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).