What Causes Vinyl Siding to Melt?


A homeowner who installs vinyl siding and thermal windows may discover the siding is melting and warping. The Vinyl Siding Institute suggests that low-e thermal windows may be the culprit; but, window suppliers extend the blame to low-grade vinyl siding and to the heat reflected by dark-colored roofing materials.

Properties of Vinyl Siding

  • Vinyl siding, a virtually maintenance-free "cladding" product, was introduced to the siding industry in the early 1960s. Composed primarily of polyvinyl chloride (PVC), acrylic resins are added to improve durability. Additionally, pigmentation and titaniuim dioxide are mixed and baked into the PVC resins. This means vinyl siding doesn't required touch-up painting, is scratch-resistant and easier to maintain.

Potential Causes of Melting

  • Though low-E thermal windows reflect heat and theoretically can magnify sunlight to generate heat levels greater than 200 degrees Fahrenheit. The LBM Journal reports that low-E windows can cause significant melting by reflecting heat. They suggest that the amount of energy generated through reflection can exceed 250 to 300 Btus (British thermal units). However, The Vinyl Siding Institute suggests that there are other factors that contribute to melting in vinyl siding. One thought is that "concavity" in double-glass window panes, combined with the sun's angle and the position of neighboring homes, creates conditions that melt vinyl siding. While the effect doesn't happen with flat, single-pane windows, it can occur with any double-pane window and isn't isolated to low-E treated thermal windows.

Properties of Low-E Thermal Windows

  • Low-E thermal windows are treated with a thin, invisible coating such as a metallic oxide layering. According to manufacturers, the coating reduces the "U-factor," or rate of heat loss. Heat generally radiates out unless a barrier or reflective material acts as a barrier. Everything radiates heat, including our bodies; when we wear winter clothing in the cold air, it reflects our body heat and prevents us from losing heat. The principle behind a protective window coating is similar. It reflects sunlight out of our homes and keeps rooms cooler; and in winter, it prevents heat from radiating through the windows. According to the LBM Journal, low-E windows can reflect up to 70 percent of the sun's radiated heat onto adjacent homes, which can cause significant melting of those homes' vinyl siding.


  • The Vinyl Siding Institute makes practical suggestions to vinyl siding homeowners. Suggestions include planting trees or installing obstacles that block reflective heat from adjacent homes. Tim Carter, syndicated columnist at Ask The Builder, suggests homeowners choose high-quality grades of vinyl siding. In fact, some vinyl siding manufacturers are incorporating a heat-reflective additive called TempRite. According to LBM Journal, vinyl siding made with TempRite can withstand temperatures approaching 225 degrees Fahrenheit. Standard vinyl siding softens and melts at 170 degrees Fahrenheit.

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