Reflective insulation and radiant barriers reduce the radiation of heat to or from the surface of a material. Radiant barriers will reflect radiant energy. A radiant barrier by itself will not affect heat conducted through the material by direct contact or heat transferred by moist air rising or covection. For this reason, trying to associate R-values with radiant barriers is difficult and inappropriate. The R-value test measures heat transfer through the material, not to or from its surface. There is no standard test designed to measure the reflection of radiated heat energy alone. Radiated heat is a significant means of heat transfer; the sun’s heat arrives by radiating through space and not by conduction or convection. At night the absence of heat (i.e. cold) is the exact same phenomenon, with the heat radiating described mathematically as the linear opposite. Radiant barriers prevent radiant heat transfer equally in both directions. 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 foil laminates.
- 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.
- 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 reflectivity 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.
Types of reflective insulation
Reflective insulation is commonly made of either aluminum foil attached to some sort of backing material or two layers of foil with foam or plastic bubbles in between creating an airspace to reduce convective heat transfer also. The aluminum foil component in reflective insulation will reduce radiant heat transfer by up to 97%. As reflective insulation incorporates an airspace to reduce convective heat flow, it carries a measurable R-Value.
Advantages
Very effective in warmer climates
No change thermal performance over time due to compaction, disintegration or moisture absorption
Thin sheets takes up less room than bulk insulation
Can act as a vapor barriers
Non-toxic/non-carcinogenic
Will not mold or mildew
Radon retarder, will limit radon penetration through the floor
Disadvantages
Must be combined with other types of insulation in very cold climates
May result in an electrical safety hazard where the foil comes into contact with faulty electrical wiring
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