Does the exterior sheathing on a double-stud wall accumulate worrisome quantities of moisture in late winter? Several researchers are now looking into this question, and Green Building Advisor has been sharing the researchers’ findings as the information becomes available.
The latest sheathing moisture measurements were made by Bill Hulstrunk, the technical manager at National Fiber in Belchertown, Massachusetts. I’d like to put an important piece of information on the table from the start: Hulstrunk is not a disinterested academic researcher; he is employed by a company that sells cellulose insulation. Clearly, his company is more inclined to share data showing that walls insulated with cellulose are performing well, and might not want to share any data that imply otherwise. That said, some of Hulstrunk’s data are thought-provoking.
Hulstrunk shared his moisture content readings in a presentation (“Hygrothermal Analysis of Superinsulated Assemblies”) at the Passive House conference in Portland, Maine, on September 22, 2014. His co-presenter was builder Chris Corson of EcoCor Construction.
Hulstrunk began his presentation by lauding the hygroscopic properties of cellulose. “Hygroscopic materials want to redistribute and equalize the moisture,” he said. “Hygroscopic materials like cellulose protect themselves and nearby materials. They can pull moisture out of nearby materials like sheathing and studs.”
Hulstrunk also provided a few cautionary statements about cellulose density. “Deep cavities require that cellulose be installed at higher densities,” he said. “I recommend 3.7 pounds per cubic foot for a 12-inch cavity or 4 pounds per cubic feet for an 18-inch cavity. Deeper cavities require more experienced installers, since multiple hose passes are required.” (For more information on this topic, see How to Install Cellulose Insulation.)
Walls without any exterior sheathing stay dry
Bill Hulstrunk set out to visit a number of homes with thick double-stud or I-joist walls insulated with dense-packed cellulose, including several…