all the air for combustion is taken from the outdoors and all exhaust products are released to the outdoors, also known as sealed combustion. Direct Vent is also known as balanced flue venting in oil furnaces and boilers.

Distribution System: The component of a heating or cooling system that delivers warmed or cooled air, or warmed water, to the living space.

Draft Hood - a device that prevents a backdraft from entering the heating unit or excessive chimney draw from affecting the operation of the boiler or furnace.

Ductless Split A/C System - A system that cools and dehumidifies air without the use of conventional duct work. The equipment location is split, with the condenser and heat pump outside of the home and the air handler and controls inside.

Efficiency Rating - the ratio of heat actually generated versus the amount of heat Theoretically possible from the amount of fuel inputted.

Flame Retention Burner: A modern oil burner which retains the flame near the mouth of the burner, for improved efficiency and operational savings.

Flue: An enclosed passage that is designed to convey hot flue gases. (Also known as a breech).

Flue Gases: The gases (eg. carbon dioxide, water vapour and nitrogen) that are formed when the fuel oil, natural gas, or propane is burned with the air. (Products of combustion are technically all of the flue gases less the nitrogen that was present before combustion).

Forced Air: A distribution system in which a fan circulates air from the heating or cooling unit to the rooms through a network of supply air and return air ducts.

Furnace: A heating appliance that warms air around a heat exchanger. The air is conveyed by fan, into a central duct system to distribute warm air to all areas of the home or building.

Heat Exchanger: A structure that transfers heat from the hot combustion gases inside the furnace heat exchanger to the circulating room air flowing across the exterior of the heat exchanger.

Heat Loss: Term used for all areas of your home where heated air may escape due to construction styles, age of house, windows, weather-stripping, etc. All homes will experience some level of heat loss.

Heat Loss Calculation: This is the means by which a heating contractor will determine the required capacity of a furnace or boiler to adequately heat the home (or building).

Heat Recovery Ventilator (HRV): A device used in central ventilation systems to reduce the amount of heat that is lost as household air is replaced with outside air. As fresh air enters the house, it is warmed as it passes through a heat exchanger, heated by the warm outgoing air stream.

Heat Transfer: the transmission of heat from the source (flame) to air or water.

Heating Capacity: the amount of usable heat produced by a heating unit.

High-boy: a term used to describe a furnace which has a small "footprint" but is tall. The blower is under the heat exchanger. This is also known as an upflow furnace.

Hot Water Boiler: a heating unit that uses water circulated throughout the home in a system of baseboard heating units, radiators, and/or in-floor radiant tubing.

Hot Water Heater: a unit with its own energy source that generates and stores hot water.

Hydronics: Hydronics, or heating with water, consists of a compact boiler (fired by any fuel) that heats water, which is distributed to a network of slim baseboard, panel or space radiators, or under floor tubing by a circulator. This term also applies to the science of heating (or cooling) with water.

Indirect Hot Water Storage Tank: a unit that works in conjunction with a boiler to generate and store domestic hot water, it does not require its own energy source.

In-floor Radiant Tubing: tubing, typically plastic or rubber, used in conjunction with heated boiler water to heat floors.

Low-boy: a term used to describe a furnace which has a low profile. The blower is located on the same level plane as the heat exchanger. This furnace style has both the return air plenum and the supply air plenum on the top of the furnace. This furnace style is sometimes called a console style furnace.

Low Water Cut-off: a device used to shut down a boiler in the event that a low water condition exists. This is required whenever radiators are located at a lower level than the boiler. Some jurisdictions require them on all boiler installations.

Natural Gas: any gas found in the earth (e.g. methane gas) as opposed to gases which are manufactured.

Nozzle: A burner component that atomizes, meters and patterns fuel oil into the heat exchanger / fire-pot.

Oil Heating: the production of heat by burning oil.

Propane: a manufactured gas typically used for cooking or heating. This is also known as L.P.G. (liquid petroleum gas).

Push Nipples: machined metal sleeves used to join adjacent sections of a boiler.

Radiant Floor Heating: Under floor heat is provided by flexible, long-lasting tubing. The continuous tubing can be placed under any flooring, and circulated hot water provides invisible heat anywhere in the home, swimming pool or driveway.

Radiant Heating: the method of heating the walls, floors or ceilings in order to transfer heat to the occupants of a room.

Radiator: a heating element, typically metal, used in conjunction with water or steam to give off heat.

Retrofit: Replacement of one or more components of an existing system.

Safety Shut-off Device: any device used to shut down a heating appliance in the event an unsafe condition exists.

Seasonal Efficiency: A performance rating that considers the heat actually delivered to the living space, the total energy available in the fuel consumed, and the impact the equipment itself has on the total heating load through an entire heating season.

S.E.E.R.: Seasonal Energy Efficiency Rating. The standards by which equipment is measured. The higher the S.E.E.R., the more efficient the equipment (especially air conditioning).

Sealed Combustion: a furnace or boiler design where all the air for combustion is taken from the outside atmosphere and all exhaust products are released to the outside atmosphere, also known as direct vent.

Steam Boiler: a heating unit designed to heat by boiling water, producing steam, and circulating it to radiators or steam baseboard units throughout the home.

Stack Damper: a device installed in the venting system that will automatically close when the appliance shuts down. This device is used to reduce the amount of warm indoor air being drawn up the chimney between heating cycles.

Supply Tapping: opening in a boiler by which hot water enters the heating system. Setback Thermostat: A programmable thermostat with a built-in timer. You can adjust it to vary household temperature automatically.

Tankless Heater: a copper coil submerged into the heated boiler water used to transfer heat to domestic water.

Venting: An opening for combustion gases to exit the house. Can be a chimney or a vent through the wall of the house. Includes all parts of the venting system - vent connector, chimney, etc.

Zone Control: A heating control system in which the space to be heated is divided into zones and each zone is controlled by a separate thermostat.

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