The integro-differential equation governing the combined conduction and radiation heat transfer in a gray medium bounded by two infinite coaxial cylindrical surfaces is solved numerically to analyse the effect of radiation in transient hot-wire measurements in porous thermal insulations. The influences of the extinction coefficient, the emissivity of the wire and the heating power are studied. It is found that the hot-wire method works accurately only in cases where the extinction coefficient exceeds a minimum value which increases with the temperature. The calculated results confirm the linear relationship between measured thermal conductivities and the heating power. The problem of reference temperature for the measured thermal conductivity is also discussed in this work. (C) 2004 Elsevier Ltd. All rights reserved.