The device of the present invention is identified as a chimney block for use in the jamb channels of window frames. The chimney block impedes or substantially reduces the vertical movement of air through the jamb channel. The reduction in air movement improves the insulating properties of the window and minimizes the amount of dust and fine dirt that might otherwise enter the jamb channel resulting in a progressively increasing force required to move the sash through the jamb channel. It consists of a vertical structural element which is secured to a planar sealing element that conforms to the cross section of the jamb channel. Legs or struts are attached in proximity to the other end of the vertical structural element to insure that the chimney block essentially remains in the same position to provide an air block.