Straw Bales & Solar Energy -- A Natural Partnership: Page 2 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.

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.

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