How you seal a casement window depends on whether it's metal, vinyl, or wood. For metal and vinyl casements, apply strips of closed-cell foam to the inside edges of the frame.

Just measure, cut it to length, and press it in place. With wood frame casements, you can use nail-on spring bronze strips.

On casements that open outward, attach the strips so the nailing flange is along the outside edge of the frame. This way the window will compress the strip as it closes. Reverse the position if the window swings inward.