THERMAL PERFORMANCE AND HYSTERESIS IN FIBROUS INSULATION EXPOSED TO MOISTURE AND STEP CHANGES IN THE COLD TEMPERATURE BOUNDARY-CONDITION

Simultaneous heat and mass transfer through a medium-density fiberglas s insulation slab is studied using a one-dimensional transient numeric al model formulated using the local volume averaging technique. The in sulation is open to ambient air at a specific humidity on the warm sid e. The cold side boundary is an impermeable cold plate at specified te mperatures. This study examines the effect of changing the cold temper ature boundary condition on the predicted heat and mass transfer. The thermal performance of fiberglass insulation is poorer as the air humi dity increases. This is particularly evident after a step change in th e cold temperature boundary condition where, for an air relative humid ity of 97%, the heat flux is 4.9 times greater than the dry case even though the accumulation of water is only 0.14% by volume (1.9% by mass ) and the average effective thermal conductivity of the insulation has increased by only 3.5%. The effects of neglecting thermal hysteresis (characterized by both the difference between the adsorption and desor ption isotherms and the heat of adsorption and desorption) in modeling transient heat and mass transfer in fiberglass insulation subject to a step change in the cold temperature boundary condition is studied. N eglecting thermal hysteresis is found to be significant at temperature s above and below freezing. The predicted mass transfer under hysteres is properties can be up to three times different than that predicted u sing non-hysteresis properties and the heat transfer can be influenced by as much as 20 to 30% for the few numerical studies presented in th is paper.