The retrofitting of a new tunnel ventilation system within an existing structure typically presents constraints around which the design mustbe engineered. This paper presents an example of such a retrofit design in which the ventilation capacity of an existing interstate tunnel issubstantially increased to address present-day life-safety requirements. A number of design alternatives are identified and evaluated leading to theselection of a preferred concept. The paper discusses the constraints encountered, and outlines the measures taken to mitigate them in order tomaximize the performance of the new tunnel ventilation system.