An insulated tuckstone that includes a basic tuckstone and an insulating layer exhibiting a thermal conductivity lower than 2.0 W·m 1·K−1 between 20° C. and 500° C., exhibiting enough rigidity to be self-supporting and covering an insulated surface of the basic tuckstone. The insulated surface is included in the lower surface of the basic tuckstone, extending into the lower transition surface and representing more than 20% of the lower transition surface. The insulating layer has a chemical composition, as a percentage by mass on the basis of the oxides, such that Al2O3+SiO2+ZrO2+CaO+Na2O+MgO+K2O+TiO2+Fe2O3+HfO2+P2O5+Y2O3>80%.