Foil-covered insulating material, the effectiveness of which is measured by the number of reflective surfaces rather than thickness.

an effective barrier against radiant heat transfer because it reflects almost all of the infrared radiation striking its surface and emits very little of the heat conducted through it

Sheet material with one or both sun faces of comparatively low heat emissivity, such as aluminum foil. When used in building construction the surfaces face air spaces, reducing the radiation across the air space.