Criteria have been investigated for the conditions at which a ＇lagging fire＇ may occur when a flammable liquid penetrates and is dispersed within insulation material surrounding a hot pipe. The conditions at which a Frank-Kamenetskii thermal ignition criterion should be replaced by one derived from the heat release rate versus fluid evaporation rate were deduced. These conditions were related to the decreasing enthalpy of vaporization of the fluid. Practical investigations were based on formal ＇cube test＇ methods for thermal ignition. The theory was tested against the behaviour of n-C16H34, n-C18H38 and n-C20H42, which represent alkanes of mid-range volatility, and also with reference to squalane (C30H62), which is representative of highly involatile alkanes.