Straw Bales & Solar Energy -- A Natural Partnership

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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.

Building with straw can completely change how we use resources in construction, how we heat and cool our homes, and how we relate to the buildings we inhabit.

Straw bale building has unique and important answers to a few broad questions that help us get to the heart of sustainable building:

  • What materials will have the lowest environmental impacts during and after construction?
  • What materials and techniques will result in the most effective building—one that is durable, efficient, safe, healthy, and comfortable?
  • What materials can be used together to make a building appealing over generations because of its resonant beauty, sense of solid shelter, and peaceful comfort?

Low-Impact Building

The straw in bales is left over from grain harvesting. Crops aren’t grown just to make straw bales: the plants are being grown for rice, wheat, barley, or oats. Once the grain is harvested, the stalks are bundled together into bales.

Straw is an abundant agricultural by-product with few uses, and using straw bales as a building material is a great example of upcycling. Unlike wood, straw is an annual crop and can often be sourced locally—frequently from less than 100 miles from a building site. That results in a small carbon footprint: other than the energy it takes to bale the leftover stalks and then deliver that bale to your job site, all of the other resources needed to produce that bale were used for the production of food.

The California Straw Building Association (CASBA) is currently studying how much carbon a straw bale wall sequesters. Plants draw carbon out of the air as they grow and lock it up, releasing it only when they decompose (or burn, which is what happens to many grain fields when the straw is not upcycled). By keeping the straw in the wall, the carbon is not released into the atmosphere (see “Carbon Sequestration” sidebar).

Plastered straw bales replace the insulation and drywall, and often the paint. They can reduce the amount of lumber needed for framing, and may even be part of the structure. Leftover straw can be used as fiber in the plaster, or used onsite as mulch without further processing. And at the end of a long life in a straw bale building, a straw bale is biodegradable.

Natural plasters used with straw bale construction also have lower impact than most wall finishes (such as cement stucco exterior or painted drywall interior), especially clay plaster. Clay is another abundant material, and can be found almost everywhere. Unlike cement—which uses a huge amount of energy during processing—clay requires little processing, so it has very low embodied energy.

When tracking the energy a building consumes, we can’t overlook the energy that’s consumed during the mining and manufacturing processes for the building materials. This embodied energy can greatly exceed a building’s operating energy. In their presentation, “The Carbon Elephants in the Room,” builders Chris Magwood and Jacob Deva Racusin compare the energy requirements and carbon emissions of embodied versus 35-year operational data for four different houses in two different climates. While operational energy use has been reduced greatly in high-performance homes, if they are constructed with conventional materials (like polyurethane foam insulation), their embodied energy can still be high. They found that the carbon embodied in the materials used to build such a home was greater than half of the home’s 35-year operational carbon emissions. Furthermore, compared to a high-performance home made of low embodied-energy (EE) materials (such as straw), the embodied carbon of the conventional home was higher than the combined 35-year operational carbon and the embodied carbon of a low EE high-performance home.

To make the best dent in total energy use and carbon emissions, we need to reduce both operational and embodied energy. The building industry is beginning to pay attention to embodied energy and carbon emissions through metrics like the Living Building Challenge and LEEDv4. We’re realizing that the energy meter shouldn’t start running after the building is built—the energy that went into making the building matters, too.

Effective Building

A building with the lightest footprint would probably be a mud and grass hut, but in many places that is not the most effective building to shelter us, to keep us safe and healthy in the long term. Straw bale buildings are high-performance. Everyone knows that a straw bale building is “natural” and “green,” but fewer people know just how effective it is!

R-value. Straw bale walls have a relatively high R-value—the measure of a material’s ability to resist the flow of heat. In the summer when it’s hot outside and cool inside, the heat will work its way through the walls. The higher the R-value of the wall, the longer it will keep that heat out.

The accepted rating for straw bales is R-1.45 per inch, and bales are 18 to 23 inches thick when laid flat. The accepted rating when the bales are on edge is R-1.76 per inch, and the bales are around 15 inches thick. The result is a wall with an insulation value between R-26 and R-33 (see “Thermal Performance” sidebar). If you ask your local energy specialist about an R-30 wall, they will probably tell you is it’s well-suited for most climates.

Thermal bridging is when a relatively non-insulating material, like wood, interrupts the insulation layer and bridges from one side of the wall to the other, more easily letting heat pass through. Because straw bale walls are so thick, penetrations are relatively shallow: the framing is on one side of the bales, usually only 4 inches deep, and electrical boxes penetrate only 4 inches. In an on-edge wall system, I-joists bridge the wall surfaces, but the wood webbing member is so thin and non-conductive that little heat is transferred.

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.

Appealing Building

Approaching sustainability requires longevity: we take care of things we love and the longer something lasts, the lower its environmental impact. Unloved buildings get knocked down; then there’s additional environmental cost to build new ones.

There’s no longevity without durability. Most conventional buildings are built to last 30 or 40 years. Compare this to straw bale structures built in the late 1890s that are still in use today.

Straw bale building owners care about their buildings because they’re charismatic. As soon as you start putting straw bales into a wall, people notice. People also notice a difference when they enter a straw bale home. Perhaps it’s the thick walls that offer a sense of security, or the hand-applied finishes that harken to a time when buildings were crafted, instead of manufactured.

Straw bale buildings invite participation. Because the materials are nontoxic, there is a tradition of getting friends and family involved in the process with work parties—days when volunteers come to help stack bales or plaster walls, not unlike an Amish barn raising. Work parties don’t make sense on every project, but can advance the construction process while engaging the community. Most people feel alienated from construction, and working on your own building is empowering: a foot in the door to further engagement. This leads to a feeling of commitment and stewardship for the building, which can lead to greater longevity.

Of course, straw bale is just the wall. For a truly healthy, high-performance building, attention must be paid to the rest of the building system, such as minimizing its overall size and its loads; incorporating renewable energy systems; selecting high-performance HVAC, electrical, and plumbing systems; and choosing nontoxic finishes throughout the building. With straw bale’s Appendix S added to the International Residential Code in 2015, and the California Straw Building Association’s (CASBA, strawbuilding.org) Detail Book soon to be published, this form of building will receive the acceptance and recognition it deserves.

Web Extras

See a load-bearing shake-table test at youtube.com/watch?v=x8Uz-2PonEk

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