Understanding Low-E Glass

by Jonathan McGaha | September 4, 2013 12:00 am

Glass has been a popular building material for centuries because it looks good and creates a strong visual and psychological connection between the indoors and out. With the advent of low-E coatings in the past several decades, glass also has become a vital weapon in the effort to cut energy use in commercial buildings; first by blocking solar heat in warm weather and trapping furnace heat in cold weather; and second, by minimizing reliance on artificial lighting and creating brighter, more comfortable work and living spaces.

 

To understand how low-E glass works, it helps to have a basic understanding of where heat and light come from on the solar energy spectrum. UV light, which contributes to the fading of interior materials such as fabrics and wall coverings, transmits energy at wavelengths of 300 to 380 nanometers. Visible light measures from about 380 to 780 nanometers, and IR light, the source of heat energy, occupies wavelengths from about 780 nanometers and up.

 

Today’s low-E glasses are coated with microscopically thin layers of metal that enable them to transmit energy from the visible light portion of the solar spectrum and to reflect or transmit infrared energy, depending on the demands of the architect and building owner.

 

Two Types of Low-E Glass

There are two major types of low-E glass: passive low-E glass and solar control, low-E glass. Each offers different performance characteristics that are determined in part by how their coatings are applied.

 

Passive Low-E Glasses

Although passive, low-E glasses are used in many of today’s most advanced green buildings, they are based on a coating technology developed in the 1980s in response to the foreign oil crises of the 1970s. These products were engineered primarily to prevent warm interior air from escaping through the glass, which enabled buildings to use less energy for heating.

 

Many passive low-glasses are made with an ultra-thin layer of tin that is baked on (pyrolytically applied) to the glass surface while it is on the float line. Others are made with a thin layer of silver that is applied to the glass in an off-line process.

 

Because passive, low-E coatings permit more heat energy from the near-IR portion of the solar spectrum to infiltrate buildings, they have higher SHGCs than solar control, low-E glasses. These two performance characteristics-the ability to trap furnace heat and transmit solar heat-often make passive low-E glasses the best choice for building envelopes in heating-dominated climates.

 

Solar Control, Low-E Glasses

Solar control, low-E coatings, which debuted in the early 1990s, are different from passive low-E coatings in several important ways.

 

First, unlike passive low-E glasses, solar control low-E glasses are formulated mostly to block heat (not trap it), thereby reducing the energy costs associated with air-conditioning. Their coatings deflect energy from the near-IR portion of the solar spectrum, which means they generally have lower SHGCs than passive low-E glasses.

 

Solar control, low-E coatings also are typically applied to the glass substrate after it has cooled. This secondary manufacturing process produces a coating that is more fragile than those on most passive low-E coatings, but also allows for more efficient and effective deposition of metal on the glass substrate.

 

In the past several years, advances in the deposition of metal to glass substrates has enabled manufacturers to add as many as three layers of silver to the coating stack, which has further heightened the ability of solar control low-e glasses to block heat without negatively impacting their ability to transmit light.

 

Energy and Environmental Performance

It goes without saying that glass specification can have a significant impact on a building’s energy performance, but what is the real impact in dollars and cents? The answer to that question was quantified in a comprehensive energy modeling study that measured the relative energy costs associated with four types of commonly specified glass on a prototype glass-walled, eight-story office building in 10 major U.S. cities.

 

The chart below shows the performance data associated with each glass type, including dual-pane tinted (non-low-E) glass:

 

Glass Performance Data

Glazing Type	Visible Light Transmittance (VLT)	Solar Heat Gain Coefficient (SHGC)	Winter Night Time U-Value	Light to Solar Gain (LSG) Ratio
Dual-pane glass (tinted, non-low-E)	62%	0.62	0.57	1.00
Passive, low-E glass (tinted)	64%	0.45	0.35	1.42
Double-silver solar control, low-E glass (clear)	70%	0.38	0.29	1.84
Triple-silver solar control, low-E glass (clear)	64%	0.27	0.29	2.37

 

The Results

The following chart shows the energy and HVAC equipment savings, averaged from the 10 cities measured in the study, generated by triple-silver-coated solar control, low-E glass; double-silver-coated solar control, low-E glass; tinted passive, low-E glass and dual-pane tinted (non-low-E) glass in the prototype office building.

 

Prototype Glass-Walled Eight-Story Office Building

Glass Type	HVAC Operating Cost	Annual Savings	HVAC Equipment Costs	Immediate Equipment Savings	1st Year Savings	30-Year Life-Cycle Savings
Dual-Pane Glass (Tinted)	$549,188	N/A	$2,153,393	­­­N/A	N/A	N/A
Passive Low-E Glass (Tinted)	$527,463	$21,725	$2,057,207	$96,186	$117,911	$769,661
Double-silver solar control, low-e glass	$508,328	$40,860	$1,948,002	$205,391	$246,251	$1,431,191
Triple-silver solar control, low-e glass	$483,134	$66,054	$1,758,210	$395,183	$461,237	$2,442,857

Based on Composite Climate Data-10 U.S. Cities (Atlanta, Boston, Chicago, Denver, Houston, Los Angeles, Philadelphia, Phoenix, St. Louis and Seattle.)

Total Floor Area: 270,000 ft²; Total Glass Area: 45,027 ft²

 

Save On Energy. Save On Equipment.

Over the 30-year lifetime of the prototype office building, passive low-E glass produced annual energy savings of $21,725 compared to tinted, non-low-E glass, plus initial HVAC equipment cost savings of $117,911 for a 30-year lifecycle savings of nearly
$770,000. With triple-silver coated, solar control, low-E glass, the life cycle savings were even more dramatic, equaling almost
$2.5 million.

 

While the results of this study may not be valid for any individual project, they do provide a broad overview of the energy savings that can be realized when low-E glasses are specified for a common architectural application. As energy modeling demonstrates, architects who design building envelopes with advanced low-E glasses can reap the rewards of an investment that pays for itself many times over.

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Paul DiCesare is manager, architectural quality assurance, flat glass at PPG Industries Inc., Pittsburgh. To learn more about specifying low-E glasses, visit www.ppgideascapes.com[1] or call (888) PPG-IDEA (774-4332).

Source URL: https://www.metalarchitecture.com/articles/understanding-low-e-glass/

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