The North Carolina Building Code Council will go before an administrative judge this week with an emergency rule the Council adopted regarding requirements for low-E windows.

The state’s building code requires low-E windows to be used for their energy-efficiency benefits, and, according to the American Architectural Manufacturers Association’s (AAMA) recently released U.S. Industry Market Studies, 81 percent of conventional residential windows in the U.S. utilize low-E glass.

However, there is some concern regarding how the window’s high reflectance could concentrate sunlight and contribute to the melting of the vinyl siding of houses and cars, as reported by multiple local newspapers across the country and documented in an investigation by local North Carolina television station WRAL.

WRAL’s most recent story caught the attention of the Council and prompted it, as chairman of the Council Dan Tingen said in an interview with WRAL last week, to adopt the emergency rule. What the Council particularly took notice of were potential fire hazards.

“When things get really, really dry, and you’ve got a temperature high enough to bubble plastic on that young lady’s car, that’s a pretty intense heat, and it wouldn’t take much more to get pine straw or some other sort of mulch around the house to catch fire,” Tingen told WRAL.

The emergency rule the council adopted would allow builders to use non-low-E glass in situations where they could foresee the reflections posing a problem, according to Barry Gupton, chief code consultant of the North Carolina Department of Insurance.

Gupton says a normal rule process would take approximately nine months to go through, but emergency rules move much more quickly. He says he expects the emergency rule regarding the low-E code requirement to go into effect in mid-July, though that’s just an estimation.

Industry experts acknowledge the potential of damaging sun reflectivity with low-E glass but argue that other factors beyond the glass itself actually dictate those effects.

“High‐performance glass used in residential windows and doors is designed to reflect the majority of the sun’s radiant heat,” says AAMA president and CEO Rich Walker. “Energy‐efficient insulating glass that utilizes low-E technology, prescribed by today’s energy codes, achieves its high performance by reflecting all but the visible portion of the solar spectrum. Certain building designs and/or site characteristics can result in situations such that the sun’s reflected heat energy is concentrated. It’s important to note that these same high‐performance products bring proven energy savings and substantial environmental benefits to a home or building.

“However, there are options for potentially mitigating these effects. The best time to address reflected sunlight and its effects is during site design and material selection. If glass-related, reflected sunlight is encountered in the field, the best way to mitigate the situation is to block the sunlight from landing on the glass by using screens, sun shades, canopies, shrubbery or trees.”

That’s where the discretion of the builder would come into play in North Carolina’s emergency rule. Low-E glass is preferred and recommended because of its obvious energy-saving benefits, but the builder would still have “the option of using non-low-E glass in locations that he can anticipate having a problem,” says Tingen.


  1. The real problem is these low-e glasses act like ‘lenses’ which magnify the heat onto a point causing vinyl to melt or other potential safety hazards. How are these lenses created? When glass is tempered, it is heated up in a furnace on a roller conveyor that has historically used radiant heat. The same kind of radiant heat that the low-e glass is designed to reflect. Many fabricators who temper these low-e glasses have outdated tempering furnaces that are transfer heat primarily by radiation. this results in either overheating the glass, and/or differentially heating the glass from on the top versus the bottom. Which then results in roller wave. Roller wave is what creates the ‘lens effect’ which are described in the article. Tempering furnaces that primarily use convection for heat transfer cost more money, but solve these issues. Window makers who utilize premium convection tempering machinery operated with quality as a priority over productivity and cost either do not have these issues or have minimized them. The industry trade association(s) need to implement guidelines which later become ANSI standards or building codes which prevent the ‘lens effect’ from occurring.

  2. As a window and door supplier for more than 40 years this condition occurred prior to LOE requirements.

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