The results of an experimental study were reported on insulation properties of two types of intumescent coating（type W and type S） under the ISO 834 fire and three non standard fire curves which are less severe than the standard fire. After heating test, expansion ratio of intumescent coating and pore structure of chars were measured and compared. Effective thermal conductivities were calculated based on temperature measurements. The results show that expansion ratios can be different when heating speeds up or slows down because more or less gas can be trapped within the coating with varied heating rate. The difference of heating rate affects the matching between timing of gas emission and viscosity of molten polymer, and therefore affects the pore size of intumescent char. The insulative properties of intumescent coating char are better when the expansion ratios are larger and the pore sizes are smaller. The constant effective thermal conductivities derived from non standard fire tests are 65% and 35% higher than that from standard fire tests for type W coating and type S coating respectively, which implies that it is unconservative using standard fire test results to guide design of intumescent coatings under non standard fire conditions which are less severe than the standard fire.