ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings, was published in 2009. The standard sets criteria for moisture design loads, hygrothermal analysis methods, and satisfactory moisture performance of the building envelope. One of the evaluation criteria specifies conditions necessary to avoid mold growth. The current standard requires that the 30-day running average relative humidity at the material surface be less than 80 % when the 30-day running average surface temperature is between 5°C (41°F) and 40°C (104°F). This criterion was intended to strike a balance between the need for simplicity to make the standard useful and the complex reality of mold growth, which varies with mold species and depends on the type of material, water activity, temperature, and other factors. Since the standard was published, many practitioners have maintained that the mold criterion is too stringent. Assemblies known to have satisfactory performance in the field do not meet the criterion under hygrothermal simulation. A recent addendum to ASHRAE Standard 160 replaced the simplified mold criterion with a state-of-the-art empirical model that describes mold growth and decline over time using a mold index. This model takes into account the sensitivity of the material, the surface temperature, and the surface relative humidity. This paper provides an overview of the mold index model and a series of comparisons between field observations of visible mold growth or lack thereof on wood-based sheathing and model predictions that use measured surface temperature and relative humidity values as inputs. The field data are from published studies on above-grade wood-frame wall assemblies and roof assemblies covering a range of climate zones. Our analysis indicates that the current 30-day criterion in ASHRAE Standard 160 fails many assemblies in which visible mold growth did not occur. In contrast, the mold index model predictions give better agreement with observations.
moisture control, building envelope, hygrothermal performance, hygrothermal analysis, simulation, mold growth, mold index, durability, failure criteria, ASHRAE Standard 160
Glass, Samuel V.
USDA Forest Service Forest Products Laboratory, Building and Fire Sciences, Madison, WI
Gatland, Stanley D.
CertainTeed Corp., Malvern Innovation Center, Malvern, PA
Building Science Corp., Westford, MA
Schumacher, Christopher J.
RDH Building Science Laboratories, Waterloo, ON