Reflectivity, or albedo, is the percentage of the sun’s energy that is reflected by a surface. Another important measurement of a roof membrane’s performance is emittance. Emittance deals with how effectively a surface releases heat; it is the percentage of absorbed energy that a material can radiate away.
Most authorities have concentrated on reflectivity as the prime measurement of energy performance of cool roofing. However, with even the most reflective materials some energy is absorbed, and if that absorbed energy is not released efficiently it can cause a roof to heat up.
There is another measurement, called the solar reflectance index (SRI), that is beginning to get some attention. SRI combines reflectivity and emittance to measure a roofs overall ability to reject solar heat. The calculation of this index is defined by ASTM E 1980-01 and is based on some rather complicated math that includes values for solar absorptance, solar flux, thermal emissivity, the Stefan Boltzmann constant, and various other coefficients. Standard black (reflectivity 5%, emittance 90%) has an index of 0, and standard white (reflectivity 80%, emittance 90%) has an index of 100. Very hot materials can actually have negative values and very cool materials can have values greater than 100.
When all is said and done, a specific value can be calculated for any roofing product. Materials with the highest SRIs are the coolest choices for roofing.
Here is a sampling of products measured by Lawrence Berkeley Labs:
|Duro-Last Cool Zone
|Atlanta Metal Products, Kynar Snow White
|White Granular Surface Bitumen
|Trocal Roofing Systems, White
|Light Gravel on BUR
If PVC and all of its products and components disappeared tomorrow would extremist organizations like Greenpeace be satisfied? Definitely not. The activists are not just anti-PVC – they are anti-plastic and anti-chemical. That affects a lot more than just PVC. Take a look at the pyramid below, originally posted on the Greenpeace web site, that shows their ranking of the “safety” of plastics, from least (top) to most (bottom).
For an objective view of PVC roofing material safety, download the Q&A document, PVC Roofing Systems: Benefits & Issues from the Duro-Last web site. We also regularly post articles on this blog regarding PVC issues.
Greenpeace’s plans are to start at the top and work down. If the activists can’t attack plastics directly they change their tactics and attack the components that go into final plastic products. Some components they like to draw attention to are chlorine, plasticizers, stabilizers, fire retardants and UV inhibitors.
If they attack any of these components they attack every single plastic in existence – plus thousands of other lifesaving products and processes. Here’s just a sampling of what would cease to exist in their present form if “just” chlorine were eliminated:
|Helmets & hardhats
||Silicon memory chips
||IV tubing & bags
There are thousands of other products and processes that have expanded and improved human existence that are threatened. So, if anyone thinks they have a competitive advantage by fueling the fires of junk science activists against their competitors, think again. Today’s competitive advantage could become tomorrow’s dead pharaoh buried under the pyramid.
Q: Who says PVC materials are safe and/or environmentally benign?
A: The following are among the many organizations that have conducted scientific studies and life cycle assessments on PVC that have arrived at neutral or positive conclusions regarding the comparative health, safety and/or environmental sustainability of PVC production, installation, use and disposal:
Q: What about concerns that PVC production results in deadly emissions of dioxin, ethylene dichloride and vinyl chloride, causing health problems among PVC workers and nearby communities?
A: According to the EPA, since adoption of a closed-loop manufacturing process in the mid-1970s vinyl chloride emissions in vinyl plants have been reduced by 99 percent and dioxin emissions from all sources have been reduced by 92 percent. During the same time frame, PVC production in the U.S. more than tripled. In 1997, CDC reported that the PVC industry had “almost completely eliminated worker exposures to vinyl chloride” as well as the incidence of cancer and other illnesses caused by exposure. More recent studies by ATSDR and others have shown that dioxin levels and the incidence of cancer in communities near PVC production facilities are no higher than the national average.