Abstract: A multiple slat insert adapted to be installed in window fixtures operable when in a closed position to insulate the window and, when in an open position, to expose the window. The insert preferably comprises a pair of elongated, spaced-apart mounting members secured within the window frame, and a plurality of elongated, generally planar slats rotatably coupled between the mounting members. The slats are of parallelogram cross section, so that when rotated to the closed, generally co-planar position, they will abut one another to provide a convective seal.