This study assesses the variability of the water vapour desorption isotherm of concrete under site conditions. Knowledge of the distribution law of this property is necessary for probabilistic modelling of the drying of concrete elements. Desorption experiments were carried out on samples of concrete mixtures, taken weekly from a construction site over a 5-month period, by means of the saturated salt solution method. Three theoretical models, used to provide physical descriptions of the measured desorption isotherms, were compared. It was found that a Gaussian law can be used to model variability of the desorption isotherm due to in situ hazards. In order to assess the effect of this statistical variability on predictions of a drying model, numerical simulations were carried out using a finite-element scheme. The results show that desorption isotherm variability has a great influence on the prediction of moisture transfer.