Insulation
Straw-bale walls have been tested for heat transmission, and have been rated as high as R-55. A conservative number, used by the California Energy Commission, is R-30. A conventional 2 x 6 wall with fiberglass insulation contains insulation rated for R-19, but when the total wall assembly is considered, scores about R-9.
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Rot
Plowed into the ground, most straw takes a year to decompose. Rice straw, which has a high silica content, takes twice that time. Straw has been used as an insulating material for many centuries, and has been found in excellent condition in Egyptian tombs thousands of years old. If kept dry, straw will not degrade. It can be said, then, that the lifetime of straw in a building could be anywhere from three weeks to nine-thousand years, depending on how the building is cared for.

Fungus (dry rot) can occur in straw at humidity levels of above 20% (percentage of dry weight). In order for significant damage to occur, these humidity levels must be maintained over a period of time--once the bales dry out, the fungus will die. Consequently intermittent moisture is not a threat to bales. Sustained high levels of moisture must be avoided, however. Experience and test results suggest that the best way to avoid sustained high moisture concentrations lies in making certain that the bales are able to transpire any accumulated moisture back into the environment.

Bale construction differs significantly from wood stud construction in how moisture travels, collects and concentrates within the wall. Moisture entering a wood-frame cavity tends to collect and concentrate within the cavity, causing the moisture level to rise quickly to a level which supports rot. Moisture entering straw-bales tends to disperse through the bale, lessening the chances of reaching high humidity levels.

Components of wood-frame construction, such as plywood sheathing, studs and blocking, work to trap moisture in walls, perpetuating humidity levels. Plastered bales, by contrast, are uniformly permeable, and so tend to disperse and breathe off moisture rather than collect moisture, just as a sponge dries. Building paper, as commonly used to cover plywood walls, could introduce a barrier to the bales transpiration to the outside and create a surface where moisture could concentrate for extended periods.

Although, in theory, a dew point can occur within the bale walls during winter months, practice indicates that walls without exterior moisture barriers perform well, even in cold climates.

There are no historical precedents of bales being used with moisture barriers, and consequently there is no data on how the two perform together. Most historical data for unwrapped bale walls demonstrates the importance of walls of maximum breathability: a mansion in Huntsville, Alabama, has successfully endured Southern humidity since 1938; a 1978 building near Rockport, Washington, receives up to 75 inches of rain a year; and an unplastered building near Tonasket, Washington, with no foundation and unplastered walls shows no apparent deterioration of the bales since 1984. Recent bale structures in northern New York (humid winters) and Nova Scotia (cold humid winters) have been monitored and demonstrate good performance in these difficult climates.

Because of the excessive amount of moisture which occurs along the splash line at the bottom of a wall, a vapor-permeable paper such as Tyvek is sometimes used to cover the lower course of bales. The conventional two layers of building paper, which are required to assure that wood-frame walls remain dry, are not recommended.

Bale walls are vulnerable to moisture from the top and bottom faces, where moisture could enter into the center of the bale and remain for extended periods. Consequently building paper over the top of the bale walls and a capillary break between the footing and the first course of bales is specified. Because moisture in the bales tends to migrate down to the lower courses of the bales, a capillary break (gravel) rather than a waterproof membrane underneath the bales is preferred.

practical moisture tips
more on moisture
tips for keeping your bales dry
Top|

Fire
Straw-bale construction is exceptionally resistant to fire. Unlike stud construction, in which a series of chimneys (stud cavities) form the wall, bales are dense and difficult to burn. If ignited they tend to smolder and burn slowly, greatly lessening the risk to life. However, they are difficult to extinguish, as embers tend to slowly tunnel through the bales. Since plaster applied to the uneven bale surfaces tends to be thicker than normally found on buildings, the bales can be said to carry an extra layer of protection.

In bale building design, the object is to keep the baled straw from becoming loose straw. When jacketed by stucco and plaster, bales are highly resistant to fire. It is therefore important that bales which could be exposed to extreme heat or flame, whether in walls or roofs, be encased in plaster sheet rock, and/or stucco netting.

Although loose straw is easy to burn and a fire hazard, baled straw chars and smolders and does not easily support a flame. This is because the straw in the bales is densely packed, inhibiting oxygen flow to fuel combustion. Bales burn like heavy timbers--they tend to char on the outside, creating an insulating layer which further inhibits combustion.

