In hot climates, benefits of attic radiant barriers include
both dollar savings and increased comfort.
Without a radiant barrier, your roof radiates
solar-generated heat to the insulation below it. The insulation absorbs the
heat and gradually transfers it to the material it touches, principally, the
ceiling. This heat transfer makes your air conditioner run longer and consume more
electricity.
An aluminum foil radiant barrier blocks 95 percent of the
heat radiated down by the roof so it can't reach the insulation.
In summer, when your roof gets very hot, a radiant barrier
cuts air-conditioning costs by blocking a sizable portion of the downward heat
gain into the building. In the warm spring and fall, radiant barriers may save
even more energy and cooling dollars by increasing your personal comfort.
During these milder seasons, outdoor air temperatures are comfortable much of
the time. Yet solar energy still heats up your roof, insulation, attic air, and
ceiling to temperatures that can make you uncomfortably warm. An attic radiant
barrier stops almost all of this downward heat transfer so that you can stay
comfortable without air conditioning during mild weather.
You may also find that radiant barriers can expand the use
of space in your home. For instance, uninsulated, unconditioned spaces such as
garages, porches, and workrooms can be more comfortable with radiant barriers.
Because radiant barriers keep attics cooler, the space is more usable for
storage.
Roof-radiated heat also warms ductwork or mechanical
equipment (air handler) in your attic. The proportion of the total heat gain
the ductwork represents, compared to heat gain to the interior of the house,
varies depending on the amount of attic and duct insulation you have.
How do radiant
barriers "block" heat transfer?
Aluminum foil, the operative material in attic radiant
barriers, has two physical properties of interest here. First, it reflects
thermal radiation very well. Second, it emits (gives off) very little heat. In
other words, aluminum is a good heat reflector and a bad heat radiator.
Your grandmother probably made use of these properties
through "kitchen physics." She covered the Thanksgiving turkey with a
loose "tent" of aluminum foil before she put it in the oven. The foil
reflected the oven's thermal radiation, so the meat cooked as evenly on top as
on the bottom. She removed the foil briefly to let the skin brown, but when she
took the bird from the oven, she "tented" it with foil again. Since
aluminum doesn't emit much heat, the turkey stayed hot until the rest of the
meal was ready. To understand the concept of not emitting heat, let's use an
analogy of a light bulb. When you turn on a light bulb, it emits light. If you
paint the light bulb black, when it is turned on, there is no light emission. A
radiant barrier has a similar effect on infrared heat. Your roof surface heats
up in the sun and will emit infrared heat. When this infrared heat heats the
radiant barrier it will not emit, or reradiate, the heat into your attic.
Cooking a turkey and painting light bulbs are simple
analogies, but the same principles of physics apply to an attic radiant
barriers. Aluminum foil across the attic airspace reflects heat radiated by the
roof. Even if the radiant barrier material has only one aluminum foil side and
that side faces down, it still stops downward heat transfer because the foil's
low emissivity will not allow it to radiate the roof's heat to the insulation
below it.
