This paper presents the results of an experimental and numerical simulation study to obtain thermal conductivity of intumescent coating after accelerated aging. Two types of intumescent coating were used. The main objective of this research was to check the accuracy of a thermal conductivity model for the fully expanded intumescent coating char, which was assumed to depend on the expansion thickness and the bubble size and distribution. By using the model and the measured expanded thicknesses and average bubble diameters for the two types of intumescent coating after different cycles of accelerated aging, the intumescent coating insulated steel plate temperatures were numerically calculated and compared with measured results. The agreement has been found to be generally satisfactory, suggesting that the thermal conductivity model is acceptable. The measured results of intumescent coating expanded thickness and bubble size show that aging has two detrimental effects on the insulation performance of intumescent coating: reducing the expansion thickness and increasing the bubble size, both leading to increased thermal conductivity. Copyright © 2012 John Wiley & Sons, Ltd.