Directly irradiated and volumetrically absorbing liquid-based solar receivers promise to enhance peak solar flux limits relative to surface absorbers and enable simpler receiver designs with integral thermal storage. However, their thermal efficiency is limited by large thermal losses at high operating temperatures. The harsh, high temperature ( > 400 degrees C) environments in liquid-based receivers impose significant material and system design constraints in mitigating heat losses, particularly for large-scale, commercial-size receivers. Here we report a new cover design that reduces thermal losses up to 51% while reducing the optical efficiency by less than 5%. The proposed design consists of floating hollow fused silica spheres and is stable in harsh high temperature environments, highly solar-transparent, reduces convective and radiative losses, and minimizes the surface area available for evaporation losses. In addition, the modular components allow easy online maintenance and unlike a single continuous window-pane, there is no limit to the surface size the spheres can cover. This transparent, insulating cover has the potential to enable much higher solar-thermal volumetric receiver temperatures and could also be used in other high temperature open bath industrial applications to provide energy savings.