A lawsuit in Oregon over melted vinyl siding is revisiting the question of what role, if any, windows might play in such events.

Michael Harney of Beaverton, Ore., is suing Associated Materials of Cuyahoga Falls, Ohio, the makers of Alside siding, for not honoring its warranty after Harney said a house he sold in October 2015 had melted siding. The complaint says Alside rejected his warranty claim because it believed the melting was caused by an unusual heat source, according to a report from Plastics News. Harney’s complaint alleges that the vinyl siding melted while being exposed to “natural conditions”  and is thus covered under the warranty. (Attorneys for Associated Materials have not yet filed a response.)

The lawsuit, which was filed in August in U.S. District Court in Portland, Ore., could become a national class action case.

“While the precise number is unknown at this time … the proposed Class may be comprised of at least thousands of members,” lawyers for the plaintiff said in their complaint, which noted that there are at least 100 potential class members who could claim damages in excess of $5 million.

A study by the National Association of Home Builders (NAHB) found that under certain rare conditions, it’s possible for windows to focus enough heat onto vinyl siding to melt or warp it.

“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. This focused light may land on adjacent building surfaces, and appear as a brilliant star-shaped spot. 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, especially those which are plastic based.”

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.

For years, many homeowners assumed that low-E glass was the main culprit in melting vinyl siding.

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 the American Architectural Manufacturers Association (AAMA).

In 2014, complaints about warped siding and even a handful of fires led the North Carolina Building Code Council to adopt an emergency rule that would let builders use non-low-E glass in situations where they could foresee reflections causing a problem. However, after intense lobbying by AAMA, the Window and Door Manufacturers Association (WDMA) and other industry groups, the North Carolina Rules Review Commission rejected the rule.

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

The change was made to ensure builders would be clear of any liability for not being in compliance with the energy conservation requirements, said Jeffrey A. Smith, communications director with the Vinyl Siding Institute.