Super-thin thermal insulation is in my sights at the moment. There are two categories: (a) multi-layer foil and fibre; and (b) paints or paint additives. The insulating effect of multi-layer systems is basically equal to the same thickness of whatever insulating fibre they use, but there is some additional advantage where they are installed with an air gap either side, since they create an extra cavity which has a certain thermal resistance. Their reflective foil will impart additional thermal resistance by preventing radiation from the hot to the cold face of the cavity. But note, however, that after filling with ordinary fibre, the hot and cold surfaces of the cavity can no longer see each other, and heat transfer is solely by conduction. So BOTH techniques eliminate radiative transfer across the cavity and the foil therefore imparts no advantage.
Insulating paints meanwhile, even if composed of material with high thermal resistivity, will have totally negligible effect because the insulating layer is microscopically thin (under 0.3mm by my calculations, based on coverage data in the advertisements). Claims that their ingredients reflect heat are unsound because those so-say reflective materials are buried in the paint layer; to reflect heat the SURFACE of the paint would need to act like a mirror to infra-red radiation.