In this paper, a method for estimating thermal resistance applicable to various shapes, thermal conductivities, and specimen thicknesses in complicated shape insulation foam was investigated. Thermal resistance was calculated by CFD simulations that reproduced the conditions measured by calibrated hot box method for specimen models with different volume loss, thermal conductivity, and specimen thickness. Based on the results, a calculation formula for estimating thermal resistance was discussed. It was confirmed that the estimated value by the calculation formula corresponds to the measured value in JIS A 1420 and the calculated value in JIS A 9521.