Greenhouse floor slab insulation retrofit (C) Daniel Friedman Steven BlissRetrofit Low-E Window Films

  • LOW-E RETROFIT ADD-ON FILMS - CONTENT: Compare Energy Efficiency of Low-E Glass vs Quad Glazing: Add-on films for low-e energy-efficient window glazing, Low-e window films for upgrading existing windows, Winter films and all-weather films retain indoor heat in cold weather, Low-e coated films for inside storm windows, Questions & answers about using add-on or retrofit films to improve window energy efficiency. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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Adding low-Emissivity film coatings to windows: this article describes low-e window films that can be added to existing windows to improve window efficiency and improve solar gain in cold weather.

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Comparing Energy Efficiency of Low-E Glass vs Quad Glazing

The text below is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Low-e Window Retrofit Film Products

Question: are there any low-e coated products for window retrofits?

Are there any low-e coated products available for retrofitting existing windows? - Ed Swiderski, Pawtuckett RI

Answer: Yes several low-e window films and an inside storm window

High passive solar gain room (C) Daniel Friedman

Two low-e window films and one inside storm window were available in the mid 1980's and more such products are currently provided by several manufacturers.

The add-on or "retrofit" low-e window films are applied to the inside of the window glass with a squeegee, using water as the adhesive. These "winter films" or "all-weather" films are tuned to retain winter heat and reject some outside heat gain.

They probably make the most sense as window energy improving retrofits in houses located north of the Mason-Dixon line.

Scotch-tint Plus All Season claims to reduce heat gain by 60 percent and wintertime heat loss through the window by 40 percent. it also blocks 98 percent of UV light that fades indoor furnishings. Scotch-tint is available from 3-M Co., St. Paul MN.

The second low-e retrofit window film is Gila River Products' [Chandler AZ] PWD-5 with roughly similar characteristics to the 3-M Scotch-tint film.

With a winter U-value of .85, this film ups the R-value of a single pane 1/8-inch thick window [glass] from around .90 to 1.18, a 31 percent improvement in window efficiency.

What is lost when a low-e film like these is applied to a window is some useful solar gain and light in winter, due to the film's low transmittance (0.55).

To our knowledge, the only low-e retrofit storm window is the Windo-Tite unit. It is an aluminum-framed single window that attaches to the inside of the primary window via swivel clips or sash locks. It has a tested U-value of .408 (r-2.45 for storm plus primary window). Amesbury Industries, [Amesbury MA] supplies the glazing frames and accessories to local distributors who make up the window units.

For more up to date information about the performance of films to increase solar collector efficiency, see SOLAR COLLECTOR FILMS

The question-and-answer article above paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

This article is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Readers should also see SUNGAIN, FILMS, LOW-E GLASS for details about use of Sungain film on windows and window film selection and installation.

A wide range of energy conservation and solar energy topics is found at SOLAR ENERGY SYSTEMS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Definition of Window Coating or Film Emissivity & Typical Emissivity Numbers for Low-E Films & Coatings

Window glazing may be factory-coated or as this article describes, an add-on adhesive low-e (low emissivity) film may be adhered to the inside or outside window surface (or both).

The emissivity of a window is a measure of its insulating performance expressed as the percentage of near-to-the-window infra-red heat in a room that is reflected back into the room (reducing heat loss from the building to outdoors), or from outside, the percentage of near infra-red-heat that is reflected back away from the window surface (reducing heat gain through the window and into the building).

In short, lower emissivity numbers mean more IR heat reflection (less heat is emitted through the window glazing) back into the occupied space. There are currently two low-e film performance ranges, depending on what product you buy.

Conventional low-e window film has a typical emissivity rating of 0.33, reflecting 0.67 or 67% of the near infra-red heat in a room back into the room interior. This film will improve the energy efficiency of a conventionally-glazed window by about 44% with the warnign that some low-e window films may produce a distracting irridescent or shiny surface dependign on the type of interior lighting used. Compact fluorescent bulbs tend to produce this irridescent sheen.

High performance low-e window film has extremely-low emissivity ratings, down as low as 0.07, reflecting 0.93 or 93% of the nearby infra-red heat back into the building interior. Expert sources we reviewed claimed a window insulating performance improvement of "up to" 92%.

Watch out: the energy savings potential of adding low-emissivity window films or buying low-e replacement windows or installing low-e windows in new construction will be limited if other building heat loss (or in hot climates un-wanted heat-gain) measures are not taken. For example, if there are signficant air leaks in the building or if the building wall and ceiling or roof insulation blankets are incomplete, the energy costs of those defects are likely to overwhelm any potential benefit of low-emissivity films or coatings on windows.

Sources of Low-E Add-On or Stick-On Window Films & Window Film Detection Instruments

As we discuss in more detail at SUNGAIN, FILMS, LOW-E GLASS, here is a quick comparison of window types and their efficiencies:

Storm Window Articles


Continue reading at WINDOW EFFICIENCY FEATURES & RATINGS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see ENERGY SAVINGS in BUILDINGS articles on ways to cut heating and cooling costs in buildings.


Or see WINDOWS & DOORS - home


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