For casement windows, you will need to apply weatherstripping around the window jamb, right beside the stops.

Use vinyl V-strip or adhesive foam (closed-cell is best) on vinyl or metal windows. First, clean the stops thoroughly. Cut the strips or foam to length so that they can run along the sides, top, and bottom of the stops, and then press the weatherstripping into place.

For wood windows, use spring metal strips. Nail the weatherstripping to the outer perimeter of the frame for casements that open outward and to the inner edge of the frame for windows that open inward.