Straw-Bale Construction

Straw-Bale Construction: A Review of Testing and Lessons Learned To Date, Building Safety Journal, May-June 2004, provides a thorough overview of straw-bale construction methods and capabilities.

Noteworthy points include:

• The material qualities of straw bales are discussed, such as size, moisture content, density, fiber length, and bale condition.
• Wall systems can be either loadbearing or nonloadbearing.
• Straw bales in walls are typically coated with various types of plasters, creating a structural composite. Depending on the plaster type, significant structural strength can be added by this outer finish material.
• Moisture or vapor barrier type materials should not be inserted between the straw bales and the outer plaster finish materials. Plaster should be bonded directly to the straw bales, and the entire assembly should remain breathable.
• Flashings or membranes are recommended at window sills, shower stalls, and top of foundation walls.
• Straw-bale walls should be protected from excessive moisture by generous roof overhangs and foundation walls that keep bales well above grade.
• Straw-bale wall assemblies provide insulation performance in the range of R-27 to R-30.
• Plastered straw-bale wall assemblies achieved fire-resistance ratings ranging from 1 to 3 hours when tested according to ASTM E119. Unplastered bales have exhibited a flame spread of 10 and a smoke development index of 350 when tested according to ASTM E84 (meeting building code requirements for fire-test characteristics of insulation materials).
• Structural tests have demonstrated bearing capacities in the range of 3300 to 4300 pounds per lineal foot, as well as significant out-of-plane and in-plane lateral force resistance (in some cases exceeding the strength of plywood-sheathed light wood framed wall construction).

More Information
EBnet provides reports and testing information related to straw-bale construction.