The outcome of the work presented here consists in predicting the moisture and heat transfer phenomena likely to occur in glass fiber insulants subjected to varying environmental conditions.This prediction is obtained by a numerical resolution of the equations governing the phenomena (Luikovʼs model) and an experimental determination of the mass transfer coefficients appearing in these equations.Comparisons between experimental and theoretical results are presented for a particular configuration (closed system with stationary boundary conditions). These comparisons show a good reproduction of the measured moisture distributions but an over estimation of the heat transfer, for reasons developed in the text.Despite its present imperfections, the procedure presented here remains good and has been used for different configurations.Further developments require additional measurement of heat and mass transfer coefficients.