A heat conduction equation with linerized radiation cooling boundary conditions is used to calculate the thermal field in a long‐strip window pane heated by thermal radiation (fire), except on narrow strips along edges built into the frame. This temperature field is used to calculate a quasi‐static thermal stress field in the pane in the first‐order planar stress approximation. Derived analytic equations of stresses are presented graphically. Thermal stresses build up at the edges in a narrow strip of a few times the pane thickness. In cool spots of the order of 100 K lower than the average temperature, stress build‐up can cause a microcrack to become a fast‐propagating fracture.