Semi-transparent thermal insulation material has very low extinction coefficient and will transmit infrared spectrum within some wavelength range. When measuring the thermal conductivity (TC) of such kinds of material with transient methods, both heat conduction and thermal radiation occur within material. Transient hot wire (HW) method is a widely used method for thermal insulation measurement. It is developed with the assumption that heat only transfers via conduction. Since thermal radiation within low density thermal insulation material is a pronounced heat transfer mode at high temperature, the TC of radiation participating medium measured by HW method might be incorrect. To reveal the inconsistency between test theory and practical heat transfer process when measuring semi-transparent thermal insulation material with HW method, the transient conduction and radiation coupled heat transfer process is simulated numerically. The uncertainty caused by thermal radiation is investigated for material with different extinction coefficient at various temperature. The deviation increases with the increment of temperature and decreases with increment of extinction coefficient compared to results obtained from the one dimensional (1D) steady state method. The numerical analysis indicates that the TC of thermal insulation material with low extinction ability measured by HW method is overestimated at high temperature. © 2021 Elsevier Ltd