The transient thermal response of a glass-fiber insulation slab with hygroscopic effects

Abstract The Brunauer-Emmett-Teller (BET) equation, representing adsorption isotherms, is used in a one-dimensional, transient vapor-diffusion model for heat and moisture transport in a typical, mediumdry density glass-fiber insulation slab to account for the hygroscopic effects (water vapor adsorption, desorption, and capillary condensation). The correction to the latent enthalpy of phase change, used in the energy equation, is derived from empirical desorption isotherms. The results obtained, for two types of boundary conditions (a closed system and an open system), show that the effects of hygroscopicity on the transient temperature distribution are significant for a slab with one boundary open to moist air. The sensitivity of hygroscopicity on the transient heat and mass transfer behavior can be depicted by a transition Fourier number, which is proportional to the adsorption capacity of the glass-fiber slab and increases as the cold side temperature and ambient relative humidity increase. Given the uncertainty in the adsorption properties of fiber-glass and experimental data, the agreement between the model and measured data is reasonable.