Emissivity of Polished Aluminium

A polished aluminium surface emits only about 5 % of the thermal radiation a black body at the same temperature would, and reflects roughly 95 % of what falls on it. That is the whole physics of radiant barriers, reflective attic foil, emergency blankets and multi-layer insulation on spacecraft: not a low conductivity, but a low emissivity. In a wall or attic cavity a foil face can cut the radiant component of heat transfer — often half the total across an air gap — by an order of magnitude.

The trap is that emissivity is a surface property with no depth to it, and the surface changes. Anodised aluminium jumps to 0.7–0.9 despite being chemically the same metal. Dust, fingerprints, oxidation and paint destroy the effect, and a radiant barrier facing a dusty attic loses much of its value within a few years — which is why they must face an air space and, ideally, face downward. Infrared thermometers make the reciprocal error: pointed at bright metal they read wildly low, because they see reflected surroundings rather than the surface, which is why technicians stick a square of matt tape on a shiny pipe before measuring it.