Abstract: A snap-on insulation barrier for window frames including an elongated base member having means for securing the base member to the frame, the base member being generally of strip configuration and having a pair of opposed rails having portions extending upwardly and outwardly from the base member, the base member having side members and the side members extending beyond the outwardly extending portions of the opposed rails a substantial distance and forming with the outwardly extending portions of the rails a pair of opposed U-slots. Also provided is a second strip having a C-shaped cross section with inturned ends for engaging in the U-slots and having inside and outside surfaces. On the outside surface of the second strip is an upstanding bead running the length thereof.