Characterizing indoor humidity for comparison studies: The moisture balance approach

If an intervention aims to reduce indoor humidity, how can the pre-intervention humidity be compared to the post-intervention humidity if these measurements are taken at different times under different outdoor conditions? For a set of houses, how can their interior humidities be compared if they are measured with different outdoor weather conditions? The approach discussed in this paper is the moisture balance approach, described and presented previously (Rose and Francisco 2004, 2010). This approach was applied to homes in a weatherization and ventilation study in the U.S. Midwest. The results seem to show that weatherization provided a statistically significant lowering of indoor humidity and that there was a difference between the control and treatment groups. However, on close review, a bias was noted when measurements were taken during mild weather, with outdoor temperatures between 10°C and 15°C. Data with outdoor temperatures >15°C were excluded from this data review. This paper explores the basis for the findings. It identifies alternative regression methods besides the method previously applied. It investigates the possible error with measurement periods of 1-week, 2-week, and 1-month measurement intervals. It closes with cautionary recommendations regarding methods for characterizing wetness with outdoor temperature corrections.