Where’s the straw?

    Some people actually expect to see little bits of straw sticking out of the walls here and there. Actually, the thick walls and incredible soundproofing of a straw bale structure make it look and feel more like adobe construction than any other style.

    The straw bale buildings found in California are actually a combination of wood or steel fame and straw. Although there are good indications that a stacked straw bales can safely support a roof, this is not yet permitted locally. So, a wood or steel post and beam “frame” is constructed to support the roof, and the walls are filled in with the straw bales. Straw bales are placed in a rows end-to-end, and the rows are stacked to make a wall. The bales are firmly bound together with wire. The entire wall is covered with a wire mesh, and stucco or some form of plaster is applied to both protect the straw from moisture and to give the wall its finished appearance.

Strawbale infill of a steel frame structure (Friends Center, Nov 2007)

Energy efficiency

    Perhaps the premier characteristic of a properly finished straw bale wall is its incredible thermal insulation.

    Thermal insulation of a wall (or anything else, for that matter) is rated by its “R” value. A standard wood wall framed with 2x4s and standard fiberglass insulation is rated R13; a 2x6 frame is rated R19 (the bigger the number, the better the insulation).

    California rather conservatively rates a straw bale wall as R30. This would be equivalent to a well-insulated wood wall more than 9 inches thick. (New Mexico uses a value of R55 for the same wall, which would be equivalent to 17 inches!).

    The result is that a well-constructed straw bale home will cost MUCH less to heat and cool.

Sounding out straw

    In addition to thermal insulation, straw bale provides amazing acoustic insulation; the thick walls simply soak up sounds from the outside. When you walk into a straw bale building you are immediately struck by how quiet it is.

Fire it up

    You would think that straw would be a fire hazard, quick to burn. Not so. To have a fire, you need fuel, oxygen and heat. Loose straw will burn in open air, although not terribly well. However, the straw bale is packed so tightly that the oxygen needed for combustion can’t enter. As a result, a straw bale structure has a minimum 2-hour fire rating; the standard for wood-frame is 1 hour.

    A number of tests have verified the substantial fire-resistance of straw bales. In one test, the “outside” of a straw bale wall was exposed to a temperature of 1,000 degrees increasing to 1,750 degrees over the period of an hour; the “inside” temperature only rose about 10 degrees. Wood would have burned; steel would have buckled. Indeed, most of the danger for straw bale structures comes from the roof or exposed eaves that can catch fire more easily if not properly constructed.

Bug out

    Lastly, straw bale owners never have to worry about termites. There is little nutrition in the straw; bugs won’t touch it, even if they could get at it.

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Perform Wall… performs

The key ingredients of Perform Wall panels are Portland cement and polystyrene beads (that’s Styrofoam to us non-chemists). The panels are about 85% polystyrene, making them light with excellent thermal insulating properties.

Perform Wall panels (right), and a close-up of an installed panel (left). Note the round channels in the panels, which are filled with steel and concrete to tie the stacked panels together.

    The panels themselves are 12 inches thick (meaning the finished wall is 12 inches wide) and come in a variety of lengths and heights. The panels are penetrated by a series of channels (see photo). To construct a wall, the panels are stacked up, steel reinforcement is run down through the channels, and the channels are filled with concrete, tying the structure together. Joints between adjacent panels are filled with a bonding material to make a strong, solid wall.

    The walls have a 4-hour fire rating, compared to a 1-hour rating for standard wood-frame construction. A Perform Wall structure also has a STC (Sound Transmission Class) rating of 50, vs. 34-39 for a wood 2x4/drywall combination, making it much quieter. Further, cement and plastic are a little low on the nutrition side, so they don’t support the growth of fungus or mold.

A PerformWall home being built. The ability to easily cut and shape the panels makes curves and angles much easier to construct than with standard wood frame methods.