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Radiant Energy Transfer and Radiant Barrier Systems in Buildings

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Radiant Energy Transfer and Radiant Barrier Systems in Buildings Radiant Energy Transfer and Radiant Barrier Systems in Buildings Author Fairey, Philip. Publication Number FSEC-DN-6-86 Copyright Copyright © Florida Solar Energy Center/University of Central Florida 1679 Clearlake Road, Cocoa, Florida 32922, USA (321) 638-1000 All rights reserved. Disclaimer The Florida Solar Energy Center/University of Central Florida nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Florida Solar Energy Center/University of Central Florida or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the Florida Solar Energy Center/University of Central Florida or any agency thereof. FSEC-DN-6-86 Design Note Radiant energy transfer and radiant barrier systems in buildings Philip Fairey Florida Solar Energy Center Principal Research Scientist Introduction energy content and respective electromagnetic wave- In Florida and other southern climates we depend lengths of both of these spectrums. on a number of strategies to keep heat out of build- On earth, regions of different temperatures that ings. Mostly, these affect heat gains by conduction or "see" each other exchange energy via infrared convection. In the average house, insulating walls and radiation in the = 4 to 40 micron wavelength band. (A ceilings primarily restricts conduction. Double-glazed micron is a millionth of a meter.) Sunlight consists of windows restrict both conductive and convective much shorter wavelengths: E 0.2 to 2.6 microns. Unlike heat gain. the visible portion of the solar spectrum (,='0.4 to 0.7 microns), infrared radiation is invisible. There are both We have largely ignored radiation - the third means "near-infrared" and "far-infrared" radiation. The latter of heat transfer - except in using window treatments and coatings that reflect, absorb or shade from solar is sometimes called "thermal" or "long-wave" radiation. 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 Energy content Energy content (W/mz rm) (wIm2.m, 1 radiant barriers. r 300 K Blackbody 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, Window" we can use the theory to practical advantage. 5 Heat transfer by all three modes - conduction, con- vection, radiation - is always from the warmer to the colder region. But while conduction and convection can be transferred only through a medium, radiation 0 .5 1.0 1.5 2.0 2.5 4 6 8 10 12 14 16 18 can be transferred across a perfect vacuum. It needs -0 Wavelength (rm) only two regions of differing temperatures that "see" r 0 > 01 each other. Radiant energy travels in a straight line 3 5 Near-infrared Far-infrared * through space; it is the main means of energy trans- Solar Spectrum Long-wave Spectrum port throughout the known universe. To better envision how radiation works, think of how your TV set operates: a transmitter emits an elec- Figure 1. Energy content and electromagnetic tromagnetic wave that travels through space; the wave wavelengths of the solar and long-wave spectrums. is "seen" by the antenna and the TV converts it to a video image. The effect of both is heat; in summer, this heat is Like a television signal, radiation is a band of elec- unwanted. Radiant barrier systems are a method of tromagnetic waves. With respect to buildings, two stopping far-infrared radiation from getting into build- radiation bands are important: the solar spectrum and ing interiors and increasing air-conditioning loads. the far-infrared spectrum. Figure 1 illustrates the 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 and - at the same time - by not radiating energy. A material that has this capability is said to have a very low emissiuity. The lower the emissivity, the better the radiant barrier. Emissivity values range from 0 to 1. Any material's emmissivity 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 Solar Absorptance radiant energy transfer. On the other hand, aluminum foil is an excellent radiant barrier. It has a low emissiv- Figure 2. Solar and far-infrared characteristics ity (0.05), therefore, it eliminates 95% of the radiant of some common opaque building materials. transfer potential. Aluminum foil is a good thermal conductor. Conse- absorbs most of this far-infrared radiation, trapping quently, it has an extremely low R-value. However, if it much of the original solar gains inside the space is placed between materials that are attempting to as heat. transfer thermal energy by radiation (rather than con- duction) and if it is separated from these materials by Roof systems an air layer, the foil effectively eliminates the normal radiant energy exchange across the airspace. This is A Florida house attic offers excellent potential for the operating principle of radiant barrier systems and it use of radiant barrier systems: first, because the roof often can be used to reduce the flow of heat through is the surface most exposed to solar radiation, and second, because most of the solar gain absorbed by building components. the roof is transmitted down to the attic floor by radia- tion. Since the attic airspace separates the hot-roof Sunlight and heat surface from the ceiling, no heat will move down by A material's response to far-infrared radiation can conduction, and the heat will not convect down from be quite different from its response to sunlight. Since the hot roof to the ceiling because heated air rises. a large percentage of sunlight is in the visible range, If you place a radiant barrier (layer of foil) in the we characterize materials bv color and claritv. We airspace between the hot roof deck and the cooler know, for example, that white paint reflects iar more attic floor (insulation), you can eliminate almost all solar radiation than does black paint. But in the far- radiant heat transfer. Studies at the Florida Solar infrared band, white paint absorbs slightly more radiation Energy Center (FSEC) indicate that, under peak day than does black paint. This surprising fact tells us that conditions, total heat transfer down through attics can we cannot judge a material's far-infrared properties be reduced by more than 40% in this way. Figure 3 by sight. Figure 2 compares the solar and far- shows air and ceiling insulation temperatures for side- infrared characteristics of some common oDaaue. by-side attic spaces monitored at FSEC. Temperature building materials. differences across the insulation are significantly Figure 2 shows only opaque materials, but transpar- reduced by the radiant barrier system. ent materials also res~onddifferentlv to solar and far- Heat transferred upward through attics (winter heat infrared radiation. Common window glass, for exam- loss) won't be affected as much because a greater part ple, transmits more than 85% of incident sunlight but of total upward heat transfer occurs by convection. absorbs more than 85% of the far-infrared energy that That is why radiant barriers in roof systems are a more strikes it. The "solar greenhouse effect" results, in effective cooling rather than heating strategy and why part, from this phenomenon. Solar energy readily they may be of great benefit to southern homeowners. passes through the glass and is absorbed by the In a typical Florida home, a radiant barrier roof could opaque surfaces within the space. When these heated cut annual cooling loads by 4-8%and peak cooling surfaces begin to radiate to cooler surfaces, the glass loads by 9% depending on the level of attic ventilation. 2 Design Note Standard roo1 side with exterior radiant barrier airspaces vented with ambient air do not perform well in winter. Unvented walls perform much better in winter, and if the radiant barrier airspace is on the interior side of the wall, its performance does not appear to depend on sea- sonal effects. In full-scale tests conducted at the FSEC Passive Cooling Laboratory seven different wall systems were tested. Five systems contained radiant barrier systems, and two did not. Interior and exterior radiant barrier systems showed significant seasonal performance vari- ations as demonstrated in Figure 4. "1) 6 8 ;0 1; i4 Ib II) Z!O ;2 ;4 Eastern Standard Time Figure 3. Unvented radiant barrier roof versus unvented standard roof (both ceilings with 6" fiberglass insulation). Good ventilation requirements, to meet the upper end of these savings, can normally be provided by contin- uous soffit and ridge vents. Wall systems The predominant heat transfer direction through walls is horizontal.
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