A hot object radiates heat into the surrounding space so it might be tempting to think that nothing would be less conducting than empty space. Nevertheless, Shanhui Fan of Stanford University and colleagues have found something even less able to conduct heat – and it contains matter.

The idea is simple: a suitable photonic crystal opaque to a band of infrared could actually block more heat flow than pure nothing. At or above room temperature, a stack of 10 silicon layers, each 1 μm thick and separated by 90 μm gaps of vacuum, would have a thermal conductance half that of empty space, with the possibility of countless future applications.