Black or White Roofs

Black Roofs and Their Time in the Sun

 

There are many misconceptions on the benefits attributed to the color of a large flat roof. In the colder Northern regions, it was once held that a black roof and its potential for increased thermal radiation absorption was the preferred color. A lot of this was due to availability and use of products like synthetic rubber and tar, which are both black by nature, than actual scientific research. The color black does absorb all wavelengths of light and reflects none, so this is why we see no color or the absence of color. Lighter colors like whites and metallic substances absorb less illuminating light, and thus heat up less.

 

Flat White RoofIt must be noted that color makes a small difference as regards to heat transfer between an object at everyday temperatures and its surroundings, since the dominant emitted wavelengths are nowhere near the visible spectrum, but rather in the far infrared. So unless the object is in direct sunlight, the color makes little difference as regards to warmth. Your roof, however, spends its entire day in direct sunlight so the visual emissivities must be taken into account.

 

Black Roofs and the Environment

Black roofs will increases thermal radiation absorption and on sunny days and are hotter than a comparable roof finished in a reflective or white membrane. The increased heat generated in the winter and its associated cost savings can be more than offset by the unwanted increased heat during the hot summer days. The trend towards white colored roofing materials in northern climates has begun to reflect this story.

 

Several factors make any potential heat gain relatively immaterial and end up costing the building owner by way of higher energy cost during the summer, thus negating any overall savings.

 

Here are some black and white facts:

 

  • The laws of physics or thermodynamics to be precise dictate that hot air (not heat) will always rise. Hot air has a higher kinetic value and thus is less dense than colder air and this is what makes for hotter temperatures nearer the top of enclosed structures. The heat or kinetic energy that is transferred to the interior of a building’s structure from the outside will influence the air nearest the roof and will remain at the top, providing minimal heat savings throughout the rest of the building.
  • In winter, there are fewer hours of sunlight, combine this with a decrease in the angle of the sun and you have less electromagnetic radiation reaching the roof to affect energy costs. In some northern areas, there is a greater than a six-hour difference between peak-summer and peak-winter sunlight, this all contributes to less sunlight available to add to a building’s potential warming.
  • In many of these northern areas, snow retention will not only provide insulation but physically turn these black roofs into white and thus eliminate any effect of heat absorption. The accumulation of snow on commercial roofs can cause additional problems for the property owner as well. See our other blog specifically detailing the effects and concerns about snow retention devices. http://blog.jottan-jorel.com/snow-retention-devices
  • The energy needed to cool a large building in the summer can be substantially greater than the small amount of assistance generated by a black roof in the winter. A white roof will not only help in cooling a building but can be considered green. White roofs due a lot in reducing “urban heat island” effect. For more information on green roofs please see our other blog on this subject. http://blog.jottan-jorel.com/green-roofs-a-choice-that-will-grow-into-the-future

Whita or black Roof

The Choice for White

 

It’s becoming increasingly clear that along with paved roads and disappearing vegetation, black roofing is a major contributor to the “urban heat island” effect. This can cause urban areas to reach temperatures three to eight degrees higher than outlying rural areas.

.

The Federal government’s response to this problem has been to mandate a  reduction in energy use for Federal office buildings by 35%. Installing a white reflective roof membrane can help reduce those energy uses and costs. In addition, many cities and virtually all 50 states have mandated cool roofing at some level, and offer tax rebates or other incentives to promote the use of cool roofing to building owners.

 

For more information on the benefits of installing a white or reflective roof you can access the Department of Energy’s “Cool Roof calculator”. This calculator when properly used can estimate your potential energy savings.

http://web.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm

 

Jottan-Jorel offers a variety of commercial roofing solutions, including white or black roofs, for your new or existing structures. Jottan-Jorel specializes in preventive maintenance, emergency repair and service along with new and reroofing services.

Back to Top