An exhaust gas treatment device, which includes an outer layer, an inner layer that is at least in part disposed within the outer layer, and a loose-fill insulation disposed in the volume between the outer layer and the inner layer, where a piece of fiber mat is disposed between the outer layer and the inner layer and forms a barrier that at least partially prevents the loss of the loose-fill insulation from the volume between the outer layer and the inner layer and a manufacturing method that includes placing a loose-fill insulation into the volume of space between an inner layer and an outer layer and positioning a piece of fiber mat between the outer layer and the inner layer to form a barrier that at least partially prevents the loss of the loose-fill insulation from the volume of space between the outer and inner layers.