Embodiments disclose an apparatus for enclosing and insulating an opening in a structure such as for an air conditioning unit or a sleeve. The apparatus includes a cover having a first section and a second section, wherein the first section is designed to be adjustable in relation to the second section; one or more insulating members positioned along an interior of the cover; two or more fasteners designed to secure the cover to an A/C unit sleeve or a structure. In another embodiment the apparatus cover is a single section and not adjustable. A method of installing the same is provided, as are other aspects.