A cover for closing an access opening that leads to an infrequently used space within a building, such as an attic, that generally provides an air seal, thermal insulation and/or acoustic insulation at the access opening. The cover may be in one or two portions, including a closure alone or a closure and a frame having an aperture that can be closed by engagement between the closure and the frame. When the cover is used alone it engages a frame or a wall circumscribing the access opening. The cover is sized and shaped to close a stairwell, or the opening at one end of a stairway, an opening in a generally vertical wall, a hatch, or a pull down ladder. The closure and frame are each made of one or more components.