It is known that proper stiffening treatments may improve the sound insulation of plate-like structures. In this study, the sound transmission loss (STL) of a thin plate with stiffening treatments is investigated using a coupled finite element and boundary element method. A stiffened-plate element used in this method allows the plate to have arbitrary elastic boundary conditions and arbitrarily located stiffeners. Numerical studies of the effects of various stiffening conditions on the STL of a window are carried out. The boundary condition of the window lies somewhere between clamped and simply supported. The results can be used to guide the design of windows with better sound insulation performance.