This study is to investigate the characteristics of critical and neutral thicknesses of an insulated oval duct by using two-dimensional numerical analysis and the one-dimensional PWTR model, coupled with accurate oval duct surfaces. Critical heat transfer occurs when the outer radius of an insulated circular duct is less than its critical insulation radius. The heat transfer rate increases with increasing the insulation thickness until the maximum value is reached at the critical insulated thickness, then it decreases until the point of neutral thickness, where the heat transfer rate is the same as that of a non-insulated duct. Thus, any insulation effort is meaningless if the insulation thickness is less than the neutral insulated thickness. These characteristics are very important for small size insulated duct, especially in conditions of low ambient air/gas convective coefficients. It shows in this study that the critical phenomena of an insulated oval duct is very similar to those of an insulated circular duct except it's critical and neutral thicknesses are smaller for larger oval axes ratio. (C) 2010 Elsevier Ltd. All rights reserved.