Cellulose fiber is a thermal insulation material made from the recycling of papers and cardboard boxes. When used in attics, it is installed as a loose-fill material blown at low densities (typically 25–35 kg/m3). Due to the weak internal structure of the material, settlement often occurs after installation. The estimation of the reduction in thickness is carried out by an experimental test following the NF EN 15,101–1/IN1 standard. The material is exposed for 4 months to alternating dry and wet phases of 14 days duration each. This accelerated aging test, considered generally as safe, is very difficult to correlate with on field measurements because of the variability of the implementation conditions and climate expositions. To scientifically validate this standard, it is essential to understand the fundamental mechanisms driving settlement. Several theoretical models have been proposed in the literature, but none adequately explains how hygrothermal conditions (temperature and humidity) affect the settlement behaviour. Our study proposes a phenomenological model based on the variation of the stiffness of the material with the humidity conditions: the material becomes less stiff under dry condition compared to wet conditions. In addition, transient transfers of humidity inside the material seem to have a key role in the settlement.