The possibility of a combustion hazard, when a flammable liquid leaks into insulation materials, is determined by the surface temperature, the volatility and loading of the fluid, the exothermicity of the combustion process, and the porosity of the matrix. Whether or not there is asymmetric heating is also influential. The oxygen diffusion rate or a reduction in its ambient concentration have a limited effect. A dimensionless, analytical, criticality criterion (u(infinity)) for ignition of flammable liquids dispersed within a hot inert matrix is compared with a dimensionless parameter (u(FK)) based on the classic Frank-Kamenetskii ignition criterion. The relationship between u(infinity) and u(FK) is explored numerically over wide ranges of conditions and the switch as a monitor of safe operating practice are calculated assuming uniform surface heating. The application of u(infinity) is most appropriate with a high loading of a relatively high volatility fluid dispersed within a matrix that presents a very high internal surface area, and the reaction is not very exothermic. The application of u(FK) is appropriate to other circumstances. It was also found that certain classes of compounds may be capable of bonding to insulation in a way that suppresses their normal evaporative loss. This may signify a hitherto unrecognized problem that certain combinations of liquids and pipe insulating materials enhance combustion hazards.