However, heat flow to and from surfaces also occurs via convection, which in some geometries is different in different directions. Reflective aluminum foil is the most common material used as a radiant barrier. It has no significant mass to absorb and retain heat. It also has very low emittance values "E-values" (typically 0.03 compared to 0.90 for most bulk insulation) which significantly reduces heat transfer by radiation. Types of radiant barriers Foil or "reflective foil laminate"s (RFL). Foil-faced polyurethane or foil-faced polyisocyanurate panels. Foil-faced polystyrene. This laminated, high density EPS is more flexible than rigid panels, works as a vapor barrier, and works as a thermal break. Uses include the underside of roof sheathing, ceilings, and on walls. For best results, this should not be used as a cavity fill type insulation. Foil-backed bubble pack. This is thin, more flexible than rigid panels, works as a vapor barrier, and resembles plastic bubble wrap with aluminum foil on both sides. Often used on cold pipes, cold ducts, and the underside of roof sheathing. Light-colored roof shingles and reflective paint. Often called cool roofs, these help to keep attics cooler in the summer and in hot climates. To maximize radiative cooling at night, they are often chosen to have high thermal emissivity, whereas their low emissivity for the solar spectrum reflects heat during the day. Metal roofs; e.g., aluminum or copper. Radiant barriers can function as a vapor barriers and serve both purposes with one product. Materials with one shiny side (such as foil-faced polystyrene) must be positioned with the shiny side facing an air space to be effective. An aluminum foil radiant barrier can be placed either way – the shiny side is created by the rolling mill during the manufacturing process and does not affect the reflective of the foil material. As radiant barriers work by reflecting infra-red energy, the aluminum foil would work just the same if both sides were dull. Reflective Insulation Insulation is a barrier material to resist/reduce substance (water, vapor, etc. ) /energy (sound, heat, electric, etc.) to transfer from one side to another. Heat/ Thermal Insulation is a barrier material to resist / block / reflect the heat energy (either one or more of the Conduction, Convection or Radiation) to transfer from one side to another. Reflective Insulation is one of the Heat/Thermal Insulation to reflect Radiation Heat (Radiant Heat) transfer from one side to