Energy Efficiency Factsheet Principles of Heat Transfer

March 2008 Version 1.1 Principles of Heat Transfer Supplement to Heat Transfer Product Reviews Many products are available that claim to save transparent medium between the surfaces ex- energy by controlling radiation heat transfer, changing radiation. Radiation exchange occurs including reflective paint additives, radiant between two surfaces when one is warmer than barriers, reflective roof coatings and low-emis- the other and they are in “view” of each other; sivity (low-e) coatings on windows. In an ideal i.e., there is nothing between the two surfaces. world, the potential energy savings due to controlling radiation losses and gains might Low-emissivity = be large. In reality, there are many factors that High Reflectivity = impact how effective these products are. Reduced Heat Transfer Radiation is a significant component of heat Radiation Versus Other Heat transfer in buildings – in both heating and Transfer Methods cooling even at typical temperatures and even First of all, heat is the energy transferred from in the absence of solar radiation – but it is espe- one body or system to another as a result of cially important for sun-exposed surfaces and a difference in temperature. Heat always mi- where there are large temperature differences grates from the hotter object to the cooler (e.g., radiant heating, refrigeration, industrial object, never the other way around. Heat is settings with warm surfaces, ice rinks, etc.) transferred by three methods: conduction, Reflective (low-emissivity) products will gener- convection, and radiation. ally be most effective in these applications. In certain applications, however, a high-emissiv- Conduction requires the physical contact of ity (non-reflective) surface will perform better. two objects. In the case of a wall, heat is con- Therefore, in evaluating a reflective product ducted through the layers within the wall from we should have some understanding of these the warmer side to the cooler side. Convection properties. is heat transfer due to fluid or airflow. A com- mon example is when warm air rises (or cool Reflectivity and emissivity are properties of a air falls) on a wall’s inside surface, inducing air surface that affect radiation heat transfer and movement. how a reflective product will perform. The fraction of radiation arriving at a surface that is Heat is transferred by radiation when surfaces reflected by it is called itsreflectivity . Another exchange electromagnetic waves, such as light, property of the surface is its emissivity, which infrared radiation, UV radiation or microwaves. essentially is the surface’s tendency to emit Radiation does not require any fluid medium radiation to other bodies. Surfaces with high or contact, but does require an air gap or other emissivity are also very absorptive, that is, they

Managed by With support from March 2008 – Page  Energy Efficiency Factsheet Principles of Heat Transfer

Figure 1 Incident Radiation on an Opaque Surface

Incident Reflected Radiation Radiation Emitted Radiation

Absorbed Radiation

For any particular wavelength of radiation striking an opaque material, the sum of radiation that is reflected and that absorbed must equal the incident radiation. The radiation emitted and the radiation absorbed must equal each other at steady state.

will readily absorb radiation striking them. • Between interior objects in a building These properties may vary depending on the (including people) and the interior wavelength of radiation falling on the surface. surfaces of exterior walls – on both For example, the surface may reflect much of hot and cold days (i.e., whenever the visible radiation (i.e., light) falling on it, you are conditioning your space to but not much of the ultraviolet (UV) radiation counter the weather outside). or infrared radiation falling on it (see Figure 1). • Between the exterior surfaces of a building and its surroundings These properties are related to each other and – on both hot and cold days (i.e., describing how they are related can be helpful whenever you are countering the in understanding them (see section titled More weather outside). on Heat Transfer Properties of Surfaces on page 6). Low-e coatings on windows save energy in most circumstances because they reduce heat Reduce Heat Transfer, transfer with the surroundings. As another or Enhance It? example, a low-emissivity ceiling – such as unpainted aluminum or a reflective aluminum Low emissivity will save energy whenever you paint product – in an ice rink may have very want to reduce heat transfer, and vice versa. good energy savings. In this case, low emissiv- Places you generally want to reduce heat ity (over all wavelengths) would reduce radia- transfer are:

