Triple Glazing with Thin Non-Structural Center Glass

Triple Glazing with Thin Non-Structural Center Glass



The U.S. window market today represents a remarkable transformation to products that use about 50% of the energy that was typical of early 1980s products, but windows still account for ~4 Q of energy use at an annual cost of $40B. The technology to convert windows to net energy neutral products exists in prototypes and niche market products but is not available in volume at competitive prices.

Residential window sales historical market share by year.
Low-e shows a rapid rise in market penetration, while triple-pane market
share is stagnant at around 1.4 percent.

Production and product integration issues have discouraged the use of traditional 3-pane insulating glass units (IGU) in residential windows. Thus, new highly insulating glazings that do not require changes in current frame designs, can be mass produced at affordable cost, and enable the thermal performance benefits of 3-pane glazings are desired.

We outline here a DOE-industry partnership with a goal to capture the ~2Q of heating energy lost by windows. The partnership involves a technology push component based on refinement and market introduction of a novel, highly insulating “thin triple” glass product that can be incorporated into almost any existing window frame and can be fabricated at modest added cost.

A companion market pull program engages a wide range of traditional market transformation actors and programs to build the demand that will encourage industry investment in the new designs. Two window manufacturers and four component suppliers are engaged, as are a number of utilities, energy efficiency organizations, and government entities with the goal of reaching mainstream markets in the next two years.


Thin Glass Illustration

This highly insulating glazing project focuses on the development of alternative center glazing layers for multiple low-e/gas-filled units. Our aim is to increase the number of technological options available to industry for center glazing layers. The two options in use today, suspended films and triple glazed glass units with low-emissivity coatings and/or insulating gas fills, are technologies that have been in use for over two decades. Using lightweight, thin, non-structural layers, the focus of this project, offers several potential manufacturing and performance advantages:

  • A single piece spacer is possible reducing labor and material costs, and potential for gas fill leakage.
  • No significant weight changes over industry standard double-pane units, thus minimal or no changes in standard operating hardware
  • Thinner center layer allows products to fit in existing cross-sections, reducing the need for revised framing systems.
  • Pressure equalization between the internal gaps is possible, reducing potential glass deflection.

The concept of the “drop-in replacement” IGU simplifies adoption of the new technology but at some sacrifice in performance. We explore in this project how use of existing frames will constrain overall performance. Center-of-glass performance much better than that obtained by thin-triple-pane IGU designs will not result in much whole window performance improvement without further enhancements to typical frames.

LBNL's prior efforts to understand the thermal performance potentials of various design options culminated with the writing of a report on this subject titled in "Highly Insulating Glazing Systems using Non-Structural Center Glazing Layers". We showed that non-structural center layers that do not create a hermetic seal at the edge have the potential to be as thermally efficient as standard designs while potentially eliminating some of the production and product integration issues that have discouraged the use of triples. Thus, flexibility exists in how a center layer can be inserted into an IG unit without compromising thermal performance. Several IR experiments on un-insulated and insulated edge cases pointed out the importance of edge insulation in minimizing edge deflection. Understanding the impacts of deflection in standard and non-standard three layer units is important both to highly insulating window development and ratings and labeling efforts.

Performance and cost

Modeled and tested IGUs incorporating non-structural thin glass center layers have been shown to perform on par thermally with traditional methods of 3-pane glazing construction. The majority of the market today is made up of 2P-lowe. These units have one low-E coating inside the IGU. The “2P-surf4” option adds a second low-E to the inner

Center of glass thermal performance based on IGU width and
gas fill. The performance ranges obtainable by double-pane low-e
(2P-lowe), double-pane low-e with roomside low-e (2P-surf4), and
thin-glass triple-pane low-e (3P) are shown. 3mm glass is used
everywhere but center-pane of triple where 0.7mm glass is used.

glazing on the room facing surface to provide modest additional insulating value. It is clear that manufacturers optimize the IGU width for double-pane with 95% Argon gas fill – the most commonly constructed IGU currently on the market. It is also clear from the data that a triple glazed unit with narrow gas spaces must switch from Argon to Krypton to provide the large, desired increase in R value, from R5 to almost R8.

“Cost effectiveness” and “paybacks” are often the first questions asked for new efficiency options. We have estimated the incremental manufacturing costs of various improvements to window thermal performance using several common IGU designs. Glass, coatings, gas fill, and the assembly (including spacers) costs were each itemized. Since 2012, when we first examined this concept, material costs have dropped dramatically. Glass cost has dropped from nearly $5/sf to less than $1/sf. Argon fill costs have remained nearly negligible and Krypton fill costs have fallen to around $0.50 per liter with waste of 3 percent. 

