Bale walls can be load-bearing (“Nebraska style”), or used as infill for a timber- or stick-frame that carries roof loads. Bales are stacked flat or on edge between framing members (for infill construction) atop a conventional concrete stem wall, and then finished with earthen or lime plaster, or cement stucco, to protect the building from the elements. Window and door openings are created with wood frames. For load-bearing walls, bales are typically compressed under a top plate to distribute the weight and help prevent uneven settling. In the original Nebraska straw bale homes, roof loads were evenly distributed around the perimeter of the building with hip roofs, and window sizes were limited. Infill designs are well-suited to complex house shapes. Straw is resistant to rot when protected from rain and snow, and is fire-resistant because its density limits combustion air (likened to trying to burn a telephone book).
Given their thickness, straw bale walls should have higher R-values and less thermal bridging than wood-framed walls insulated with fiberglass or cellulose. In tests at the Oak Ridge National Laboratory, a 19-inch-thick straw bale wall had an R-value of 27.5, or 1.45 per inch. By comparison, a 2-by-6 wall insulated with standard fiberglass batts (and no exterior insulation) would have an R-value of roughly 20 at the center of the wall (less when thermal bridging is taken into account for a whole-wall measurement).
Straw bale construction is not covered in the International Residential Code, meaning that prospective builders need approval from local code officials.
Building a double-stud wall is an easily understood approach to achieving an energy-efficient envelope, using the same techniques and materials that go into a conventionally built house. Two parallel-framed walls are framed from dimensional lumber—one load-bearing and the other a nonweight-bearing partition. The walls are separated by a gap and studs in the two walls can be staggered—or not. Then the wall is filled with insulation, which could be cellulose, fiberglass batts, or something else, which can result in R-40 or above.
This strategy is simple and effective, and used on many high-performance houses, including a Habitat for Humanity house in Wheat Ridge, Colorado, that was designed by Habitat and the National Renewable Energy Laboratory. These designers had two goals: create a net zero-energy home, with the house producing as much energy as it used each year, and have Habitat volunteers build it, not professionals.
The designers spaced the two 2-by-4 walls 3 1/2 inches apart. Fiberglass batts were installed vertically in both stud walls, with a third layer of batts placed horizontally in the space between the two walls. Even though fiberglass batts can be one of the least-effective types of insulation when poorly installed in a conventionally framed house, they proved highly effective here.
Other builders might construct the outer, load-bearing wall from 2-by-6s, for example, to give even more room for insulation. The gap between the two walls eliminates the path for thermal bridging, so double-stud walls don’t need a layer of rigid foam insulation over the sheathing, which simplifies construction considerably while holding costs down.
A trade-off is that some floor space is lost to the second framed wall. In houses with very small footprints, this could be an issue. And, since the framing is duplicated, it doubles the amount of time it takes to frame the house. Applying advanced framing techniques reduces the amount of lumber and construction time. Finally, because of the extra wall depth, detailing doors and windows is a little more complicated. Windows set to the outside of the wall require extra-deep jamb extensions, and some builders flare window openings so walls don’t look too thick, which is also an extra step.