Colloidal crystals typically consist of sub-micron sized monodisperse particles, which are densely packed on a face centered cubic lattice. While many properties of this material class have been studied over the past decades, little is known about their thermal transport properties. The high amount of interfaces and their small interparticle contact area should result in efficient thermal insulation. Using laser flash analysis we report for the first time on the temperature dependent thermal conductivity of a freestanding 366 nm polystyrene (PS) colloidal crystal. Macroscopic monoliths of these samples were fabricated by colloidal self-assembly. We demonstrate a very low thermal conductivity kappa of 51 mW K-1 m(-1) (kappa of bulk PS similar to 140 mW m(-1)). Remarkably, this low thermal conductivity is reached at a comparatively high density of 750 kg m(-3). It can be further increased by almost 300% upon film formation and loss of the colloidal mesostructure. Additionally, this open porous structure is largely independent of the surrounding atmosphere. This can be rationalized by the small size (similar to 100 nm) of the pores present within this colloidal crystal. (C) 2015 Elsevier Inc. All rights reserved.