{"id":5622,"date":"2016-12-02T08:00:35","date_gmt":"2016-12-02T14:00:35","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=5622"},"modified":"2016-12-09T14:27:44","modified_gmt":"2016-12-09T20:27:44","slug":"pc-architectural-coatings-that-reduce-heating-and-cooling","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/5622\/pc-architectural-coatings-that-reduce-heating-and-cooling\/","title":{"rendered":"Architectural Coatings that Reduce Heating and Cooling Costs"},"content":{"rendered":"

According to the U.S. Energy Information Service, 40 percent of all US energy consumption is used for heating and cooling residential and commercial buildings. For homeowners, 25 percent of their average energy bill is for cooling. Considering these facts, consumers appreciate any efficiencies coatings formulators can offer.<\/p>\n

Heat transfer mechanisms<\/strong><\/p>\n

Prior to considering how coatings can be engineered to save heating and cooling costs, it is instructive to examine heat transfer<\/em><\/strong> mechanisms: radiation, conduction, and convection<\/em><\/strong>.<\/p>\n

Radiation<\/em><\/p>\n

As figure 1 indicates, radiation<\/em><\/strong> is the emission and propagation of light energy in the form of rays or waves through space:<\/p>\n

\"architectural-fig1\"
Figure 1 \u2013 Radiation light spectrum1<\/sup><\/figcaption><\/figure>\n

As figure 2 illustrates, pigments can absorb or reflect solar infrared energy<\/em><\/strong> based on their color.\u00a0 For example, if the pigment absorbs<\/em> infrared (IR) energy (such as conventional darker pigments), we see heat build-up of the coated substrate. If the pigment reflects<\/em> IR light (such as white and lighter colors), we see a lower increase in temperature.<\/p>\n

To illustrate, the surface of a steel building at an ambient air temperature of 20\u00b0 C will remain at about 20\u00b0 C when painted white, whereas the surface will be about 35\u00b0 C when painted black.<\/p>\n

\"architectural-fig2\"
Fig. 2 \u2013 Surface Temperature of Paints using Conventional Pigments in Sunlight<\/figcaption><\/figure>\n

In summer, heat gain occurs through radiation<\/strong>, and in winter, heat loss occurs by conduction<\/em><\/strong> (solid-to-solid contact). Heat transfer by air movement is called convection <\/em><\/strong>(heat transfer via liquids or gases).<\/p>\n

Emissivity<\/em><\/strong> is the ratio of radiation emitted by a surface to the radiation emitted by a black body at the same temperature. A material\u2019s surface emissivity is a measure of the energy emitted when a surface is directly viewed. Generally, emissivity is measured indirectly by assuming 1- reflectivity, hence a highly reflective material has low emissivity.<\/p>\n

\"architectural-fig3\"
Figure 3. \u2013 Low emissive window glass reflects IR radiation to reduce cooling cost<\/figcaption><\/figure>\n

Figures 3 and 4 illustrate how a low emissivity layer in window glass can help save energy as a result of air conditioning. In the illustration, visible light is transmitted through window glass, but much of the IR heat in the form of solar radiation is reflected, due to the glass surface\u2019s low emissivity layer.<\/p>\n

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Figure 4 \u2013 Energy savings through the use of IR reflective exterior coatings and low emissivity interior coatings2<\/sup><\/figcaption><\/figure>\n

Emissivity is a number between 0 to 1. For example, if a surface has emissivity of 0.1, it means this surface is reflecting<\/em> 90 percent of the energy and conversely, the object will not absorb<\/em> 90 percent of the energy in the form of heat. A mirrored surface may reflect 98 percent of the energy, while absorbing 2 percent of the energy.<\/p>\n

On the other hand, a black body surface will absorb 98 percent of the energy and reflect only 2 percent, and thus absorb the energy in the form of heat. Among the variables that affect emissivity are geometrical, black body, the uniformity of the black body, temperature, wavelength, and emission angle.<\/p>\n

Heat conduction and convection<\/em><\/p>\n

\"architectural-fig5\"
Fig. 5 \u2013 Transfer of heat through conduction3<\/sup><\/figcaption><\/figure>\n

Conduction <\/em><\/strong>is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred. An example of conduction heat transfer is the melting of ice cubes in your hand as heat transfers from your skin to the ice or the flow of heat from a hot object to a cold object in direct contact (figure 5).<\/p>\n

As illustrated in figure 6, convection<\/strong><\/em> is the transfer of heat through gases or liquids from a warmer spot to a cooler spot.\u00a0 Radiation heat transfer at temperatures representative of building components, occurs in the long wave IR portion of radiation spectrum. Due to this energy distribution, radiation heat transfer is directly proportional to the surface emissivity in the long wave IR spectrum.\u00a0 A highly conductive material has a surface with low emissivity and thus reflects IR radiation well.<\/p>\n

\"architectural-fig6\"
Figure 6 \u2013 Transfer of heat through convection4<\/sup><\/figcaption><\/figure>\n

For example, interior building heat loss is minimized by surfaces that provide low emissivity and a highly conductive surface, as the heat flow from an interior warm wall flows to the exterior colder surface through conduction. <\/em><\/strong><\/p>\n

To reduce heat transfer via conduction,<\/em><\/strong> a coating must have a high insulating quality. The insulating capability of a material is measured with thermal conductivity (k)<\/a>. Low thermal conductivity is equivalent to high insulating capability. Other important properties of insulating materials are product density (\u03c1) and specific heat capacity (c).<\/p>\n

In conclusion, coatings can reduce heat transfer caused by convection, conduction <\/strong><\/em>or radiation.<\/em><\/strong> The use of IR reflective pigments in an exterior paint system can prevent radiant heat transfer<\/em><\/strong>, providing a cooler exterior surface and less conduction<\/em><\/strong> heat transfer in the summer. Interior coatings that provide low emissivity can also reduce heat transfer from radiant heat loss in the winter months, as illustrated in figure 4, above.<\/p>\n\n\n\n\n\n
Issue<\/strong><\/td>\nMechanism\u00a0<\/strong><\/td>\n\u00a0Energy Saving Approach<\/strong><\/td>\n<\/tr>\n
Solar heating\u00a0 in summer <\/strong><\/p>\n

(high air conditioning cost)<\/strong><\/td>\n

\n
    \n
  • Radiation from IR light spectrum<\/li>\n
  • Conduction of heat from exterior surface to cooler interior by solid to solid contact<\/li>\n<\/ul>\n<\/td>\n
\n
    \n
  • Use light colors<\/li>\n
  • Use solar reflective pigments<\/li>\n<\/ul>\n<\/td>\n<\/tr>\n
Interior heat loss in winter<\/strong><\/p>\n

(high heating cost)<\/strong><\/p>\n

\u00a0<\/strong><\/td>\n

\n
    \n
  • Radiation from long wave interior IR light<\/li>\n
  • Conduction of heat from warm interior wall surface to exterior cold surface<\/li>\n<\/ul>\n<\/td>\n
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