Diagram of heat reflectance and light absorption through the thermochromic film.

Thermochromic Windows

Thermochromic windows are the most advanced, yet simplest, dynamic window technology available and are quickly growing in popularity and being installed in many commercial, retail and residential buildings throughout the world. The thermochromic glass simply uses heat from direct sunlight to tint the windows when necessary. The more direct and intense the sunlight is on the glass the darker it will become. This allows the windows to drastically reduce the heat load coming into the building and because the glass transmission adapts continuously over a range of temperatures, a natural balance and maximum use of daylighting is achieved. By design, thermochromic windows help reduce glare, fading and noise, and increase safety.

Diagram of a typical vertical insulated glass unit for a Suntuitive® thermochromic window. Image courtesy of Pleotint, LLC.

Thermochromic windows consist on a common safety interlayer, made from extruding special thermochromic materials into polyvinyl butyral (PVB), laminated between two pieces of heat strengthened or tempered glass and placed into an insulated glass unit (IGU) with a low emissivity coating. A warm edge spacer is also recommended. Due to the laminated design, a thermochromic glass laminate can be used as a building block to meet most building codes and design requirements. The interlayer can be laminated to nearly any tint or thickness of glass and used with high-performance Low-e coatings and specialty glass. Operable or fixed windows, doors and skylights can all be made using thermochromic glass.

Senior Living Facility in Wyoming, Michigan uses Suntuitive® Glass to reduce heat gain and provide natural daylighting. Photo courtesy of Pleotint, LLC.

As part of efficient building design, thermochromic windows can help manage a building's changing needs for passive solar heat gain and natural daylight. All together this can lower costs associated with heating, air conditioning and artificial lighting. Thermochromic windows do not require wires, power supplies or control equipment and can be installed by glazing contractors, just like conventional windows. These features, along with lower cost, make thermochromic windows very attractive as compared to electrochromic and other electro-optic approaches to variable tinting windows.

A clear insulated glass unit incorporating a thermochromic laminate and Low-e coating can have a visible light transmission between 54% and 8%, a solar heat gain coefficient from 0.36 and 0.16 and a U-value of 0.24. Performance can be chosen by selecting various glass tints, Low-e coatings and air spaces. The windows simply tint to a desirable level based on sun exposure every day of the year, any time of the day, and for any orientation on the building.

Thermochromic windows, or Suntuitive® Glass, have been commercially available since 2010 from Pleotint, LLC. As of mid-2015, units as large as 64" x 144" have been produced with the thermochromic interlayer and installed throughout the world. Data for Suntuitive interlayer is available in the International Glazing Database (IGDB) and ratings are available through the National Fenestration Rating Council (NFRC).

Suntuitive® Glass windows installed at an educational facility in Keller, Texas, USA. Photo courtesy of Pleotint, LLC.

A Quick Note on Thermotropic Windows
Windows are known that change between clear and light scattering with changing temperature. This change is based on lower critical solution temperature (LCST) phase change materials that in the past were inadvertently called thermochromic. These materials are better described as thermotropic since there is a phase change or change in state of the materials and there is no change in "chroma" in these systems. In addition, there are systems based on liquid crystals between crossed polarizers that are sometimes called thermochromic but these liquid crystal systems are also better described as thermotropic.


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