A building closure for mounting within an opening in a building wall structure includes a frame for mounting within the wall structure opening and at least one closure unit for mounting within the frame. The frame has at least one interior chamber extending entirely around the frame and a nailing flange for securement to the building wall structure such that at least a portion of the frame is disposed within the building wall opening. There is a cavity between at least one exterior surface of the frame and the inner periphery of the building wall opening, with the cavity extending along at least a top portion of the frame. A barrier extends along at least the top portion of the frame to retard passage of moisture from the cavity to the building interior. At least one moisture weep opening is disposed in the top portion of the frame from the cavity to the interior chamber of the frame such that moisture in the cavity flows through the weep opening into the chamber, within the chamber around the frame and then out of a drain opening in a bottom portion of the frame.