Radiant Energy Transfer and Radiant Barrier Systems in Buildings
Written by Philip Fairey
INTRODUCTION
In Florida and other southern climates we depend on a number of strategies to keep heat
out of buildings. Mostly, these affect heat gains by conduction or convection. In the
average house, insulating walls and ceilings primarily restricts conduction. Double-glazed
windows restrict both conductive and convective heat gain. We have largely ignored
radiation (the third means of heat transfer) except in using window treatments and
coatings that reflect, absorb or shade from solar energy. But research points to exciting
potential for reducing heat gain in buildings by controlling radiation transfer in walls
and ceilings through the use of radiant barriers.
RADIANT ENERGY TRANSFER
The transfer of radiant energy through building components is difficult to understand.
It's even hard to simulate with computers. But if we grasp some basic facts about how
radiant energy is transferred, we can use the theory to practical advantage. Heat transfer
by all three modesconduction, convection, radiationis always from the warmer to
the colder region. But while conduction and convection can be transferred only through a
medium, radiation can be transferred across a perfect vacuum. It needs only two regions of
differing temperatures that "see" each other. Radiant energy travels in a
straight line through space; it is the main means of energy transport throughout the known
universe. To better envision how radiation works, think of how your TV set operates: a
transmitter emits an electromagnetic wave that travels through space; the wave is
"seen" by the antenna and the TV converts it to a video image. Like a television
signal, radiation is a band of electromagnetic waves. With respect to buildings, two
radiation bands are important: the solar spectrum and the far-infrared spectrum. On earth,
regions of different temperatures that see" each other exchange energy via infrared
radiation in the 4 to 40 micron wavelength band. (A micron is a millionth of a meter.)
Sunlight consists of much shorter wavelengths: 0.2 to 2.6 microns. Unlike the visible
portion of the solar spectrum ( 0.4 to 0.7 microns), infrared radiation is invisible.
There are both "near-infrared" and "far-infrared" radiation. The
latter is sometimes called "thermal" or "long-wave" radiation. The
effect of both is heat; in summer, this heat is unwanted. Radiant barrier systems are a
method of stopping far-infrared radiation from getting into building interiors and
increasing air-conditioning loads.
RADIANT BARRIER SYSTEMS
Radiant barrier systems comprise an airspace with one or more of its boundaries
functioning as a radiant barrier. Radiant Barriers are materials that restrict the
transfer of far-infrared radiation across an airspace. They do this by reflecting the
radiation that strikes them andat the same timeby not radiating energy. A material
that has this capability is said to have a very low emissivity. The lower the emissivity,
the better the radiant barrier. Emissivity values range from 0 to 1. Any material's
emissivity plus transmissivity plus reflectivity must always equal one. A material with an
opaque surface has a transmissivity of zero, so its emissivity equals one minus its
reflectivity. Materials that radiate very well and absorb a large percentage of the
radiation that strikes them have high emissivities. Most common building materials,
including glass and paints of all colors, have high emissivities near 0.9. Such materials
are capable of transferring 90% of their radiant energy potential. These materials are
ineffective barriers to radiant energy transfer. On the other hand, aluminum foil is an
excellent radiant barrier. It has a low emissivity (0.05), therefore, it eliminates 95% of
the radiant transfer potential. Aluminum foil is a good thermal conductor. Consequently,
it has an extremely low R-value. However, if it is placed between materials that are
attempting to transfer thermal energy by radiation (rather than conduction) and if it is
separated from these materials by an air layer, the foil effectively eliminates the normal
radiant energy exchange across the airspace. This is the operating principle of radiant
barrier systems and it often can be used to reduce the flow of heat through building
components.
SUNLIGHT AND HEAT
A material's response to far-infrared radiation can be quite different from its response
to gunfight. Since a large percentage of sunlight is in the visible range, we characterize
materials by color and clarity. We know, for example, that white paint reflects far more
solar radiation than does black paint. But in the farinfrared band, white paint absorbs
slightly more radiation than does black paint. This surprising fact tells us that we
cannot judge a material's far-infrared properties by sight. Common window glass, for
example, transmits more than 85% of incident sunlight but absorbs more than 85% of the
far-infrared energy that strikes it. The "solar greenhouse effect" results, in
part, from this phenomenon. Solar energy readily passes through the glass and is absorbed
by the opaque surfaces within the space. When these heated surfaces begin to radiate to
cooler surfaces, the glass absorbs most of this far-infrared radiation, trapping much of
the original solar gains inside the space as heat.
