from what you're describing it sounds like you're referring to the casement. It's the thing that looks like a box, the the air-conditioner slides into. To answer your question - this a/c doesn't need that. It already comes in a shell that doesn't require that. (Note the air vents on the side of the unit itself) This unit is for a window installation. If it was for a wall unit - it wouldn't have those vents, it would vent through the back.