Unintended Hygrothermal Implications Resulting from Code Requirements for Continuous Insulation and NFPA 285 Testing

Abstract

The 2009 edition of the International Energy Conservation Code (IECC) prescriptively requires continuous insulation for above-grade metal-framed walls in most Climate Zones. Continuous insulation improves the hygrothermal performance by creating a more balanced thermal gradient, where a smaller proportion of the total temperature change occurs across the stud cavity and the potential for dropping below the dew point temperature within the stud cavity is reduced. For years, extruded polystyrene (XPS) was the most common material selected for this application—chosen because of its nonabsorptive and water resistance properties. Recently, closed cell spray polyurethane foam (SPF) insulation has become another common choice for this material. Both of these insulations offer considerable vapor resistance. In a seemingly unrelated requirement, Section 1403.5 of the 2012 edition of the International Building Code began requiring National Fire Prevention Association (NFPA) 285 testing for most buildings if the water resistive barrier was combustible. This new requirement immediately drew manufacturers’, code officials’, and designers’ attention to a long-standing International Building Code Chapter 26 stipulation that required NFPA 285 testing when an assembly included foam plastic insulation. Although it was technically required previously, walls including XPS or SPF insulation were now subject to NFPA 285 testing because of this renewed attention. In order to avoid this testing requirement, many designers began specifying mineral insulations, such as rock wool. These insulations are highly vapor permeable and drastically alter how vapor diffuses across a wall assembly as compared to XPS or SPF. Oftentimes, this switch is made without examining the material properties of the air and water resistive barriers and other materials making up a wall assembly. This paper compares and contrasts the results of a parametric study on the continuous insulation type and the long-term hygrothermal behavior as predicted using transient one-dimensional hygrothermal analyses. The study focuses on performance in mixed humid climate zones subjected to a bidirectional vapor drive. This paper also examines related hygrothermal impacts of the industry’s insulation switch and discusses the recent industry responses to the requirements.