Abstract
The diffusion of water vapor through permeable retarders applied over thermal insulation in low-temperature service is a function of vapor pressure differentials. For permeable insulation materials of a fibrous nature, with an installed vapor retarder on the warm side, the annual intake of water by vapor diffusion is readily calculated. It has been observed that for sprayed-on polyurethane foam insulation, the actual intake is substantially less than calculated. Field data from eight and eleven-year-old installations of spray-applied polyurethane foam, covered with vapor-retardant mastic on large storage tanks containing agricultural liquid ammonia held at −33°C (−28°F), show that the calculated level of water intake was not attained and that the insulation material continues to perform without significant change in thermal transmission. The author hypothesizes that the cellular nature of the insulation restricts and changes the diffusion mechanism and that the absence of free air spaces within the insulation attenuates moisture diffusion paths.
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