An exhaust treatment device includes an inner shell spaced apart from an outer shell by a first gap. An inlet assembly includes a first inner cap and a first outer cap spaced apart by a second gap. An outlet assembly includes a second inner cap and a second outer cap spaced apart from each other by a third gap. A substrate is located within the inner shell. An insulating mat is positioned between the substrate and the inner shell and includes a reduced diameter portion compressing the insulating mat between the inner shall and the substrate. Insulation material fills the first, second and third gaps that are in communication with each other.