AbstractLow density rigid foams, reaction products of polyether polyols with tolylene diisocyanate, may transmit moisture vapor at rates from 0.3 to more than 8 perm‐inches at 23°C. Analysis of experimental data provided conservative design correlations showing that rate constants could be predicted only when a foam's polymer composition and physical structure were considered jointly. Among commercially acceptable rigid foam systems, cell structure and polymer aromaticity dominated the control of permeability levels. Permeabilities were lowest when transfer was directed perpendicular to the rise of highly orthotropic closed‐cell foams based on aromatic polyols of low equivalent weight. Permeabilities increased with effective cell diameter and also as foam density and closed‐cell content were r
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