Case Study: Low-Emissivity Windows

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By Jeffrey Rissman and Hallie Kennan

Improving the energy efficiency of buildings has great potential to boost the U.S. economy, improve public health, and protect the environment. The energy used by buildings costs $418 billion annually and accounts for 39% of all U.S. carbon dioxide emissions. A major driver of building energy consumption is heat and cooling loss through the walls, roof, and windows. Low‑emissivity (“low‑e”) windows use a transparent coating that blocks infrared radiation, keeping heat outside the building on hot days and keeping it inside the building on cold days. Relative to an ordinary, single-pane window, the best low‑e windows can reduce heat loss by 85%.

Since the 1970s, the government has used four primary mechanisms to drive low‑e window technology development and commercialization: basic research and seed investments; computer tools for simulating window performance; standardized testing procedures and performance ratings; and educational outreach to manufacturers and consumers.

Basic Research and Seed Investments: From 1976 –1983, the government spent $2 million ($5.5 million in current dollars) to start a window research program at Lawrence Berkeley National Laboratory (LBNL). LBNL issued contracts to private firms to develop low‑e technology and made its own facilities available for prototype testing. DOE provided $700,000 of seed funding to the start-up company Suntek Research Associates (later renamed Southwall Technologies), and LBNL worked in partnership with the company during its earliest stages. In 1981, Southwall released the first mass-market low‑e window product, and their commercial success changed major window and glass manufacturers’ opinions on low‑e window marketability.

Computer Tools for Simulating Window Performance: In addition to direct research support and physical prototype testing, Lawrence Berkeley National Lab created WINDOW, a computer model for simulating window performance. Virtual design and optimization of a product with simulation tools is much faster and cheaper than the traditional method of iterative prototype construction and laboratory testing. The software was released for free and quickly became an important tool used by private companies to efficiently test window designs. Today, over 80% of all residential window designs are modeled using LBNL’s simulation tools.

Standardized Testing Procedures and Performance Ratings: In the 1980s, there was no standardized system for evaluating and rating window performance. As a result, manufacturers found it difficult to convince consumers of the windows’ benefits. To address this issue, window and glass manufacturers formed the National Fenestration Rating Council (NFRC) in 1989. The NFRC developed testing procedures, ratings, and a labeling system for energy efficient windows, which were officially sanctioned by the U.S. government. NFRC’s system was a necessary precursor to voluntary standards, such as ENERGY STAR, and mandatory standards, such as state and local building codes, that helped drive adoption of low‑e windows in the 1990s and 2000s.

Educational Outreach to Manufacturers and Consumers: The U.S. government took steps to inform manufacturers and the public about the benefits of low‑e window technology. LBNL staff informed key decision-makers by presenting their research results at industry trade shows and in private meetings with code officials, utilities, and research and marketing staffs from major window manufacturers. In 1998, the federal government added windows to ENERGY STAR, a voluntary labeling program that helps customers identify energy-efficient products. The ENERGY STAR program has been tremendously successful at driving adoption of energy efficient windows whose market share climbed from 35% to 81% of residential window sales. However, both industry and EPA have observed that the growth in market share is partially due to the slow rise in standards. In order to promote continued innovation, EPA is currently planning to reduce market share from 81% to 41% with windows’ next specification revision.

Low-e windows are now an integral part of the U.S. building sector. DOE-sponsored research investments from 1976 –1983 totaling just $2 million ($5.5 million in current dollars, about $0.7 million/year), along with annual investments of similar magnitude during the rest of the 1980s and 1990s, resulted in a net savings of more than $8 billion by 2000 ($10.7 billion in current dollars).

The history of low‑e windows demonstrates the importance of the government playing an active role in developing new energy technologies. The government has the laboratories, staff, and financial resources to investigate many technologies simultaneously, and government can take a long-term view, making high-risk, high-reward bets that would not pay off in time to satisfy investors in private companies. Moreover, the government’s role in creating performance and certification standards is critical. Objective performance metrics give consumers the information they need to choose between products and enable states and localities to integrate the technology into their building codes. Voluntary standards and labeling programs help consumers distinguish between more and less efficient products and should be tightened as the technology develops to incentivize continued innovation. By taking an active role in energy efficiency R&D, from project inception all the way through ongoing standard-setting and labeling, the government can work with the private sector to achieve economic benefits, further U.S. technological leadership, improve public health, and protect the environment.


The views expressed in this case are those of the AEIC staff and do not necessarily reflect the views of the AEIC principals.