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This article describes different types of energy-efficient window glazing products: low-transmission films, low-e glass, coated glass, reflective fiolms, high transmission glass, low emissivity films etc. Window glazing add-on films: this article also discusses sources for window films and coating to control heat gain, heat loss, heat transmission - high-transmission 3M Sungain film. We list and discuss window glazing energy products and answer: what are the differences in function & use among low-transmission films, low-E glass, coated reflective films & high transmission, low emissivity films or reduced-iron-content glass?
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Also see LOW-E VS QUAD-GLAZING for a comparison of the benefits of Sungain films vs. multiple-glazed windows. For retrofit or "DIY" Low-E window films see LOW-E RETROFIT ADD-ON FILMS. For more up to date information about the performance of films to increase solar collector efficiency, see SOLAR COLLECTOR FILMS.
Sungain Window FIlmSources
I've had little luck [back in 1984] locating quad glazing with Sungain™ film (Solar Age, 2/83, 9/83) locally. Do you have any information on where I can buy quad-glazed glass window units? -- Rick Essman, Carson City NV
High-transmission Sungain film is made by the 3M Company. According to M.J. Johnson of 3M's Energy Control Products, quad-pane and tri-pane units and windows with Sungain film were [by the mid 1980's in the U.S.] available from several manufacturers, and 3M continued to add more of them across North America by 1985.
Sungain is a thin plastic film, discussed in Popular Science Magazine in 1982 and produced by 3M Corporation. Sungain plastic film admits more heat than glass through the same area.
Quad-glazed units (glass only):
Complete windows with quad-pane Sungain glazing
Check with the glazing and window manufacturers for prices and availability.
Window Glazing Energy Products: What are the Differences in Function & Use Among Low-Transmission Films, Low-E glass, Coated Reflective Films & High Transmission, Low Emissivity Films or Reduced-Iron-Content Glass?
Definition & Uses of of Low Transmission Window Films and Low-E Glass
Low-transmission films such as Scotchtint™ were developed to block solar transmission through window glass into the building interior, avoiding un-wanted heat gain in some buildings. Low-transmission, heat reflecting films would not help solar collector efficiency.
Low-E glass, low-emissivity glass, low infrared reflectance glass (these all mean the same thing) produced by Airco Temescal, Berkeley CA, and Guardian Industries, Carelton Michigan, uses a transparent coating that reflects heat. Infrared energy is long-wave energy (longer than visible light), and is in essence, radiant heat. If we reflect radiant heat from a window surface we are reducing the heat that passes through the window.
Practically speaking, in winter indoor heat may be reflected back into the interior, reducing heat loss through low-e glazed windows by radiation loss by 1/3 to 1/2. Of course air bypass leaks in a building can easily overcome the energy savings from special window glazing. See AIR LEAK MINIMIZATION and AIR SEAL STRATEGIES.
In summer, heat from bright sunlight may be reflected back outdoors, reducing the heat transmitted to the building interior. Thus low-e glass or Heat Mirror film coated glass may reduce heating costs in the heating season and cooling costs (by letting in less solar heat) during the cooling season.
Low-e glass, compared with un-treated glass, has a slight bluish color.
Coated plastic reflective films, such as Heat Mirror, produced by Southwall Corp., Palo Alto, CA, also reflect heat as does the low-e glass mentioned above.
Low transmission glazing films such as Heat Mirror (a 2-mil polyester reflective film) and Low-E glass have been installed on airplane windshields to increase pilot protection, comfort, and visibility since World War II. -- op cit. A special advantage on airplanes was the fact that the glass is electrically conductive: sending an electrical current through the glass could be used to remove fogging, frost, or ice.
The Southwall Corporation was formed by the original developers of coated plastic reflective films for these applications, including John Brooks, Sean Wellesley-miller, and physicist Day Chahroudi who had observed low-E and coated windshield technology, working at MIT, adapted the process to produce a transparent insulation leading to a whole industry of special glazing intended for use in solar and energy conservation applications.
Heat mirror film coated glass, compared with un-treated glass, looks about the same.
1. Popular Science Magazine in 1982
2. U-value is the measurement of heat transmission through a material - it's the reciprocal of R-value. R-value is a material's resistance to heat transfer.
Definition & Uses of High Transmission, Low Emissivity Films
The high-transmission, low-emissivity films such as Heat Mirror™ or 3-M's Sungain™ might boost solar collector efficiencies in some applications, particularly high-temperature collectors in cold climates. In this case, the added insulation value of the film might offset the transmission losses. The actual efficiency gains for the solar collector may still not justify the added expense.
As these products were developed for building glazings (windows), their durability in solar collector applications is in question. The effects of high temperatures, thermal cycling, and high UV exposure on the window glazing film had (in the 1980's) only been studied in a preliminary way. 3M informed us that its Sungain film will become brittle at temperatures above 250 degF., prohibiting its use in some solar collector applications.
Reduced-iron-content glass, such as Solakleer, produced by General Glass, International, New Rochelle, NY, also admits more heat than ordinary glass.
For more up to date information about the performance of films to increase solar collector efficiency, see SOLAR COLLECTOR FILMS
The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.
Frequently Asked Questions (FAQs)
Questions & answers or comments about where to buy & how to use high-transmission Sungain type window films.
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Technical Reviewers & References
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