Some homeowners try to reduce their maintenance time and costs by cladding their homes with vinyl. Others try to reduce their heating and cooling bills by installing energy efficient low-e windows. And if one of these types of homeowners lives close enough to the other, and the angle of the sun and the time of year is just right, these estimable goals can result in a fire.

Low-e windows are double-or triple paned with a thin metal or metallic oxide coating applied to one or two of the inner or outer surfaces, and in some cases, an inert gas with low thermal conductivity, such as argon or krypton, is funneled between the panes. The window’s emissivity refers to its tendency to radiate absorbed heat. The transparent glass allows sunlight to pass through but the metallic coating and gas layer reduces the transmission of the sun’s heat. In colder climates, low-e windows improve a home’s thermal insulation, keeping heat in. In warmer climates, low-e windows can keep a home cool by keeping the sun’s heat out.

The construction of low-e windows also increases its reflectivity – untreated glass windows reflect 10 percent of the sun’s rays; low-e windows reflect 30 to 50 percent of sunlight.

Low-e window heat-damage incidents are artifacts of modern construction. Vinyl siding was introduced in the late 1950s, and low-e windows came into the marketplace in the early 1980s. Now both are ubiquitous. According the U.S. Census Bureau’s Survey of Construction, vinyl siding has been the most commonly used exterior cladding on new single family homes since 1994, which the National Association of Home Builders reports, was installed on more than a third of new and existing homes in 2014. Today, a combination of government incentives and local building codes have resulted in low-e windows capturing 80 percent of the U.S. residential market and 50 percent of the commercial market share.

The prismatic heat of sunlight reflected on curved glass has been noted on a large scale. In Las Vegas, the Vdara Hotel’s 57-story structure with a curved exterior threw a concentrated beam of heat, burning hotel guests arrayed around the swimming pool area below, causing one news outlet to dub it a “death ray.” In London, wags re-christened the so-called the Walkie-Talkie building, a $400 million skyscraper with a concave design, the Walkie-Scorchie building, after it was blamed for focusing rays of light powerful enough to melt cars on the street below.

But cases involving individual homes and low-e windows started popping up in 2007, according to the International Association of Certified Home Inspectors. Forensic engineer and Adjunct Professor of Alternative Energy at Western New England University. Curt Freedman first encountered the phenomenon when he was called to investigate a case of melted vinyl siding on a home in Hasting on the Hudson, New York. This spurred Freedman to start collecting exemplar windows and do his own testing to refine his understanding of the phenomenon.

It takes several factors aligning to create a scenario in which the reflected light from a low-e window can produce enough heat to melt vinyl or ignite brush. In the former case, there must be 15 to 30 feet of proximity to the window, Freedman says. The angle of the sun must be low enough to reflect the sunlight onto an adjacent building – which is why these incidents tend to happen in the colder months. Finally, the window itself must be concave – created, in part, by a difference in the barometric pressure between the interior of the glass panes and the outside air pressure. This effect is more pronounced in the cooler months, as the gases contract and pull the pane in.  

According to a National Home Builders of America white paper: “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.”

“The barometric pressure may have some role but what I believe the more major role is temperature – the colder the temperature, the more the window is sucked in and it changes the curvature of the glass,” he says.

Some theorize that low-e windows featuring argon gas are more prone to this curvature, because over time, argon escapes from the void between the panes, but air molecules, which are bigger, can’t get in to replace the inert gas. But Freedman says this phenomenon also occurs in low-e windows with air between the panes.

Some of the recent cases Freedman has investigated include:

• A report of fire damage in January 2015 on the north exterior wall of a home in Whitman, Mass. The fire department found smoke seeping into the second floor, and opened up the interior walls in two bedrooms looking for the source. The fire was extinguished, but firefighters found other hot spots that needed to be wet down. When investigators failed to find fault with the building’s electrical or heating system components, the state Fire Marshall determined that the mostly likely cause was the concentrated reflection of sunlight coming from an adjacent home’s second floor windows. The building’s homeowners told fire officials that it was the second time the vinyl siding on that side of the house had been melted and replaced.

• In November 2016, a neighbor’s low-e window burned a four-foot square patch of mulch, in Waxhaw, North Carolina.

• On January 28, 2017, the state Forest Service extinguished a fire at an Advance, North Carolina residence that destroyed an old dog house and burned a patch of ground covered in pine straw. Four days later, the department responded to another fire in the same area and determined that it was set by the reflection of sunlight off the neighbor’s second-story window. Fire investigators measured the temperature at the cross-shaped reflection at 341 ºF.

• On January 6, 2018, low-e windows were blamed for burning through the wooden shingles of an adjacent house in Somerville, Mass. through to the framing underneath. It was the second incident in six years.

 

Besides general consternation, melted vinyl siding has resulted in at least one civil action. In 2016, an Oregon man sued Associated Materials of Cuyahoga Falls, Ohio, the makers of Alside siding, for rejecting a warranty claim for melted vinyl siding, citing an unusual heat source as the root cause.

For Freedman’s part, he continues to assist fire investigators who suspect that a low-e window might be the ignition source in some fires. His goal is to educate fire departments to recognize the phenomenon.

“There is nothing in the fire investigation books about concentrated sunlight fire. No one’s really published on this topic,” he says. “The issue of reflected light off windows needs to be better understood, for example, what causes wood to burn from reflected sunlight? They need to recognize when these sunlight induced-fire events occur, and also distinguish legitimate cases of solar induced fires from arson.”