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Thermal conductivity measurements were made on samples of an experimental glass fibrous insulation ranging in density from 0.5 to 8.4 pcf. Tests were carried out at atmospheric pressure with four different gases in the insulation samples, and the thermal conductivity with air was studied over a pressure range of 1 micron to 760 mm of Hg. Gas conduction is the most important mechanism of heat transfer. A theory of gas conduction in fibrous insulations was developed, and agrees well with experimental results. Theoretical considerations of heat transfer by radiation were confirmed by the experimental thermal conductivity values at low pressures. Methods are discussed of producing considerable reduction in the thermal conductivity of fibrous insulations.