Many green builders today are embracing the latest innovations in closed-cell spray foam and structural insulated panels to create energy-efficient structures. Others, however, see different writing on the wall.
In New York, builders are discussing the merits of sand and sawdust mortar mixes for stacked cordwood. They are using blower doors and infrared cameras to study the energy performance of straw bales in Vermont, and are standardizing ways to stack and insulate tires packed with earth in New Mexico.
Researchers also want to understand the performance of walls built with native materials. In the English countryside, they are measuring the heat loss from century-old, rock-sided homes.
It’s tempting to dismiss alternative wall systems as remnants of the 1960s back-to-the-land movement. But in 2011, alternative walls have matured into a very active realm of building science, although one that does not show up in magazine ads or at big-box stores.
Research is being driven by small-scale entrepreneurs and trial and error in the field, rather than large corporations. Knowledge typically is conveyed online and at workshops.
In general, alternative walls place a greater emphasis on sustainability and local materials. They exploit the properties of thermal mass, rather than pure R-value, to achieve comfort and energy efficiency.
For mainstream builders, the research under way on alternative walls opens a window to evolving, green technologies that are both centuries old and very up to date. One common lesson has emerged: The performance of any building material—whether cordwood or closed-cell foam—is compromised when best practices such as air sealing and moisture control fall short.
Straw bales
Stacking bales of straw and plastering them is an age-old practice. Rising fuel prices have put the spotlight on R-value, with a 14 or 18-inch-thick bale wall rated at roughly R-28. That sounds impressive, but in recent years, builders have been using blower doors, infrared cameras and moisture probes to study the true thermal performance and water-transfer characteristics of straw bale homes.
Some of the most thoughtful work is being done by Jacob Deva Racusin of New Frameworks Natural Building LLC in Middlesex, Vermont. Racusin studied seven homes in early 2011 that his company had built. The goal is to develop a testing program to document performance, as well as identify strengths and weaknesses in construction practices.
All the projects featured timber frame structural walls, with straw bales used as infill walls. Using the standard blower-door test to simulate a 20 mph wind (depressurized to 50 kpa), researchers recorded volumetric, per-hour air changes ranging from 2.50 to 6 ach50.
Roof assemblies emerged as primary air-leak locations, around chimney and plumbing vent stacks, for instance. Window and door edges also were problems. These gaps stand out in thermal imaging, performed while the homes were depressurized. Testers used a smoke pencil to verify issues spotted by the infrared camera and to pinpoint airflow.
In the Northeast, most straw bale buildings are infill walls, paired with a timber frame or post and beam frame. Starting the bale wall 18 inches off the ground and overhanging the roof 24 inches have become accepted building practices to limit moisture problems.
Credit: Jacob Deva Racusin, New Frameworks Natural Building LLC
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