Theoretical relationships have been developed to describe the heat transfer by combined fluid conduction-convection through air-permeable insulation with vertical air spaces adjacent to both surfaces. The fluid conduction-convection is shown to be a function of fluid properties, air flow coefficient of the insulation, insulation height and thickness, and temperature difference. A correlation in terms of dimensionless groups has been derived. Results of measurements on a 4-ft high insulation specimen over a temperature difference range from 30 to 90F were in agreement with the theory. The relationships suggest that very low air flow coefficients are required to minimize convection effects and, in practice, an air barrier in contact with the insulation is required if air spaces occur on both sides.