Understanding Concentrated Solar Reflection

Concentrated sunlight reflecting off certain surfaces — like energy efficient windows — and onto vinyl siding can create enough heat to melt the siding surface. While it’s not common, it is important to know how to prevent a home's vinyl siding from melting due to window reflection.

With increased attention on cases of siding distortion, we’re committed to addressing concerns from homeowners, builders, remodelers, architects, code officials, or the media. Below are answers to some of the most common questions about siding distortion from solar reflection.

What is siding distortion from window reflection?

Siding distortion — sometimes described as “melted” siding or "solar distortion"— is caused by concentrated sunlight reflected from windows. In every case, the evidence is overwhelming and unavoidable that energy efficient windows are capable of concentrating and reflecting solar energy, which causes damage to a wide variety of materials. The important takeaway here is that this “phenomenon” is not caused in any way by vinyl siding. It’s caused by newer “low-e” windows that are installed on nearby structures where the light happens to reflect in a particular way, according to Vinyl Verified®.

How to prevent siding distortion from solar reflection?

Damage to property caused by concentrated sunlight is infrequent and occurs only under a unique set of conditions. Homeowners can protect their personal property in such instances by blocking the path of sunlight, either to the window or between the window and the siding. This includes the strategic placement of trees, bushes, or other landscaping elements and the use of sunshades above windows. Screening applied to windows may reduce the total amount of reflected energy and help to avoid the problem.

Can siding distortion from solar reflection affect my house?

The chances of siding distortion or melting occurring on any given house are extremely small. Vinyl siding is the most popular choice of exterior cladding, and has been for about two decades. There are tens of millions of houses sided with vinyl. Only a tiny percentage of those houses have been affected by exposure to concentrated sunlight reflected from windows.

A very narrow set of conditions needs to come together in order to cause the problem, and this happens very infrequently. If your house has not been affected in the past, it is not likely to be affected in the future. This can change, however, if windows are replaced (either on your home or your neighbor’s) with ones that are more likely to concentrate the sunlight, or if something that blocked sunlight is removed.

How close to the window does the vinyl siding need to be in order to be affected by the solar reflection?

It is not possible to predict what distance is at greatest risk or whether there is a “safe” distance from the window. This is a topic of on-going research. The range or “zone” of distance from the window in which the reflected beam is most concentrated depends mostly on the curvature of the glass. The curve can be different for different windows, and can even change in the same window under different temperature and atmospheric pressure conditions.

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.

Does the heated vinyl siding release dioxin or other chemicals?

When vinyl siding is distorted and melted by the heat, the vinyl siding is changing shape, but does not break down or release toxic chemicals at the temperatures reached in reflected sunlight cases. The combustion and other conditions necessary to create dioxin do not occur with reflected sunlight.

Does lighter-colored, thicker or more expensive siding resist these high temperatures better?

No, the thickness, color, or “quality” of the siding are not likely to be major factors. Many of the conditions that affect the risk of distortion from concentrated sunlight are still being studied. Concentrated reflected sunlight has caused distortion of light and dark siding, and even the thickest and most expensive profiles. These factors may make relatively small differences in the occurrence of distortion, but their effects are overwhelmed by the large amount of energy transmitted in a beam of concentrated sunlight. Effective solutions must involve preventing the windows from concentrating the reflection, or by intercepting or diffusing the beam before it strikes the siding.

Does the type of material behind the siding—such as a light, dark or shiny underlayment—affect the likelihood of distortion?

No. Based on current laboratory research and field observation, there’s no evidence that the nature of the underlayment has a significant effect on distortion.

Is this kind of distortion covered by the siding manufacturer’s warranty?

The terms of each siding manufacturer’s warranty are likely to be different, so you should refer to the warranty for the specific product. Warranties generally cover manufacturing defects that lead to unacceptable performance under normal conditions for the intended use. Damage caused by excessive or abusive conditions are not normally covered by warranties. Thus while temperatures created by normal environmental conditions, including exposure to the sun, would fall within most warranty terms, the excessively high temperatures caused by a fire—or by unnaturally concentrated sunlight—would not. But, follow up with the siding manufacturer to determine the specifics.