The present study investigates efficacy of ZnO based transparent heat mirror covers as thermal loss mitigators in 'direct volumetrically absorbing' solar thermal platforms. Comprehensive experimental and theoretical modeling frameworks have been developed to understand and quantify the heat loss mechanisms. Detailed analysis reveals that performance characteristics are strong functions of the 'side' of the glass that has been coated (i.e. whether 'receiver facing' (RF) or 'sky facing' (SF) sides of the cover has been coated). Results show that employing ZnO based heat mirror as a cover significantly reduces the thermal losses relative to uncoated glass cover (27.72% and 21.44% reduction for RF and SF side coated heat mirrors respectively). Moreover, fundamental performance limits of ideal heat mirrors have also been determined for both RF and SF side coated cover configurations. Relative to the uncoated glass covers, ideal heat mirror covers (viz., RF and SF side coated) promise 47.42% and 36.07% thermal loss reduction respectively (@ 400 °C receiver surface temperature and 1.5 μm cut-off wavelength). Overall, the present work represents a significant step in improving the existing volumetric absorpion based solar thermal systems; particularly aiming at intermediate temperature applications (viz. industrial and domestical heating/cooling).