An insulated door comprising a door shell having spaced first and second exterior panels. The door includes a plurality of stiffeners bonded to a liner panel, with thermal insulation between adjacent stiffeners, disposed within the door shell. The liner panel may extend along one side of the stiffeners and the thermal insulation along the liner panel between the stiffeners and the first and second exterior panels. The stiffeners and the liner panel are preferably made of steel and the stiffeners are welded to the liner panel. The stiffeners are preferably not welded to the first and second exterior panels of the door shell. The liner panel is preferably insertable into the first or second exterior panels of the door shell. The insulation material fills substantially all of the space between the liner panel and one of the first or second exterior panels.