ROOF SYSTEMS
A Florida house attic offers excellent potential for use of radiant barrier systems: first
because the roof is the surface most exposed to solar radiation, and second, because most
of the solar gain absorbed by the roof is transmitted down to the attic floor by
radiation. Since the attic airspace separates the hot roof surface from the ceiling, no
heat will move down by conduction, and the heat will not convect down from the hot roof to
the ceiling because heated air rises. If you place a radiant barrier (layer of foil) in
the airspace between the hot roof deck and the cooler attic floor (insulation), you can
eliminate almost all radiant heat transfer. Studies at the Florida Solar Energy Center
(FSEC) indicate that, under peak day conditions, total heat transfer down through attics
can be reduced by more than 40% in this way.
Heat transferred upward through attics (winter heat loss) won't be affected as much
because a greater part of total upward heat transfer occurs by convection. That is why
radiant barriers in roof systems are a more effective cooling rather than heating strategy
and why they may be of great benefit to southern homeowners.
WALL SYSTEMS
The predominant heat transfer direction through walls is horizontal. Therefore, the
overall performance of radiant barrier systems in walls is less dependent on heat flow
direction than it is in roofs. But the seasonal performance of a radiant barrier wall
system differs for other reasons. In both winter and summer, the radiant barrier airspace
will drastically reduce the "sol-air effect." The sol-air effect, caused by
solar radiation striking the exterior surface of the building, raises the surface
temperature above that of the ambient air. This increases heat gain into the building in
summer but reduces heat loss in winter. Radiant barriers drastically reduce this sol-air
effect.
BUILDING TYPE
Exterior radiant barriers best apply to skin-load-dominated buildings such as homes,
rather than internal-load-dominated structures such as office buildings. Multistory
commercial buildings are not good candidates because they do not have dominating roof
loads. Even multistory residences located in borderline climates may not warrant radiant
barrier protection. Certain large buildings, however, can benefit from radiant barrier
construction. For example, some open-bay manufacturing buildings will benefit from
interior radiant barriers if radiant transfer between the occupants and the building skin
otherwise dominates comfort conditions. In the same way, radiant barriers can be used in
agricultural buildings that shelter livestock (most living things have high emissivities).
Radiant barriers can reduce energy consumption and/or improve comfort in many buildings.
But the radiant barrier strategy and construction technique will have to respond to
individual building needs.
FOR FURTHER INFORMATION
Fairey, Philip, "Designing and Installing Radiant Barrier Systems," Design Note
7, Florida Solar Energy Center, Cape Canaveral, FL, June 1984. Fairey, Philip,
"Effects of Infrared Radiation Barriers on Effective Thermal Resistance of Building
Envelopes," proceedings of the ASHRAE/DOE Conference on Thermal Performance of the
Exterior Envelopes of Buildings II, Las -Vegas, NV, December 1982. Fairey, Philip, et al.,
"The Thermal Performance of Selected Building Envelope Components in Warm,Humid
Climates," proceedings of the 1983 ASME Solar Division Conference, Orlando, FL, April
1983. Van Straaten, J. F., Thermal Performance of Buildings, Elsevier Publishing Company,
New York (1967) pp. 142-160. Wilkes, G. B., "Reflective Insulation," Journal of
Industrial and Engineering Chemistry, 31D:832. July 1939.
Footnotes
1. This document is FSEC Publication DN-6, provided for the Energy Resource CD-ROM by the
Florida Energy Extension Service, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: May 1994. First published:
1986.
2. Philip Fairey, Principal Research Scientist, Florida Solar Energy Center, State
University System of Florida, 300 State Road 401, Cape Canaveral, Florida 32920. ©
Copyright 1984, Florida Solar Energy Center. The Florida Energy Extension Service receives
funding from the Energy Office, Department of Community Affairs, and is operated by the
University of Florida's Institute of Food and Agricultural Sciences through the
Cooperative Extension Service. The information contained herein is the product of the
Florida Energy Extension Service and does not necessarily reflect the view of the Florida
Energy office.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action
employer authorized to provide research, educational information and other services only
to individuals and institutions that function without regard to race color, sex, age,
handicap, or national origin. For information on obtaining other extension publications,
contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences /
University of Florida / Christine Taylor Waddill, Dean
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