A recent Associated Press caption reported: “Tower’s sun glare burns Jaguar, spurs hunt for solution.”
There have been many reports of glare on newly constructed roofs that have required remediation, but in this case the heat source was from reflective window glass, not just from white or aluminum roof coatings on a roof surface.
What makes this incident special is the magnitude of the heat load. The article went on to state that a Jaguar parked in London in the path of the reflected heat “for a couple of hours” had melted the Jaguar’s mirror, panels, and hood ornament and burned a hole in the welcome mat of a barbershop across the street.
That excessive heat load is an entirely new issue – not just for luxury cars parked near highly reflective facades, but for roofs too, as too much heat causes premature aging.
What the Standards Say
ASTM Specification for Poly (Vinyl Chloride) Sheet Roofing (D4434) requires heat aging at 176 degrees F. for 56 days. This roughly relates to chemical kinetics that states once material has reached its energy of activation, the rate of acceleration doubles for every rise of 10 degrees C. Standard D6754 for Ketone Ethylene Ester (KEE) also uses 176 degrees F. for 56 days.
Heat aging for Thermoplastic Polyolefin Based Sheet Roofing (TPO) D6678 is somewhat more severe – 240 degrees F. for 224 days, perhaps recognizing the demand for more efficient roofing systems and as a result of far more thermal insulation directly under the roof membrane.
All of the above polymers are designated as “thermoplastic,” and their seams can be heat welded using a hot-air welding gun. The heat exposure is very brief whether hand-welded or machine-welded. Many of these materials can now be seamed using pressure-sensitive cold adhesives. However, we lack accurate data on reflected heat. If the London building (which will not be completed until 2014) is any indication, we need to use materials that can accommodate these new magnitudes of heat load.