Development of a method to relate the moisture content of a building material to its water activity

Subjective indicators of building dampness consistently have been linked to health, but they are, at best, semi-quantitative, and objective and quantitative assessments of dampness are also needed to study dampness-related health effects. Investigators can readily and non-destructively measure the "moisture content" (MC) of building materials with hand-held moisture meters. However, MC does not indicate the amount of the water in a material that is available to microorganisms for growth, that is, the "water activity" (A_w ). Unfortunately, A_w has not been readily measurable in the field and is not relatable to MC unless previously determined experimentally, because for the same moisture meter reading, A_w can differ across materials as well as during moisture adsorption vs desorption. To determine the A_w s that correspond to MC levels, stable air relative humidities were generated in a glove box above saturated, aqueous salt solutions, and the A_w of gypsum board and the relative humidity of the chamber air were tracked until they reached equilibrium. Strong correlations were observed between meter readings and gravimetrically determined MC (r=.91-1.00), among readings with three moisture meters (r=.87-.98), and between meter readings and gypsum board A_w (r=.77-.99).