A piece of conventional wisdom often encountered in the restorative drying profession is: “By day 2 the relative humidity should be below 40%.” This was boasted to me as recently as March of this year by my AMRT training instructor, and the mitigation software I use is indeed programmed to display a warning symbol when RH >40% is entered.
Attribution of this tenet seems to me to be a tad mysterious. One can scour the last three editions of the ANSI/IICRC S500 Standard for Professional Water Damage Restoration looking for its mention to no avail. Perhaps it was lifted from ASHRAE or CPSC recommended indoor humidity ranges, or refers to the limited capabilities of the old conventional refrigerant dehumidifiers. I’ve also suspected it may be an oversimplification intended to guide those just beginning their restoration career. Or perhaps, like minimum grain depression, it was imposed by adjusters and meekly accepted by our industry in appeasement.