March 2008 – Page  Energy Efficiency Factsheet Principles of Heat Transfer tion heat transfer between the warmer ceiling The Complication with and the cold surface of the ice. Sun-Exposed Surfaces Solar radiation is composed primarily of visible On the other hand, a surface used as a radiant and near-infrared radiation. Far-infrared radia- heater – such as a radiant floor or a radiator – is tion has longer wavelengths than near-infrared an example of where high emissivity is benefi- and is largely outside the solar band (see Fig- cial because we want to enhance heat transfer ure 2). (Both near- and far-IR are felt as heat.) from the radiator. Another example of where Therefore, whenever we are trying to under- low-emissivity coatings will increase energy stand how a material behaves when exposed to use is a building that requires cooling even on solar radiation, its emissivity in each of these a cool day because, in this case, you generally three bands is more important than its overall want to enhance heat transfer (i.e., you are not value summed over the entire spectrum. For a countering the weather outside). For example, “cool roof,” we want to reflect solar radiation low-e surfaces on windows or walls are bad in – and whatever heat we do absorb we want to rooms with high internal gains, such as com- emit back to the surroundings. Therefore, we puter server or telephone switching rooms, want high reflectivity in the visible and near- because you generally want to get rid of heat infrared bands, but low reflectivity (i.e., high at all times. The only time low-e surfaces will emissivity) in the far-infrared bands. help you in a server room is when it gets so cold outside that you have to start heating (i.e., If you just look at a single number for emissiv- when you must counter the weather outside). ity you will miss an important characteristic of

Figure 2 The Solar Spectrum

Visible 2000

Near 1500 Infrared /micrometers) 2 1000

Far Infrared 500 (~5 to ~100 micrometers)

Power Density (W/m 0 0.50 0.75 1.00 1.25 1.50 Wavelength (micrometers) Solar radiation is composed primarily of visible and near-infrared radiation. Far-infraredFar-infrared radiation radiation is largely is largely outside outside the the solar solar spectrum, spectrum; that is that is, solar radiation is very low at such long wavelengths. solar radiation is very low at such long wavelengths.

March 2008 – Page  Energy Efficiency Factsheet Principles of Heat Transfer certain materials. For example, both unpainted • Keep in mind also that the size of the air metal surfaces and white surfaces have high gap matters. Some reflective products, reflectivity when summed over all bands and such as reflective bubble insulation, have so both reflect much of the solar radiation an air gap built into the construction striking them. But when you look at each of of the material. If this air gap is small, the three bands separately, you can see why however, its effect on radiation heat unpainted metal roofs become hot in the sun transfer will also be less important but white surfaces stay cooler. White roofs have and the reflective product will be less high emissivity in the far-infrared band, while effective. unpainted metal roofs have low emissivity in this band. • In order for a reflective product to remain effective, its reflectivity must Tips for Evaluating Products remain high. Reflectivity declines if the and Their Application product is dirty and often declines with age and exposure to the sun or weather Most surfaces found in buildings – such as over time. In contrast, maintaining walls painted with conventional paints, surface quality is not a concern with carpeted or finished floors, and standard roof- insulation products, such as fiberglass or ing materials – have high emissivity. Glass that rigid foam insulation. does not have a low-e coating also has high emissivity. In most situations, we don’t have • In addition, obtaining a high initial to specify high-emissivity surfaces, since these reflectivity can be difficult. Paint addi- are typical. On the other hand, reflective or tives, for example, are generally limited low-emissivity surfaces must generally be in their ability to make large changes in specified. The following tips will help in reflectivity and emissivity. Experimen- selecting and maintaining reflective or low- tal data for one product indicated that emissivity surfaces. reflectivity was changed by 15% or less. • While R-values are used to quantify an • For many applications of reflective prod- insulation’s resistance to conduction heat ucts, both the solar reflectivity and the transfer, they cannot be used to describe far- infrared emissivity are important. a reflective product’s effect on radiation A high infrared emissivity will be an heat transfer. Sometimes manufacturers advantage in certain situations (e.g., a use an “equivalent R-value” to compare hot roof or a radiant heater) and a det- the reflective product to an insulation, riment in many other situations (e.g., but such equivalent R-values are only interior and exterior walls that are not valid under the specific conditions of the sun exposed.) When analyzing any test in which they were determined. In reflective product, we must consider the general “equivalent R-values” are not use- particular conditions under which it will ful in evaluating a product’s impact on be used and whether heat transfer from energy use. Be skeptical of any claim of the surface to its environment is a a high R-value or even a high equivalent benefit or detriment in that case. R-value for reflective products. • Considering the whole building, keep • Because heat transfer by radiation re- in mind that heat gains and losses of quires an air gap or other transparent the building component on which these medium, reflective products have no products are applied generally comprise effect at all when placed in direct contact only a relatively small part of the overall with an opaque surface. As an example, heat gains and losses in a building. If a the foil of a foil-faced rigid foam insu- reflective coating is applied to an exterior lation has no effect whatsoever if the wall, for example, consider that walls insulation fills the entire cavity.