In bale building design, therefore, the object is to keep the baled straw from becoming loose straw. This could occur if a fire or high heat source was able to heat the bale twine and cause it to burn or melt. The bales would then loose compaction and flakes or chunks of unbaled straw could come loose and fall out out of the structure, and become engulfed in flame. (Polypropylene twine is preferred in bale structures over baling wire, which is subject to rust).

When jacketed by stucco and plaster, bales are highly resistant to fire. When the bales are also wrapped with wire lath, the potential danger of burned and busted baling twine is, of course, greatly reduced. It is therefore important that bales which could be exposed to extreme heat or flame, whether in walls or roofs, be encased in plaster, sheet rock, and/or wire.

During construction, a great deal of loose straw can accumulate around the job site. Tradespeople are often not accustomed to the extra precautions necessary when working around straw. The job site should therefore be kept clear of accumulations of loose straw, and plumbers, welders and others should be cautioned when working close to bare bales. Fire extinguishers and water should be handy at all times. Although brief encounters with flames (say from a plumbers torch) will only singe the bales, a combination of loose straw, high winds, exposed twine and carelessness could result in a disastrous fire.

Generally, straw building can be considerably safer than conventional stud construction, and examples of the resilience and fire resistance of bale buildings are impressive: In New Mexico, an unattended candle burned the ceiling of a wall niche, which burned the wall above it before the owner returned and called the fire department. The firemen told the owner that if the same fire had begun in a conventional building, the entire building would have been seriously involved by the time the owner returned home. Instead, the only repairs necessary were minor plaster patching.

An exterior bale bench went though a fire storm in San Luis Obispo County in California. The fire destroyed the adjacent house and woods, yet the bale bench remains undamaged to this day. (Actually, two benches were involved. The bottom 2x4 plate of one was left protruding from the plaster. During the fire the 2x4 ignited and a few days after the fire the owner noticed that embers had slowly eaten their way through the bales inside the bench).
Top|

Pests
Compared with wood, there are few termites who like straw. At least once, termites entered a building, left the straw alone, and ate the wood windows. Consequently, the normal precautions used with wood construction should be followed.

Bales provide fewer spaces for pests than conventional wood framing, and should rodents enter a wall at a break in the plaster coating, they would be likely to make a place to stay. It would be very difficult for pests to travel through the bales, however. Unlike hay, straw contains very little nutritional substance and will not, in itself, support a pest population. Conventional precautions against pests should be more than adequate for straw-bale.

Some concerns have been raised about hay fever, toxins and pesticide residues. Of course, once the bales are encased in plaster it would be difficult for irritants to transfer into the dwelling. Clean, dry bales contain few molds or pests. And pesticides, at least for rice straw, are used early in the growth cycle, then discontinued. Mature rice contains extremely small amounts of pesticide residue.
Top|

Plaster
Straw bales provide an excellent mechanical key for plaster and stucco, and reinforcement is generally not needed for attachment of plaster to the walls. Reinforcement may be desired when stucco is used as part of the structural system, or as assurance against hairline cracking. A variety of techniques for attaching the netting are used, including stitching with baling twine and attachment through the bales with galvanized wire ties and large staples. Because of the natural undulations of the bales an irregular pattern of attachment rather than a simple grid works best, if care is taken that the netting is uniformly secure. Lime-based or earthen plasters are recommended for their ability to breathe.
Top|

Earthquakes
Compared to wood-frame structures, bale buildings are resilient and flexible. We believe bale walls will absorb forces of a seismic event, and have design bale walls as a secondary system to absorb energy during an earthquake and to provide a back-up structural system in the event of failure of the post and beam system. The two systems are integrated by notching the posts into the bales and then wrapping the entire wall with stucco netting and plaster.
Top|

Pipes, Wires and Cabinets
When running plumbing in straw-bale walls, pipes that could leak or sweat are carried in sleeves, as a precaution.

Electrical boxes and fixtures can be attached in a variety of ways. Flexible and rigid conduit may be used, and romex, particularly UF romex, is often laid between courses of bales.

Several methods are used for attaching cabinets, ledgers, intersecting walls, shelves, etc. depending upon the amount of strength required. These techniques include embedded bolts and stakes; wire or string ties; and simple wall anchors. For extreme strength requirements, bolts through a ledger sandwich, similar to seismic reinforcing bolts in masonry walls, may be used.

