Straw Bales & Solar Energy -- A Natural Partnership: Page 3 of 4

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Straw bale walls can be part of a whole-house plan to achieve high energy efficiency while keeping embodied energy low.
In a post-and-beam structure, bales are notched around the structural uprights, preventing thermal bridging. (See "Straw Bale Walls" sidebar.)
In an I-joist structure, bales are fit between the joists, which reduces thermal bridging compared to a conventionally framed structure. (See "Straw Bale Walls" sidebar.)
A bale wall will rest on a raised and insulated double sill.
Author and builder Rebecca Tasker (center) helps homeowners ceremonially set the first bale.
Exterior stucco can be lime-cement or, for reduced embodied energy, earthen-based, and any plaster used must be vapor-permeable.
Bale wall systems have fewer thermal bridges that cause heat flow across the assembly.
Well-sized roof overhangs protect the walls, and shade south-facing walls and windows from the intense summer sun. (See "Southern (Oregon) Comfort in a Solar Straw Bale Home" sidebar.)
A double airlock entry reduces air exchanges by stopping blow-through when the outer door is opened. (See "Southern (Oregon) Comfort in a Solar Straw Bale Home" sidebar.)
Clerestory windows let in natural light and admit solar heat during winter. (See "Southern (Oregon) Comfort in a Solar Straw Bale Home" sidebar.)
Thick walls make deep windowsills, for an old-world feel. (See "Southern (Oregon) Comfort in a Solar Straw Bale Home" sidebar.)
High ceilings paired with well-placed windows promote convective cooling. (See "Southern (Oregon) Comfort in a Solar Straw Bale Home" sidebar.)
A natural earthen floor covers R-15 insulation and provides thermal mass for storing passive solar gain. (See "Southern (Oregon) Comfort in a Solar Straw Bale Home" sidebar.)
Multiple layers of natural plaster, both interior and exterior, mitigate diurnal temperature swings inside the building.
Thick walls make for deep door and window openings. There are several structural and aesthetic ways to approach this.
Thick walls make for deep door and window openings. There are several structural and aesthetic ways to approach this.
The relatively simple, but labor-intensive, aspect of building with bales fosters community involvement with “bale raising” parties.

Air leakage. Straw bale walls are pretty good at preventing air leakage, too. Because plastered straw bale walls have fewer edges than materials like plywood and sheetrock, there are fewer seams to seal. Well-built straw bale homes have reached Passive House standards for air-tightness—a maximum of 0.6 air changes per hour at 50 Pascals pressure (ACH50). As with any tight building envelope, attention must be paid to getting fresh air through natural or mechanical ventilation.

Thermal mass provides heat storage. Imagine a rock sitting in the sun all day—the rock stays warm after the sun sets and the air temperature drops, because the rock has a lot of thermal mass and air has very little. The plaster on straw bale walls is 1-inch-thick evenly distributed thermal mass. This helps slow the transfer of heat through the wall and also slows changes in temperature, so a warm room stays warm longer.

Fire & seismic resistance. Plastered straw bale walls have a high fire-resistance rating: 1 hour for clay plaster and 2 hours for lime-cement, which both compare favorably to conventional building. Straw bales are so dense, it’s like trying to burn a phone book—there isn’t enough oxygen available for combustion: they just smolder, allowing a lot of time before the walls are compromised. For comparison, 0.5-inch-thick gypsum wallboard has a 15-minute rating. To achieve a two-hour fire-resistance rating for a conventionally framed exterior wall, you’d need an assembly with 1 inch of exterior cement stucco over 5/8-inch-thick fire-retardant sheathing over retardant-treated 2-by-6 wood studs, with two layers of 5/8-inch-thick fire-resistant gypsum wallboard on the interior.

Earthquakes are an important consideration in some locations. Seismic testing on straw bale wall assemblies demonstrate that they’re up to the challenge. Because both the natural plasters and straw bales are flexible, they do well in earthquakes because they “bend” more than conventional materials before they break.

Vapor permeability. Straw bale walls are also vapor-permeable, which means that they allow water vapor to pass through them, though they don’t allow air or liquid water to enter. People are damp—a family of four can produce as much as 2 gallons of moisture a day from breathing, showering, and cooking. If we choose a wall system that traps that moisture, we get moldy, sick buildings. If a building can’t deal passively with moisture, we have to mechanically vent it.

The old adage about moisture was that “buildings have to breathe,” but that’s misleading because breathing entails air moving in and out—and we don’t want air leaks. A better way to put it is that “buildings need to transpire.” Clay or lime-plastered straw bale walls allow moisture to pass through without allowing air to pass through.

Clay plaster also has hygric mass, which is like thermal mass but for moisture—it “stores” moisture like a rock stores heat. Clay plasters can draw excess moisture out of the air when it is humid and store it until the air dries, then re-release it. This evens out spikes in humidity, making people feel more comfortable, and it reduces the risk of condensation and mold. Better indoor air quality is achieved using these nontoxic, zero-VOC materials that don’t trap moisture, reduce the risk of mold, and balance humidity.

Pair this super-insulated, low embodied energy, thermally massive wall system with passive solar design, and you get a structure that has relatively small heating and cooling loads. Climate-appropriate glazing on the south side; roof overhangs to limit summer heat gain; and reduced glazing, where summer sun or winter wind impacts interior temperatures, are important. Windows that encourage a thermal siphon for nighttime cooling can help moderate interior temperatures, too. A well-designed straw bale building can be comfortable year-round with little energy used for heating and cooling.

Other Considerations

One potential drawback to straw bale building is the thickness of the wall, which can be significant. Matts Myhrman, one of the straw bale building revival’s early leaders, famously quipped, “You can have anything you want in a straw bale house, except skinny walls.”

Another disadvantage is that straw bale building often has a higher up-front cost, and unfamiliarity of designers and builders with this system can add to costs. A well-designed and well-managed straw bale project can cost 10% to 15% more per square foot to build than a conventional home.

But be sure to compare apples to apples. If you compare straw bale to other well-insulated wall systems that can achieve R-30—such as double-stud and cellulose—the costs are very similar. And the main difference between a well-insulated wall system and conventional wall is that the energy bills will be significantly less over the building’s life, making up for the higher up-front cost. If you consider the embodied energy “cost” of building an R-30 wall with bales versus other materials, straw bale is less. In his book, Making Better Buildings, Chris Magwood compares the embodied energy (EE) of various wall systems of a sample house. A wood-framed wall insulated to R-30 with cellulose, with drywall on the interior and OSB and lime-cement stucco on the exterior adds up to 40,497 megajoules (MJ). The EE of a bale-laid-flat, post-and-beam straw-bale wall with 2-by-4 framing at doors and windows, and lime-cement plaster on the interior and exterior, was about half, at 19,145 MJ.

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