Sodium sulfate is one of the most damaging salts for porous building materials. In our quest to increase the durability of civil structures and cultural heritage we need to understand its crystallization behavior. In this paper we investigate the cooling-induced growth of the metastable heptahydrate crystal phase by combining nuclear magnetic resonance, for non-destructive measurement of the concentration, with time-lapse microscopy, to visualize the crystal growth. The growth rate is found to be controlled by interface attachment kinetics. The kinetic growth parameter Gk ranges from 0.001 to 0.007 mm/s for single crystals in a temperature range of 4.8–13 °C.