Abstract: The present invention relates to a cover for closing a vent of a building, which is particularly useful for covering air conditioning vents to prevent heat loss and/or air leakage. The vent cover of the present invention comprises a sheet, preferably of a polymeric material such as polyvinyl chloride (PVC). The sheet has sink key hole slides adjacent to the side edges so that fasteners, such as screws, can be used to secure the cover in place. In a preferred embodiment, the cover is adjustable such that its size can be adjusted to accommodate different vent sizes. The adjustable cover comprise at least two adjustable members that snaps together to form the cover. When the two adjustable members snap together, they form a cover having sink keyhole slides and/or flaps. However, the size of the resulting cover is adjustable by sliding the two halves toward or away from each other.