Abstract: A cover assembly for an air conditioning unit of the type which projects outwardly from a chamber wall opening includes a cover member with an imperforate outer end wall, and imperforate top, bottom and side walls, and a cover-supporting flange at its open inner end, in combination with an intermediate adapter frame having inner and outer support surfaces and a central opening dimensioned to surround the air conditioning unit. The inner adapter support surface can be fitted in relatively fixed and sealed engagement against a chamber wall surrounding such a wall opening with the aid of a wall-sealing gasket and first fastening means, while the cover member can be selectively and removably assembled in sealing engagement with the outer support surface of the adapter frame with the aid of a cover-sealing gasket and cooperating manually-operable fastening means on the cover member and adapter frame.