A spate of spontaneous fires near a home in North Carolina is reigniting debate about the role windows might play in such events.

In January, Nancy Monda’s security cameras captured flames erupting in her yard in Davie County in central North Carolina, according to a report from WRAL-TV in Raleigh. The fires broke out several more times. One almost engulfed a propane tank, which could have led to an explosion.

According to the report, a firefighter investigating one of the incidents noticed intense light and heat reflecting off a neighbor’s new low-e windows. They’re now suspected of being the source of the fires, which have also affected another neighbor.

This week, Monda took her case to the North Carolina Building Code Council.

“A known fire hazard lurking in neighborhoods throughout the state, frankly country for that matter, but it’s causing fires in heavily populated family neighborhoods,” Monda told the council, according to WRAL. “Yes, I find that to be a huge problem.”

Council Chair Dan Tingen said replacing the windows or adding screens would be an immediate fix, but he said a long-term fix is not as simple.

“Obviously there is going to be a lot of concern over (Monda’s) problem and the fact that other people have experienced a similar problem,” Tingen told WRAL. “In order to overcome the larger question of how to eliminate this completely, in new construction, it’s going to be a difficult task to get your arms around.”

The North Carolina Building Code Council has dealt with this problem before.

In 2014, the council adopted an emergency rule that would let builders use non-low-E glass after several complaints about warped siding and fires. However, after intense lobbying by the American Architectural Manufacturers Association (AAMA), the Window and Door Manufacturers Association (WDMA) and other industry groups, the North Carolina Rules Review Commission rejected the rule.

But in the fall of 2016, the North Carolina Building Code Council changed the energy conservation section of the state building code to allow the substitution of windows when they have been either shown to cause a problem relating to concentrated solar reflection or it can be determined that they could cause a problem. The permanent rule took effect in January 2016.

The change was made to ensure builders wouldn’t be liable for not complying with the energy conservation requirements, said Jeffrey A. Smith, communications director with the Vinyl Siding Institute.

Curvature Can Cause Fires

For years, many homeowners assumed that low-E glass was the main culprit in fires or melted siding. However, a study by the National Association of Home Builders (NAHB) found that under certain rare conditions, it’s possible for windows to develop a curvature that can focus enough heat to melt or warp vinyl siding or even cause small fires.

“Glass in double-paned windows may on occasion slightly warp or deflect due to a difference in barometric pressure between the interior of the glass panes and the outside air pressure,” a 2014 summary of earlier NAHB research says. “This can create a concavity in the glass. Such a concavity is a normal response to pressure differences, does not affect the performance of the window, and does not constitute a defective window condition. However, the concavity may focus sunlight reflected from the window in a fashion similar to the effect seen when light passes through a magnifying glass. … The concentrated heat generated by the focused reflected sunlight results in surface temperatures well above that encountered from direct sunlight, and has the capability of causing damage to exposed materials.”

Other factors that can affect distortion include foliage, the angle of the sun and how close houses are to one another, the report says.

To prevent concavity, the NAHB report urges window manufacturers to use capillary tubes between the two lites of glass.

“The capillary tube connects the interior space between the window panes to the outside air, permitting a gradual equalization of barometric pressure, and thereby lessening the possibility that a concavity will develop in the glass,” the report says.

However, capillary tubes can’t be used in windows with argon filling, because the gas will leak out.

Additionally, the NAHB report says manufacturers could switch to double-strength glass to prevent the concavity problem. Most windows are made with two pieces of glass that are 3/32 of an inch thick. Glass that’s 1/8 of an inch thick would maintain a flatter surface.

Low-E glass has been standard in building codes for years. Today, it’s in more than 81 percent of all residential windows, according to AAMA.