An exhaust system is provided that includes an exhaust aftertreatment unit, first and second exhaust pathway in communication with and upstream of the exhaust aftertreatment unit, a thermally activated flow control device operable in a first and second mode, and a thermal storage device. In the first mode, the flow control device permits exhaust to flow to the aftertreatment unit through the first pathway and inhibits flow through the second pathway. In the second mode, the flow control device permits exhaust flow to the aftertreatment unit through the second pathway and inhibits flow through the first pathway. The flow control device may switch between the first and second modes based on a change of temperature. The thermal storage device is within the second pathway, stores thermal mass, and provides thermal insulation to enable a catalyst of the aftertreatment unit to maintain a predetermined temperature for a predetermined time.