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 aluminum foil 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 Installed in the Ceiling of an Attic as a Radiant Barrier
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.
Click here for a comprehensive article on attic insulation.
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.
Click here for a comprehensive article on the Physics of Foil.
| 
