Temperature, humidity and aging have significant influence on the thermal performance of insulation materials. The assumption of constant material properties frequently leads to the wrong results for simulating heat transfer through insulation materials. Therefore, the impact of hygrothermal conditions and aging on the long-term behavior of insulation materials should be investigated. The objective of this work is to evaluate the influence of coupled hygrothermal conditions and aging on the thermal resistivity of organic foamy materials. A model of aging conversion coefficient for two organic foamy materials is proposed. The thermal resistivity ratio (TRR) is calculated using the coupled heat and moisture transfer model which introduces effective thermal conductivity for expanded polystyrene and extruded polystyrene. The comparison of predicted values with experimental data shows a good agreement. This paper provides an estimation approach to the aging conversion coefficient that affects the thermal conductivity of the organic foamy materials. (C) 2013 Elsevier B.V. All rights reserved.