The thermal conductivity of evacuated pure and opacified silica aerogel powders was measured in a guarded hot plate apparatus for temperatures from 10 K up to 275 K and external loads from 1.5*10⁴ Pa to 10⁵ Pa. Infrared optical measurements of the extinction give the possibility to separate contributions from solid thermal conduction and radiative thermal transport. Measured thermal conductivities range from 29 to 80*10^-6 W m^-1 K^-1 at a temperature of 15 K and from 0.9 to 3.1*10^-3 W m^-1 K^-1 at a temperature of 250 K. The solid thermal conductivities lambda _powder of the powders were compared with those of the monolithic silica aerogel ( lambda _monolith) the powder grains consist of. The ratio g'= lambda _monolith/ lambda _powder was found to be temperature independent in the investigated temperature range. Thus g' can be interpreted as a geometrically induced reduction factor. The mean thermal conductivities averaged over the application temperatures range from 0.7 to 2.2*10^-3 W m^-1 K^-1 for boundary temperatures of 77.6 and 290 K and from 0.5 to 1.8*10^-3 W m^-1 K^-1 for boundary temperatures of 20.4 and 290 K. The investigated silica aerogel powders show a large improvement potential for cryogenic thermal powder insulations. They are well suited to substitute expanded Perlite as cryo insulation.