Glass in 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. 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. The heat generated by the focused reflected sunlight has proven sufficient to visibly damage and distort vinyl siding on nearby houses. Any double paned window may cause this effect, but double paned low-e windows have a higher reflectivity quotient which can exacerbate the reflected light/vinyl distortion phenomenon.
The Vinyl Siding Institute (VSI) states that temperature ranges beginning at 160-165 degrees Fahrenheit can soften normal grades of vinyl siding. Darker colors absorb more heat, and will soften before lighter colors of siding. Heat generated from double paned low-e window reflected sunlight has been measured in excess of 200 degrees Fahrenheit, more than sufficient to soften and distort any normal grade or color of vinyl siding. There have been some reports of reflected sunlight damage to materials other than vinyl siding. Occasional wood discoloration and charring, and damage to paint and other plastics (e.g.,decking, window lineals, trim), have all been reported.
The use of double paned low-e windows will not necessarily result in any damaging reflected sunlight incident. A combination of contributing factors must be present before the effect occurs or causes damage to any nearby materials, including vinyl siding. The presence of the concavity in the double glass panes (resulting in the magnifying glass effect with a focused light beam) appears to be the primary cause of the heat generation, more so than the mere increased reflectivity of the low-e window. The angle of the sun is also a factor. A low angle of sunlight (such as might occur in late fall, winter, or early spring) is more likely to produce the effect. Other factors, such as proximity to the adjoining house, wind speed, air temperature, and the presence of buffering foliage are all said to have an impact on whether a damaging reflected sunlight effect does in fact occur.
Vinyl siding and insulating windows both have very large market penetration. Vinyl siding has been the most used siding product on new single-family homes in the U.S. every year since 1994. It was applied to 35% of all new homes built during that time frame. The majority of new vinylsided homes are in the south (40%), midwest (35%), and northeast (19%) (U.S. Census Bureau 2009). Based on sales data and projections from 1999 to 2019, approximately 45% of residential vinyl siding is, or will be, used in the new construction market; the remainder will be used for retrofits and repairs (Freedonia Group, Inc. 2009).
The U.S. Department of Energy and model energy codes have made improved residential energy efficiency a national priority. Therefore, standards have called for increasingly stringent energy efficiency requirements for windows. The performance levels achieved by insulating windows are mandated by nearly all local building energy codes for both new replacement in existing buildings. Windows are typically designed to meet the requirements of the International Code (IECC), the most commonly adopted model energy code in the U.S. for residential buildings. To meet IECC’s U-factor and solar heat gain coefficient (SHGC) requirements insulating double-pane windows with low-emissivity (low-e) coatings are often used.
If you have had and fires or incidents involving Low - E Windows and Vinyl Siding? I would love to hear from you. Please email me @email@example.com
A special thanks to Chief Kevin A. Gallagher for bringing this issue to my attention.
Lt. John Shafer