Abstract: A two-piece fastener system for installing roofing insulation. The fastener system includes a round disk having a hub extending outwardly from the center of one side. The outwardly extending end of the hub includes an opening adapted to accommodate a threaded fastener or nail. A thermally insulating plug is driven into the cavity through an opening at its opposite end to abut against a head of the fastener, preventing it from backing out. In one embodiment, the plug is retained by a lip that extends thereabove, and in a second embodiment, is retained by engagement of a ridge in a corresponding groove.