This article presents a temperature-temperature thermal characterization method for the measurement of the thermal diffusivity of insulating materials at high temperature. This novel method, noted 4L, is a transient absolute measurement method based on the estimation of the transfer function at the center of a symmetrical stack composed of two specimen of an insulating material sandwiched between two conductive metallic plates. Two different direct 1D semi-analytical models were developed. The first one considers purely conductive opaque materials and the second one takes into account the coupling between radiative and conductive heat transfer modes for purely scattering semi-transparent materials. The first model was used to estimate the thermal diffusivity of two different opaque insulating materials (calcium silicate boards) up to 800°C with accuracy better than 10%. The second model was used to estimate the thermal diffusivity of a semi-transparent insulating material (ceramic foam) up to 1000°C with an accuracy of approximately 10%. The second model was used to identify significant estimation errors occurring if a purely conductive model is used for semi-transparent materials. The conducto-radiative model was also used to estimate an average value of the mean extinction coefficient of the semi-transparent material. Nomenclature thermal diffusivity [m 2 s-1 ] specific heat of the sample [J kg-1 K-1 ] thickness of the samples [m] transfer function (real space) transfer function (Laplace space) directional monochromatic radiative intensity [W m-1 ] () black body monochromatic radiative intensity at [W m-1 ] refractive index Δ′ ⃗ → Δ ⃗ scattering phase function Laplace parameter [s-1 ] conduction heat flux [W m-2 ]