Effects of exfiltration on moisture and frost accumulation in a fibrous insulation

Experiments were performed of heat and moisture transport, coupled with air exfiltration, through a glass-fiber slab. The measured data, showing a coupling effect of air movement with moisture and frost accumulation, are compared to a numerical model. A constant airflow, at a rate compatible to typical building envelope air exfiltration rates, passes through the test slab from the warm side of the slab (at a room temperature and for various humidity levels) to its cold side (below the triple point of water). Through temperature, heat flux and moisture accumulation measurements, the increase in heat loss due to the transport of moisture by moist air is demonstrated. A one-dimensional, numerical model, including convection and vapor diffusion in porous media, is developed to simulate the process under the test, conditions. Comparisons between the experimental and numerical results are favorable. It is shown that the increase in moisture accumulation under frosting condition could be an order of magnitude higher than for the temperature range above freezing. The conduction heal, loss can be up to 300% higher with air exfiltration, depending on the exfiltration rates, as compared to the moisture diffusion-only process (i.e., no exfiltration) under the same temperature boundary conditions.