Good predictions of the extent of crack growth during thermal shock can normally be obtained by calculating the crack driving force for a single crack while assuming that the cracking has no effect on the temperature distribution in the body. However, experiments have shown that sometimes cracks grow much deeper than predicted. To ascertain whether this might be due to cooling by water penetration into the cracks, the effect of cooling along the crack faces on the crack driving force is calculated and predictions for the extent of cracking are compared with experiments on soda–lime glass. It is confirmed that cooling along the crack faces causes self‐driven crack propagation. The effect of cooling in the cracks is only important when the fracture energy is low compared with the severity of the thermal shock, consistent with the observation that excessive cracking tends to be observed only in materials with low fracture energies.