Novel fire resistant insulating structural building elements are provided which are comprised of unitized structures such as walls, roofs, ceilings, and floors in combination with a layer of plastic foam insulation to which is in turn bonded a coating of gypsum plaster by a layer of bonding agent. The novel elements of the invention find particular utility in the rapid and economic construction of thermally insulated and fire resistant buildings. Additionally, a method is disclosed for a facile and economic way of providing thermal barrier protection for exposed plastic foam insulation in existing buildings by the application of a bonding agent and a plaster coat on said foam.