We investigated the reduction of gas heat conduction in evacuated flat-plate collectors with operating pressures between 10(3) and 10(4) Pa. In stationary heat loss experiments with a real-size model the pressure dependency of the thermal losses was examined for air, krypton and for a nanostructured SiO2-aerogel powder insulation between absorber and rear side of the casing. The theory for pressure dependent gaseous conduction was validated. Due to its high molecular mass, krypton as filling gas reduces gaseous conduction by 65% in the continuum range with respect to air. In a low emissive EFPC where gas conduction losses are comparable to radiative losses, the reduction in the total losses is about 30%. In principle, with unpacked aerogel the heat conduction could be reduced further: the nanostructures affect the gaseous heat conduction to be nearly totally suppressed below 10(2) Pa, because the mean free path of the gas is large compared to the pore dimensions of the aerogel. However, the applied packaging of PTFE-fabric compensates the excellent properties of the insulant due to solid conduction in the fabric.