Insulation
Straw-bale walls have been tested for heat transmission, and have been rated as high as R-55. A conservative number, used by the California Energy Commission, is R-30. A conventional 2 x 6 wall with fiberglass insulation contains insulation rated for R-19, but when the total wall assembly is considered, scores about R-9.
Top|

Rot
Plowed into the ground, most straw takes a year to decompose. Rice straw, which has a high silica content, takes twice that time. Straw has been used as an insulating material for many centuries, and has been found in excellent condition in Egyptian tombs thousands of years old. If kept dry, straw will not degrade. It can be said, then, that the lifetime of straw in a building could be anywhere from three weeks to nine-thousand years, depending on how the building is cared for.

Fungus (dry rot) can occur in straw at humidity levels of above 20% (percentage of dry weight). In order for significant damage to occur, these humidity levels must be maintained over a period of time--once the bales dry out, the fungus will die. Consequently intermittent moisture is not a threat to bales. Sustained high levels of moisture must be avoided, however. Experience and test results suggest that the best way to avoid sustained high moisture concentrations lies in making certain that the bales are able to transpire any accumulated moisture back into the environment.

Bale construction differs significantly from wood stud construction in how moisture travels, collects and concentrates within the wall. Moisture entering a wood-frame cavity tends to collect and concentrate within the cavity, causing the moisture level to rise quickly to a level which supports rot. Moisture entering straw-bales tends to disperse through the bale, lessening the chances of reaching high humidity levels.

Components of wood-frame construction, such as plywood sheathing, studs and blocking, work to trap moisture in walls, perpetuating humidity levels. Plastered bales, by contrast, are uniformly permeable, and so tend to disperse and breathe off moisture rather than collect moisture, just as a sponge dries. Building paper, as commonly used to cover plywood walls, could introduce a barrier to the bales transpiration to the outside and create a surface where moisture could concentrate for extended periods.

Although, in theory, a dew point can occur within the bale walls during winter months, practice indicates that walls without exterior moisture barriers perform well, even in cold climates.

There are no historical precedents of bales being used with moisture barriers, and consequently there is no data on how the two perform together. Most historical data for unwrapped bale walls demonstrates the importance of walls of maximum breathability: a mansion in Huntsville, Alabama, has successfully endured Southern humidity since 1938; a 1978 building near Rockport, Washington, receives up to 75 inches of rain a year; and an unplastered building near Tonasket, Washington, with no foundation and unplastered walls shows no apparent deterioration of the bales since 1984. Recent bale structures in northern New York (humid winters) and Nova Scotia (cold humid winters) have been monitored and demonstrate good performance in these difficult climates.

Because of the excessive amount of moisture which occurs along the splash line at the bottom of a wall, a vapor-permeable paper such as Tyvek is sometimes used to cover the lower course of bales. The conventional two layers of building paper, which are required to assure that wood-frame walls remain dry, are not recommended.

Bale walls are vulnerable to moisture from the top and bottom faces, where moisture could enter into the center of the bale and remain for extended periods. Consequently building paper over the top of the bale walls and a capillary break between the footing and the first course of bales is specified. Because moisture in the bales tends to migrate down to the lower courses of the bales, a capillary break (gravel) rather than a waterproof membrane underneath the bales is preferred.

practical moisture tips
more on moisture
tips for keeping your bales dry
Top|

Fire
Straw-bale construction is exceptionally resistant to fire. Unlike stud construction, in which a series of chimneys (stud cavities) form the wall, bales are dense and difficult to burn. If ignited they tend to smolder and burn slowly, greatly lessening the risk to life. However, they are difficult to extinguish, as embers tend to slowly tunnel through the bales. Since plaster applied to the uneven bale surfaces tends to be thicker than normally found on buildings, the bales can be said to carry an extra layer of protection.

In bale building design, the object is to keep the baled straw from becoming loose straw. When jacketed by stucco and plaster, bales are highly resistant to fire. It is therefore important that bales which could be exposed to extreme heat or flame, whether in walls or roofs, be encased in plaster sheet rock, and/or stucco netting.

Although loose straw is easy to burn and a fire hazard, baled straw chars and smolders and does not easily support a flame. This is because the straw in the bales is densely packed, inhibiting oxygen flow to fuel combustion. Bales burn like heavy timbers--they tend to char on the outside, creating an insulating layer which further inhibits combustion.

