DC-2792 -- A Study of Water Vapor Transmission through Insulation under Steady State and Transient Conditions PDF

A guarded hot box has been built that allows one-dimensional moisture and temperature gradients to be independently set across a test section. Local values of temperature and humidity were continuously monitored within the test section, which was made of expanded polystyrene insulation.

Comparison of a transient test without phase change to an analytical solution for diffusion allowed the diffusion coefficient, D, to be determined for water vapor through the air-filled insulation. The diffusion coefficient, D, was found to be almost two orders of magnitude less than the value for water vapor through air without the insulation present.

Steady-state tests were carried out with a concentration and temperature gradient that caused condensation to take place within the insulation. The extent of the condensation zone can be estimated from the intersection of the saturation line corresponding to the local temperature and a straight line between the concentrations at the two surfaces of the sample.

Condensation tests run with time-varying boundary conditions revealed that the extent of the condensation zone during condensation and dry out is not well represented by steady-state results. In some instances, the width of the condensation zone increased during dry-out conditions, probably due to capillary effects.