Among the many anomalous properties of water, the formation of amorphous ice is one of its most intriguing phenomena. When water vapour is deposited at very low temperatures, it forms an amorphous solid known as amorphous solid water (ASW). Here, we combine X-ray free-electron laser pump-probe diffraction, continuum heat-transfer modelling, and molecular dynamics simulations to investigate the transient thermal response of ASW-films on a platinum substrate. Picosecond laser pulses heat the Pt from 100 K to ~700 K and maintain elevated temperatures for several nanoseconds. Contrary to expectations based on diffusive thermal transport, the hundreds-nanometer-thick ASW layer shows no detectable structural change within tens of nanoseconds. Simulations suggest a mechanism in which a nanometric vapour layer spontaneously forms at the interface, suppressing thermal contact and insulating the ice. The experimental data are consistent with a nanometric (~6 nm) interfacial gap, indicating that the anisotropic scattering originates from a vapour-nucleated interfacial layer. This finding highlights a general non-equilibrium mechanism of interfacial thermal decoupling that may extend beyond the specific ASW-metal system studied here.
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