In bale building design, therefore, the object is to keep the baled straw from becoming loose straw. This could occur if a fire or high heat source was able to heat the bale twine and cause it to burn or melt. The bales would then loose compaction and flakes or chunks of unbaled straw could come loose and fall out out of the structure, and become engulfed in flame. (Polypropylene twine is preferred in bale structures over baling wire, which is subject to rust).

When jacketed by stucco and plaster, bales are highly resistant to fire. When the bales are also wrapped with wire lath, the potential danger of burned and busted baling twine is, of course, greatly reduced. It is therefore important that bales which could be exposed to extreme heat or flame, whether in walls or roofs, be encased in plaster, sheet rock, and/or wire.

During construction, a great deal of loose straw can accumulate around the job site. Tradespeople are often not accustomed to the extra precautions necessary when working around straw. The job site should therefore be kept clear of accumulations of loose straw, and plumbers, welders and others should be cautioned when working close to bare bales. Fire extinguishers and water should be handy at all times. Although brief encounters with flames (say from a plumbers torch) will only singe the bales, a combination of loose straw, high winds, exposed twine and carelessness could result in a disastrous fire.

Generally, straw building can be considerably safer than conventional stud construction, and examples of the resilience and fire resistance of bale buildings are impressive: In New Mexico, an unattended candle burned the ceiling of a wall niche, which burned the wall above it before the owner returned and called the fire department. The firemen told the owner that if the same fire had begun in a conventional building, the entire building would have been seriously involved by the time the owner returned home. Instead, the only repairs necessary were minor plaster patching.

An exterior bale bench went though a fire storm in San Luis Obispo County in California. The fire destroyed the adjacent house and woods, yet the bale bench remains undamaged to this day. (Actually, two benches were involved. The bottom 2x4 plate of one was left protruding from the plaster. During the fire the 2x4 ignited and a few days after the fire the owner noticed that embers had slowly eaten their way through the bales inside the bench).
Top|

Pests
Compared with wood, there are few termites who like straw. At least once, termites entered a building, left the straw alone, and ate the wood windows. Consequently, the normal precautions used with wood construction should be followed.

Bales provide fewer spaces for pests than conventional wood framing, and should rodents enter a wall at a break in the plaster coating, they would be likely to make a place to stay. It would be very difficult for pests to travel through the bales, however. Unlike hay, straw contains very little nutritional substance and will not, in itself, support a pest population. Conventional precautions against pests should be more than adequate for straw-bale.

Some concerns have been raised about hay fever, toxins and pesticide residues. Of course, once the bales are encased in plaster it would be difficult for irritants to transfer into the dwelling. Clean, dry bales contain few molds or pests. And pesticides, at least for rice straw, are used early in the growth cycle, then discontinued. Mature rice contains extremely small amounts of pesticide residue.
Top|

Plaster
Straw bales provide an excellent mechanical key for plaster and stucco, and reinforcement is generally not needed for attachment of plaster to the walls. Reinforcement may be desired when stucco is used as part of the structural system, or as assurance against hairline cracking. A variety of techniques for attaching the netting are used, including stitching with baling twine and attachment through the bales with galvanized wire ties and large staples. Because of the natural undulations of the bales an irregular pattern of attachment rather than a simple grid works best, if care is taken that the netting is uniformly secure. Lime-based or earthen plasters are recommended for their ability to breathe.
Top|

Earthquakes
Compared to wood-frame structures, bale buildings are resilient and flexible. We believe bale walls will absorb forces of a seismic event, and have design bale walls as a secondary system to absorb energy during an earthquake and to provide a back-up structural system in the event of failure of the post and beam system. The two systems are integrated by notching the posts into the bales and then wrapping the entire wall with stucco netting and plaster.
Top|

Pipes, Wires and Cabinets
When running plumbing in straw-bale walls, pipes that could leak or sweat are carried in sleeves, as a precaution.

Electrical boxes and fixtures can be attached in a variety of ways. Flexible and rigid conduit may be used, and romex, particularly UF romex, is often laid between courses of bales.

Several methods are used for attaching cabinets, ledgers, intersecting walls, shelves, etc. depending upon the amount of strength required. These techniques include embedded bolts and stakes; wire or string ties; and simple wall anchors. For extreme strength requirements, bolts through a ledger sandwich, similar to seismic reinforcing bolts in masonry walls, may be used.