Positron annihilation lifetime spectroscopy data are often used to get information on the typical sizes of sub nanometric holes forming the free volume in polymers. To this purpose, the cavities are modelled as spheres or, more generally, using geometries which assume an isotropic expansion with the temperature. However, this guess could be unrealistic owing to the irregular shape of holes and the constrained movements of the macromolecules. In this work positron and dilatometric data are used to estimate the free volume fraction in atactic polypropylene. Comparison with the prediction of the Simha-Somcynsky theory supplies information on the thermal dependence of the volume of holes, whose behaviour can be interpreted in terms of anisotropic expansion.