Lagging fires, also known as insulation fires, pose significant risks in the process industry, arising from the self-heating of leaked combustible liquids within insulation materials. These fires typically initiate through oxidation, leading to smouldering that can escalate into larger fires or serve as ignition sources for other flammable materials. Statistics from the Swedish Contingency Agency was collected for a period of ten years an revealed an average of ten fires per year. Following discussions with six industries, this is likely an underestimation. A small-scale test method, based on isothermal calorimetry, has been developed to estimate a system’s self-ignition temperature. The small-scale test method resulted in apparent heat generation rates, which were used to model lagging on a DN50 pipe. To find an ignition criterion, real-scale tests were conducted. A system comprised of rapeseed oil and industrial lagging was used. The small-scale method was used to study and compare the reactivity of different C18-substances. It was found that substances with conjugated double bonds are more prone to self-heat than substances with a single double bond, whin in turn are more reactive than substances lacking a double bond. Thermal power from rapeseed oil mixed with either mineral wool, glass wool, or mineral wool treated at 500 °C were studied with isothermal calorimetry. One of the materials, as well as the heat-treated mineral wool were found to increase the maximal thermal power. This highlights that different laggings, or elevated temperatures, may affect the risk of a lagging fire. It was also found that copper catalyses the autoxidation and thus increases the risk of a lagging fire.