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Radiation is the transfer of heat (infra-red radiant energy) from a hot surface to a cold surface through air or a vacuum. Radiation is the dominant method of heat transfer in a building accounting for 65-85 percent of all heat transfer through walls, ceilings, attic and floors. Reflective insulation is an effective barrier against radiant heat transfer because it reflects back almost all of the infrared radiation striking its surface and emits very little of the heat conducted through it. Reflective insulation products also incorporate trapped air spaces as part of the system to retard the convective flow of heated air the way fiberglass insulation does. The total thermal performance of the reflective system varies with the size and number of enclosed reflective spaces within the building cavity.
The thermal resistance of insulation is measured in R-value, or resistance to the movement of heated air by convection. The higher the R-value the more effective the insulation will be in resisting convective heat transfer. R-value does not measure an insulation's ability to resist radiant heat. Mass insulation like fiberglass or foam board primarily slows convective heat transfer.
In contrast, reflective foil insulation facing an airspace like an attic, wall cavity, or crawlspace creates a radiant heat barrier. Aluminum foil reflects back 97% of radiant energy that strikes it. The generally accepted definition of a radiant barrier system specifies that the reflective material face an open air space. Radiant barriers that have no central layer of foam or bubbles to trap air are not insulation per se, and by definition, have no R-value. The idea is that a radiant barrier facing an enclosed air space (like an attic or a wall cavity) becomes a "reflective insulation" with a measurable R- value. Despite the advances in space technology in insulation systems based on understanding and modifying the effects of radiation, no universally accepted laboratory method has yet been devised to measure and report the resistance to heat flow of a multi-layer foil to properly compare reflective insulation to the R-values of mass insulation.
Reflective insulation is most effective in reducing the downward flow of heat through the roof in summer and through the floor in winter. Reflective insulation systems are typically located between roof rafters, floor joists, and wall studs.
Reflective insulation is effective in floor systems above unheated basements and ventilated crawl spaces. Heat is lost through floors primarily by radiation (up to 93%). When reflective insulation is installed in the ground floors and crawl spaces of cold buildings, it prevents the indoor heat rays from penetrating down, reflecting the heat back into the building, thereby warming the floor. Since aluminum is non-permeable, it is unaffected by ground vapors. A properly installed reflective insulation system in the floor will act as a vapor barrier, significantly retarding penetration of ground moisture into the house above.
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