Thermal crystallization experiments have been carried out on nanocrystalline silicon films. From the thermal activation of the process, it is concluded that the amorphous phase crystallizes by solid phase epitaxy around the preexisting crystallites. However, and in contrast with the usual epitaxial crystallization of wafers partially amorphized by ion implantation or ball-milled powders, the epitaxial growth is inhibited for most of the amorphous-crystalline interface. Our analysis indicates that a small fraction of the interface contributes to the epitaxial growth. Although the influence of oxidation and of Si-H groups located at the interface cannot be completely excluded, this behavior is explained in terms of a high density of microvoids located at the a-c interface. This result has implications for the models of electrical conduction of nc-Si:H.