An appropriate thermal insulation of fresh fish packaging can substantially reduce negative effects of poor temperature management in chill chains. The aim of the current study is to experimentally and numerically investigate the performance of two types of EPS (expanded polystyrene) boxes in protecting superchilled fresh fish products subjected to temperature fluctuations, simulating conditions during transport. One EPS box type is a new improved version designed by utilising numerical heat transfer modelling for minimising the maximum product temperature during thermal load. This box weighed 11% less than the older box type. The performance of the boxes was evaluated by means of temperature monitoring and sensory evaluation. The thermal insulation of the new boxes was significantly better compared to the old boxes. According to sensory evaluation, storage in the new boxes resulted in approximately 2 days longer storage life. A satisfactory agreement between numerical results and experimental results was obtained.