Relative Humidity and Water Activity
Relative Humidity
One of the concepts that was probably over simplified in the past related to the idea of Relative Humidity. It was often stated something like this, "Relative Humidity is relative to the temperature of the air; the warmer the air is, the more moisture it can hold". The reality of the process involved looks toward the vapor pressure, not the temperature.
"[R]elative humidity is relative to vapor pressure, NOT temperature. The rate at which the molecules are released is NOT dependent on the temperature of the air... but the vapor pressure of the liquid verses the vapor pressure of the air! (Accurate Science: Relative humidity is the vapor pressure divided by the saturation vapor pressure (more accurately called equilibrium vapor pressure), multiplied by 100. Displays percentage RH)" (Leadership In Restorative Drying, page 86)
"The true and accurate definition of relative humidity is the ratio of the actual partial vapor pressure of water to the equilibrium vapor pressure at that temperature. Relative Humidity is also correctly called Relative Vapor Pressure." (Leadership In Restorative Drying, page 87)
"The reason why we considered psychrometry using dry bulb and wet bulb temperatures is that it explains the mechanics behind psychrometric calculations. A scientifically sound drying strategy requies the full understanding of the mechanics and calculations behind molecular phase changes." (Leadership In Restorative Drying, page 89)
Water Activity
Relative vapor pressure or Relative Humidity at the surface of hygroscopic materialsis an important metric to consider because of its implications in the drying process and for considering the risk of microbial growth in that microenvironment.
"When we look at scientific documents that speak of a microrganisms' necessary quantity of moisture to support its growth, it is expressed in something called "water activity"..." (Leadership In Restorative Drying, page 89)
This is a very important calculation because as restorers we are trying to minimize or elimnate secondary damage such as mold growth and we need to alter the environment in such a way based on accurate data to achieve clear objectives.
"The amount of free water available to microorganisms for growth on a substrate (food source) or microenvironment is described as water activity (aw). Water activity is the ratio of the water-vapor pressure in the substrate to the water-vapor pressure of pure water at the same temperature. It can be compared to the equilibrium relative humidity (ERH) of a material. Many fungi have a minimum requirement of 0.88 aw or 88% ERH. Some have a limit that can be considerably below 0.80 aw (80% ERH), meaning that bthey require less water to germinate and multiply. These lower-water-activity, or 'xerophilic' fungi are often 'primary colonizers,' and are best represented by some species of Peicillium and Aspergillus, including Aspergillus versicolor, Eurotitium and Wallemia. 'Secondary colonizers' typically have between 0.80-0.90 aw (80-90% ERH), and include molds such as Cladosporium, Paecilomyces, Scopulariopsis and a variety of Aspergillus and Penicillium species. Finally, extremely wet microenvironments, particularly those with cellulose-based materials (e.g. wallpaper, gypsum board, and books), favor the growth of 'tertiary-colonizer' fungi with >0.90 aw (90% ERH), such as Stachybotrys, Acremonium, Ulocladium, Fusarium, Trichoderma, Chaetomium, and a variety of yeasts (Domsch et al. 1993, Pitt and Hocking 1999)." (IICRC S520, Standard and Reference Guide for Professional Mold Remediation, Second Edition, 2008, page 69)
"Although water activity is the moisture parameter that allows for mold growth, it can be correlated with moisture content of materials. Different building materials will support mold growth at different moisture contents. For example, at 80% ERH (0.80aw), the moisture content of softwood is around 17% which is suffiecient to support mold growth (Fl below the dew point temperature though the mechanism f losing its thermal energyannigan and Morey 1996)." (IICRC S520, Standard and Reference Guide for Professional Mold Remediation, Second Edition, 2008, page 69)
For a "Wood EMC (Equilibrium Moisture Content)" chart see Leadership In Restorative Drying, pages 128-129.
"When the water activity is less than 0.70, it is usually determined to be suffciently dry to prevent the growth of fungi." (Leadership In Restorative Drying, page 89)