March 2008 – Page  Energy Efficiency Factsheet Principles of Heat Transfer

typically account for about a quarter of surface properties: overall residential building losses, while sun-exposed walls will be a fraction of Edwards, D.K., V.E. Denny and A.F. that. Heating and cooling loads due to Mills, Transfer Processes: An Introduction to roofs are typically a greater share of the Diffusion, Convection and Radiation, 2nd total and have more sun exposure, and Edition, McGraw-Hill, 1975. so have greater potential for reduction. Reagan, J.A. and Acklam, D.M., “So- • Reflective barriers are not generally lar Reflectivity of Common Building considered a good application in houses Materials and its Influences on the Roof located in climates where heating is Heat Gain of Typical Southwestern U.S. dominant. Residences,” Energy and Buildings, No. 2, Elsevier Sequoia, Netherlands, 1979. • Also consider the person interacting with the building as a system. Controlling A handy calculator for comparing heat loss radiation from occupants to building through roofs with different surfaces is avail- surfaces often improves comfort. This able at www.roofcalc.com/. can result in additional energy savings since, as comfort is improved, occupants are more likely to reduce thermostat set- Product and Technology Reviews tings in the winter and increase them in The EnergyIdeas Clearinghouse publishes the summer. Such savings are often not Product and Technology Reviews for Northwest captured in analytical studies or labora- electric utilities. The reviews describe the tory results. technology, discuss available data, and suggest additional testing to verify energy saving • The Federal Trade Commission, Bureau claims. Products reviewed include: of Consumer Protection, publishes and enforces R-value testing and labeling • Ultra Concrete Barrier rFOIL™ requirements for home insulation. For www.EnergyIdeas.org/documents/ more information about the rules go to Factsheets/PTR/UltraCBF_rFOIL.pdf www.ftc.gov/bcp/conline/edcams/eande/ • Insuladd® index.html. www.EnergyIdeas.org/documents/ Factsheets/PTR/Insuladd.pdf Reflectivity and Emissivity Values Values for reflectivity and emissivity of Additional Information materials, paints, reflective coatings and low- Northwest electric utilities can contact the emissivity products are often available from EnergyIdeas Clearinghouse for additional manufacturers or on websites of organizations information on this or other energy technolo- such the Florida Solar Energy Center (FSEC) gies or products. Contact: www.fsec.ucf.edu. For example, see the FSEC Phone: 1-800-872-3568 report, “Laboratory Testing of the Reflectance Email: [email protected] Properties of Roofing Materials” www.fsec.ucf. Website: www.EnergyIdeas.org edu/en/publications/html/fsec-cr-670-00.

Data for a variety of surfaces can be found at Author Carolyn Roos, Ph.D. http://pdf.directindustry.com/pdf/impac-infrared/ WSU Extension Energy Program mikron-catalogue/4798-11038-_370.html, (pages 370-374) a website compiled by DirectIndustry, and at www.tak2000.com/data/finish.htm, a web- © 2008 Washington State University site compiled by K&K Associates. The following Extension Energy Program. references are also useful sources of data for WSUEEP08-006, updated March 2008