Estimated incremental manufacturer
material costs over double-pane low-e
glazing. Costs have dropped sharply in
the last few years.

There are several potential challenges that need to be addressed before thin glass can be brought to market, including a business case for the new technology, material cost, manufacturability, durability and dedicated spacer development. LBNL works with glass and window manufacturers by providing design assistance, product testing, and thermal modeling as part of the development effort.

The designs proposed in this project build on proven approaches to insulating glass manufacturing. By using technologies that are currently available from several vendors, this project reduces the business and market risk. Proven heat transfer reduction techniques are the foundation for the variations being researched. In principle our approach can be implemented without major investments in production facilities, by using existing materials that are combined and assembled in novel ways.​


Simple payback over a double clear window based on current
incremental cost estimates. MN: Minneapolis; DC: Washington DC;
SC: Charleston; TX: Houston; UT: Salt Lake City; CA: Los Angeles.
2P-lowe: double glazed, low-e; 2P-surf4: double low-e with extra
low-e on room facing surface; 3P-tg:  triple, thin glass; 3P-opt:
triple standard glazing, larger spacing


















Relevant Publications

Hart, R., Selkowitz, S. & Curcija, C. "Thermal performance and potential annual energy impact of retrofit thin-glass triple-pane glazing in US residential buildings." Build. Simul. (2019) 12: 79.

Selkowitz, S., R. Hart, and C. Curcija. "Breaking the 20 Year Logjam to Better Insulating Windows." Proceedings of the 2018 ACEEE Summer Study on Energy Efficiency in Buildings, August 2018, Pacific Grove, CA.

Van Den Bergh, SofieRobert HartBjørn Petter Jelle, and Arlid Gustavsen"Window Spacers and Edge Seals in Insulating Glass Units: A State-of-the-Art Review and Future Perspectives." Energy and Buildings 58 (2013) 263–280. LBNL-6122E.

Jelle, Bjørn PetterAndrew HyndArlid GustavsenDariush K ArastehHowdy Goudey, and Robert Hart"Fenestration of Today and Tomorrow: A State-of-the-Art Review and Future Research Opportunities." Solar Energy Materials and Solar Cells 96 (2011) 1-28. LBNL-5304E.

Gustavsen, ArlidSteinar GrynningDariush K ArastehBjørn Petter Jelle, and Howdy Goudey"Key Elements of and Materials Performance Targets for Highly Insulating Window Frames." Energy and Buildings 43.10 (2011) 2583-2594. LBNL-5099E.

Gustavsen, ArlidGoce TalevDariush K ArastehHowdy GoudeyChristian KohlerSivert Uvsløkk, and Bjørn Petter Jelle"Experimental and Numerical Examination of the Thermal Transmittance of High Performance Window Frames." Thermal Performance of the Exterior Envelopes of Whole Buildings XI International Conference, December 5-9, 2010. Clearwater Beach, FL, 2010. LBNL-3886E.

Arasteh, Dariush KHowdy Goudey, and Christian Kohler"Highly Insulating Glazing Systems using Non-Structural Center Glazing Layers." 2008 Annual ASHRAE Meeting. Salt Lake City, UT, 2008. LBNL-611E.

Arasteh, Dariush KDragan C CurcijaYu Joe HuangCharlie Huizenga, and Christian Kohler"Evaluating Fenestration Products for Zero-Energy Buildings: Issues for Discussion." SimBuild 2006: Building Sustainability and Performance Through Simulation. Cambridge, MA, 2006. LBNL-61249.

Arasteh, Dariush KHowdy GoudeyYu Joe HuangChristian Kohler, and Robin Mitchell"Performance Criteria for Residential Zero Energy Windows." 2007 ASHRAE Winter Meeting. Dallas, TX, 2006. LBNL-59190.

Apte, Joshua S, and Dariush K ArastehWindow-Related Energy Consumption in the US Residential and Commercial Building Stock. 2006. LBNL-60146.

Arasteh, Dariush KStephen E Selkowitz, and John R Wolfe"The Design and Testing of a Highly Insulating Glazing System for Use with Conventional Window Systems." Journal of Solar Energy Engineering 111.1 (1989) 44-53. LBL-24903 Revised.

Selkowitz, Stephen E. et al., United States Statutory Invention Registration No. H975, entitled "Thermal Insulated Glazing Unit", published Nov. 5, 1991

Principal Investigator(s)