Kinetic studies at 1123 K show a dramatic decrease of Tg during the heat treatment of 5 min. This occurs due to the Na rich domains contributing to a large decrease of glass network connectivity. After 5 min Tg increases slightly. This is attributed to both the depletion of network modifiers from the glass matrix due to formation of Na2SrSi2O6 crystal and the diffusion of Na+ ions into the interior, resulting in a more homogenous distribution of Na in the foam glass.