I first learned about this factor from a technical bulletin on the AZEK web site. First, what is low E glass? E stands for “Emissivity.” Emissivity can be described as percentage of incoming radiant heat that is NOT reflected. For example Aluminum Foil would reflect 97% of radiant heat from the Sun. It fails to reflect just 3% and therefore has very low thermal emissivity of .03. Asphalt has hight thermal emissivity of .88 because it fails to reflect 88% of incoming radiant solar heat, which is why it can get so hot on a warm sunny day.
Here is the strange thing about regular window glass. It fails to reflect even more heat than asphalt … 91%. (high thermal emissivity of .91) because the solar heat passes through it. This is why traditional home builders in New England wanted more and bigger windows on the sunny side of the house, at least in the Winter.
So what does this have to do with composite decking?
Azek, maker of composite decking, wants you to realize you may encounter a problem if you have windows and doors with specially made Low-E glass. This is glass with a micro coating that reflects most of the heat while not obstructing light. It is a high tech way to make thermally insulated glass to keep an interior space from heating up in the summer or from losing heat in the winter. A house with sliding glass doors opening on to a deck has a large glass surface perpendicular to the deck surface. Sunlight with its thermal radiation hits at an angle and could reflect directly onto the deck’s surface… except that with regular window glass, 91% of the heat passes through into the kitchen. Low E-glass would reflect the heat onto the deck, and that is the concern.
An Azek technical bulletin specifically warns customers about E-Glass in "AZEK Building Products Technical Bulletin #060310 Effects of Excessive and Unusual Heat Sources on AZEK® Products:
This is by no means a criticism of Azek composite decking. Excessive heat from Low-E glass is potentially a problem for composite decking made by any manufacturer. We applaud Azek for making customers aware of the issue. Unlike Trex, Azek does not specifically warn its customers about using a fire pit on a composite deck. They probably assume you are already concerned about that. It should be noted that Azek decking meets exacting building codes for heat and flame resistance.
The Azek bulletin refers the reader to the Vinyl Siding Institute web site where I found the following FAQ items on a page about “Concentrated Solar Reflection.:”
"Is the damage from solar reflection and heat distortion limited to vinyl siding?
No. Other materials can be raised to temperatures known to cause short-term or long-term damage from exposure to intensely concentrated sunlight. Anything that falls in the path of that reflected beam can be harmed including cups, bags, pool covers, car parts, painted surfaces, and cedar shingles are a few of the materials reported in the media to have been damaged. People who have found themselves in the path of such beams of have reported extreme discomfort and the inability to remain exposed for more than a few seconds without enduring the risk of burns. Eye damage from even a brief exposure is highly likely."
"Does this happen only in the summer, or only in hot climates?
The phenomenon can occur anywhere the sun shines, in any season. Even when the air is cool in winter, if the sun’s energy is concentrated onto a small surface area, that location can become far hotter than the surrounding area. But in many cases the relationships of the angle of the sun, the direction of the window and the location of the house wall are “just right” only at certain times of the year."
"Can the high temperatures created by concentrated sunlight set my siding on fire?
No. The ignition temperature of vinyl siding is approximately 720-750 °F (380-400 °C). This is well above any temperatures reported to have been caused by even the most extreme cases of concentrated sunlight reflected from windows. In fact, the ignition temperature of wood is lower than that of vinyl siding, so wood materials such as siding, fencing or decking would be at greater risk of ignition than vinyl siding."
What does vinyl siding have in common with composite decking? Vinyl.