March 2008 – Page  Energy Efficiency Factsheet Principles of Heat Transfer

Even More on Heat is shiny if it reflects much of ment at the longer, infrared the visible radiation (i.e., light) wavelengths. As an example, Transfer Properties that falls on it or, equivalently, an unpainted aluminum or tin of Surfaces does not absorb much light. roof with 70% solar reflectivity A surface may also be “shiny” and 4% far-infrared emissivity Reflectivity, emissivity, transmis- (i.e., have high reflectivity and will be warmer on a sunny day sivity and absorptivity are prop- low absorptivity) to non-visible than a white metal roof with erties of a surface that affect parts of the spectrum, such as 70% total solar reflectivity and radiation heat transfer. UV radiation or infrared radia- 85% far-infrared emissivity (“cat tion. Remember we can think on a hot tin roof,” not “cat on a At any particular wavelength of of emissivity and absorptivity hot white roof”). If on the other radiation, these properties are as approximately equal, so low hand, we want to enhance solar all related to each other. How- absorptivity also means low heating of a surface, a surface ever, these relations do not emissivity. with the opposite properties of generally hold when talking a good “cool roof” is desired. about radiation at different In evaluating sun-exposed That is, for a good solar absorb- wavelengths; that is, a surface building products or products er on, say, the tubes of a solar that is reflective to radiation in used in solar applications, we water heater, we want low solar one part of the electromagnetic are often particularly concerned reflectivity as well as low far-in- spectrum may not be reflective with a surface’s “total solar frared emissivity. to a different part of the spec- reflectivity” and its “far-infra- trum. In fact, different behavior red emissivity” because both ––––––– in different parts of the spec- these properties affect radiation 1 Technically, this is true only at steady trum can be very important in heat transfer of a sun-exposed state but is a useful approximation for the effectiveness of a reflective surface. Total solar reflectivity is non-steady state. It is also true only product. the surface’s reflectivity just to when speaking of total absorptivity (i.e., absorptivity over all wavelengths) and those wavelengths that com- total emissivity. This relationship is derived A useful relationship to under- pose solar radiation; i.e., it is the from an energy balance on the surface. stand is that the absorptivity fraction of solar radiation2 that If a body does not emit all the energy and emissivity of a surface are is reflected by the surface and it absorbs, then its temperature will not approximately equal1 (see Figure not absorbed by it. Far-infrared be constant and hence it will not be at steady state. 1). This means a surface that emissivity, usually referred to in readily absorbs radiation just as product data as simply infrared 2 Solar radiation consists primarily of readily reemits the energy it emissivity, is the surface’s ability 46% visible, 49% near infrared, and 5% absorbs back to the environ- to emit radiation in the far-infra- ultraviolet radiation. Other components, ment. In other words, a dark red wavelengths, which we feel such as far-infrared radiation, make up a surface absorbs heat easily, but as heat3. tiny fraction. also easily reemits energy (gets 3 Total solar reflectivity and far-infrared rid of its heat and cools down) Importantly, a surface absorbing emissivity are not simply related and so when the surface is hot and the solar radiation (of which only must each be experimentally determined surroundings are cooler. about half is infrared radia- independently of each other. Summed tion) will reemit that radiation over the entire spectrum, reflectivity and Another useful relationship at a longer wavelength (100% emissivity must equal one. However, total solar reflectivity and infrared emissivity do when thinking about these infrared radiation). For this rea- not generally equal one since they pertain properties is that for any par- son, to keep roofs cool in sunny to radiation of different wavelengths; that ticular wavelength of radiation, weather when the surface of the is, they each pertain to different frac- the sum of reflectivity and roof gets hotter than its environ- tions of the spectrum of electro-magnetic absorptivity must equal one for ment, it is desirable for the roof radiation. Reflective paint additives, for an opaque object (see Figure 2). example, may increase the total solar re- surface to have both high solar flectivity of the paint, but may or may not That is, a surface that readily reflectivity and high infrared decrease the infrared emissivity, depend- reflects a particular wavelength emissivity. This way, the portion ing on the paint product. of radiation does not absorb of solar energy that is absorbed much of that radiation, and vice by the surface is then readily versa. For example, a surface reemitted back to the environ-

March 2008 – Page