The thermal conductivity of pyrogenic silica powder boards mixed with a silicon carbide opacifier is measured as a function of gas pressure and of temperature. At room temperature and gas pressures around 1 to 5 mbar the thermal conductivity is about 0.004 W m(-1) K-1. At a mean temperature of 300 degreesC the evacuated panel has a thermal conductivity well below 0.010 W m(-1) K-1, because of the silicon carbide opacifier. Several square metres of vacuum panels have been encapsulated in laminated aluminium envelopes and tested as thermal insulations for building facades.

An evacuated thermal insulation panel has been developed consisting of glass panes as covers and a powder filling. Several powder materials and mixtures of powders have been testing regarding their thermal conductivity and the critical gas pressure for the onset of gas conductivity. With an opacified precipitated silica powder a thermal conductivity of less than 0.003 W/mK could be reached if evacuated below 0.1 mbar. A value of 0.004 W/mK has been measured at an internal gas pressure of 1 mbar. A crucial point of panel design is the rim seal which should provide a negligible gas permeation and a small thermal heat transfer coefficient. With very thin stainless steel foils these requirements could be met.