Radiative heat transfer in SiO2aerogel is determined from the spectral infrared extinction (absorption) E(λ). From E( λ) we derive a temperature dependent Rosse-land mean Ē(T) for extinction. A suitable diffusion model for the infrared radiation allows to calculate the temperature dependent radiative loss coefficient kraa(T). For a 1 cm thick aerogel board and a mean temperature within the insulation of T = 280 K, a value as low as krad,≃ 0.16W/m2K results. Between T = 250 and 450K a good approximation israd∞Tα, with α = 5.6.