The strawbale is a terrific source of insulation. Its R-value is comparable to dense-packed cellulose – after all, straw is basically cellulose. It’s easy to build with, coming in a Lego-like package that’s easy to handle and stack. Surprisingly, it’s more fire-resistant than fiberglass batts – the material is packed so densely that no oxygen can flow. And best of all, it’s a naturally occurring byproduct with a low embodied energy and no toxins to mess with air quality.
One can build strawbale walls strong enough to support a roof, and to date the majority of strawbale construction has taken this form, as seen previously on Tiny Tuesday. There’s an appealing economy of material when the insulation is also the structural system, but it requires specially trained builders to build strawbale walls with the proper reinforcing, and few building codes recognize it. A different system holds more promise to bring strawbales into the mainstream.
Straw cell hybrid construction combines the insulation properties of the straw with the familiarity and constructability of stick frame. Dimensional lumber, either 2×4 or 2×6, is used to frame exterior walls and support roofs. Then, directly inside of the studs, stacked strawbales (typically 18 inches wide) form a nonstructural insulation layer. Finally, the lumber cavity is filled with dense-packed cellulose. The total thickness of insulation is 22 to 24 inches, for an R-value of 45 to 50.
Strawbales provide a flexible interior surface: builders can carve out niches and round off corners, finishing the walls with plaster. Meanwhile, the exterior wall is dimensional lumber spaced 16 or 24 inches on center, enabling the use of plywood or OSB sheathing and wood or vinyl siding. The assembly requires about half the lumber needed for double-wall construction.
Determining the return on investment for straw-cell construction – or for any non-traditional building method – is difficult. One must evaluate the cost of materials and labor, the land value (24-inch-thick walls steal a lot of potential floor space), and personal values such as embodied energy and occupant health. Consider this a work in progress!
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