Modular Concerns: The Dangers of Sunlight Reflected Off Energy-Efficient Windows

By Kevin A. Gallagher

Back in October 2013, I wrote an article for this column on the very real phenomenon of sunlight reflecting off energy efficient windows and causing damage to vinyl siding. As that article noted, the nationwide push for greater energy efficiency has resulted in the adoption of stricter energy codes. Reducing our carbon footprint is the goal, but sometimes this can result in unexpected consequences.

The October 2013 article discussed several examples of vinyl siding becoming distorted in some cases and melted in others. The amount of heat reflected off these windows is—in some cases—great enough to cause damage from considerable distances (photo 1).

(1) Photos by author.


Fortunately, this is not one of those pesky issues from which the various industries are running away. The National Association of Home Builders recognizes the problem, and the Vinyl Siding Institute has issued an advisory on solar reflection and heat distortion. The problem, it appears, is getting to a solution.

As stated in the previous article, the ignition of vinyl siding by reflected sunlight may not be the primary concern. The Vinyl Siding Institute’s report on solar reflection and heat distortion states the following:

There does not appear to be a real concern about this concentrated heat energy at these levels igniting vinyl siding. The authors of the University of California report state, “The auto-ignition temperature of vinyl siding varies with the specific vinyl compound formulation and test conditions, but it is typically reported around 730 degrees F. For comparison, the auto-ignition temperature of wood, another common siding material, is typically reported in the temperature range of 400 to 500 degrees F.” Well, this seems like good news. However, the authors of the report strike this concern, stating, “Vinyl siding will shrink, char, and expose its backing material well before it reaches its auto-ignition temperature. Therefore, if any fire hazards from highly concentrated radiation need to be considered, the focus should be on materials surrounding and backing the vinyl siding, not the siding itself.”

The concerns, however, are other materials igniting because of reflected heat from vinyl windows (photo 2). I mentioned the old cigarette in a dried-out flower box origin and cause with which we are all familiar. The damage from that scenario can be excessive.



The question was asked in October, “If this phenomenon has been known to melt plastic parts on automobiles, what other damage can be caused by this focused heat source?”

One answer came from a community north of Boston, Massachusetts.

On July 1, 2010, at 0301 hours, the Melrose (MA) Fire Department (MFD) was dispatched to a reported house fire (2½-story wood frame). Two engines and a ladder were dispatched. Prior to arrival, dispatch updated the responding units that the bushes outside the home were burning and that all occupants had exited the structure. Firefighters arrived to find the homeowner and a neighbor wetting down the front exterior of the house with a garden house. The first-arriving engine advanced a line and wet the entire front exterior while the crew of the ladder entered the residence, checked for extension, and found only a slight smoke condition on the second floor. Their inspection found charred trim boards on the outside soffit with some charring of the underlying framing.

The homeowner advised the fire officers that he and his wife were awoken by shouts that their house was on fire and banging on their door. Looking outside, he reported that he saw the bushes in front of his house on fire with the fire in contact with his home. A neighbor was able to extend a garden house, and the two extinguished the fire.


(3, 4). Typical high-energy windows.


(5) Performance rating sticker attached to energy-efficient window.


Scenario: At three o’clock in the morning you are dispatched to a fire in the bushes of side A of a two-story residential property. The fire is out on arrival. What are you thinking is the cause? Discarded smoking material? Probably. Failure of underground electrical system? Possibly. Arson? Under consideration.

How about a smoldering fire caused by reflected sunlight off of an energy efficient window?  The report says that the fire was called in at 0301 hours. How could the sunlight-window-reflected-concentrated- heat phenomenon play a role at this fire?

Because of some very good investigative work by Captain Ed Collina of the MFD Investigative Unit, an anomaly was found and examined closely. Following is from his report:

“I photographed the front of the house including the grass and mulch area. I found an area of grass that was dried while the rest of the grass looked healthy. There was an area of mulch burnt that lead to a golden thread juniper shrub/tree planted in front of the house. This shrub/tree caught fire, the flames extended to exterior of house.

“I sifted through the mulch in the burnt area and found no evidence of any underground electrical as well as no evidence of smoking material.

“While looking at the fried grass I noticed evidence of the dried grass had burnt. During this part of the investigation I noticed two bright spots being reflected down off of a second floor window. This reflection was in the dried grass area and moved in an eastward direction as the sun moved in a westward direction. At 1215 hours, the first bright reflection met the first evidence of the point of origin. I placed the backside of my hand on the reflection and it was extremely hot, I removed my hand after it started to burn.”

The homeowner informed investigators that the only window in the house that had been replaced was the double pane, low E window that was reflecting concentrated sunlight during this investigation.

The investigative report continues:

“He (homeowner) stated that the sun reflects off his second floor window and makes a spot on his lawn like a magnifying glass. His wife reported that during the day she smelled smoke like a brush fire.”

Thanks to the extensive investigation by the MFD and a very thorough and detailed report by Captain John White, we have evidence of a structure fire caused by reflected sunlight!

To recap, shortly after 1200 hours, sunlight reflecting off of one energy-efficient window generates enough concentrated heat energy to burn a dried-out area of lawn. The occupant will not see smoke or fire but notes the smell of burning brush during the day. Smoldering continues during the next 15 hours until it reaches shrubs. Enough heat energy is present to ignite the shrubs with the subsequent fire extending to the exterior of the house. Extension to the interior stopped by civilian extinguishment actions. (The MFD fire report states that, while battery smoke detectors were present, they would have failed to work because of the batteries having been removed.)

Back in October, I asked, if reflected light off the type of windows that were mandated by the building codes is sufficient enough to melt vinyl siding, what other damage can be caused by this focused heat source?

I’m sure the folks from the MFD and two very fortunate residents of its city would answer that question with one word: Plenty!


KEVIN A. GALLAGHER has served with the Acushnet (MA) Fire & EMS Department since 1986, where he was appointed chief in 2003. He has an associate degree in fire science and a bachelor’s degree in political science. He is an adjunct instructor in the fire science program at Bristol Community College. He is the Fire Chiefs Association of Massachusetts representative to the Board of Building Regulations and Standards, responsible for overseeing the state’s building code. He has contributed articles to Fire Engineering and taught at FDIC on modular construction.


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