Are White Roofs Cooler and More Energy Efficient than Non-White Roofs?

Recently, a team set out to determine if white roofs are indeed cooler and more energy efficient than non-white roofs. A roof in Tampa, FL was a perfect choice for this in-depth cool roof study.

The team installed HOBO data loggers from Onset Computer Corporation directly on the roof surfaces and in the low-traffic, non-air conditioned stairwells of both a new TPO single-ply roofing system just installed on a building and a building next door in the same corporate plaza (which still has the original ballasted EPDM roof system in place.) It was a perfect “before and after” condition, being that the results would be based on identical weather conditions as opposed to data from one year to the next on the same building, where many different variables could negatively affect the results.

The temperature sensors on the roof surfaces were secured under the exact materials used in each of the current roof systems, as to not allow for the direct sunlight to impact the readings and to give an accurate surface temperature on each building. The temperature sensors placed in the non-air conditioned stairwells were secured three feet from the roof hatch and directly to the concrete roof deck to measure the energy efficiency of the roof systems (in other words, they measured how much heat was being transferred through the concrete deck into the inside of the building).

The sensors measured both the roof surface and stairwell temperatures on both buildings every hour, 24 hours per day from August 6th to September 4th. During the data collection period, both buildings had primarily hot sunny days, with a few rain storms sprinkled in which allowed for near perfect test conditions.

The conditions of this study were near perfect for multiple reasons. First, they are virtually identical buildings that are side-by-side. Second, one building had the old roof system and the other had the new cool roof system –a before-and-after analysis at the same time and under the same conditions could be completed.

Conclusion #1: White Roofs Reduce Thermal Shock
The first thing that caught our attention was the significant reduction of thermal shock, which is a direct result of the more consistent cool roof surface and internal building temperatures of the new TPO roof.

Thermal shock was minimized by slowing down the rate of temperature change (both up and down) on the roof surface, which in turn nearly eliminated the rate of temperature change inside the building. The only other way to minimize thermal shock is to use materials with much greater strength, increased thermal conductivity and reduced coefficient of thermal expansion – but that approach is more expensive and doesn’t guarantee as favorable of results.

The rate of change in temperature inside the older EPDM building was expected – it heats up fast, and cools down even faster. But even more surprising was just how regulated the temperature inside the new TPO roof building is. The variation in temperature in the EPDM stairwell is 3.75 times that of the new roof building stairwell. In fact, there is so little temperature variation in the new roof building stairwell now that we are led to assume that internal thermal shock may be eliminated completely, and that maintaining a constant internal temperature is much easier as well!

Between 10:00pm and 6:00am, the buildings were very similar in temperature internally, but the EPDM roof building is always hotter than the TPO roof building. At 8:00am the EPDM building begins to heat up dramatically while the TPO roof building remains virtually unchanged. Later, between 5:00pm and 8:00pm, the EPDM roof building cools down even faster than it heats up – and again the TPO roof building keeps a constant internal temperature.

Conclusion #2: White Roofs Increase Energy Efficiency
Over the course of a typical hot August day in Tampa, FL, a white roof is 4.62% cooler (more energy efficient) than a non-white roof. During the hottest point in the day inside the building (5:00pm), a white roof is 8.49% cooler (more energy efficient) than a non-white roof. During the maximum internal temperature increase hours of 7:00am – 5:00pm, a white roof is 6.97% cooler (more energy efficient) than a non-white roof.

Based on this data, we would suspect that the August kilowatt consumption of the building with the new TPO roof should have reduced by approximately 7.00% or more from the previous year (when it had a ballasted EPDM roof), assuming all other items remaining equal.

In August 2010 (before the new roof was installed), the facility consumed 719,000 kilowatts of electricity. According to the maintenance supervisor, all other items remained virtually unchanged from 2010, and that the installation of the TPO roof system was the only substantial difference in 2011. In August 2011, the kilowatt consumption was 663,000 – a 7.79% decrease (or $6,450) in electricity required to cool the building (much more energy efficient.)

Again, this is on a building with a concrete roof deck. If the building had a steel or plywood roof deck, the energy savings would be even greater.

Conclusion #3: White Roofs Reduce Roof Surface Temperatures
The TPO cool roof is on average 12 degrees (13.57%) cooler than the EPDM roof. But that includes overnight when there is no direct sunlight.

At 12:00pm is the largest difference in roof surface temperature over the course of our study, when the average outside temperature was 90.66°F. The EPDM roof was on average 156.28°F at that time, but the TPO roof was only 108.59°F. That’s a difference of 49.69 degrees - quite significant to the long-term survival of rooftop equipment (as well as the building’s facilities workers).

The rate of roof surface temperature increase between 6:00am and 8:00am were similar on both buildings, but the EPDM roof kept getting hotter while the TPO building started to level off. Also, the TPO building cooled off more gradually than the EPDM building, which heats up really fast and becomes extremely hot, and then cools down at a faster rate (although it never gets cooler than the TPO roof). It isn’t until 7:00pm when they come back to the same rate of temperature change. This is another indication of reduction in thermal shock with the TPO cool roof.

For more information on cool roofing systems and how they can impact your facility and bottom line, visit www.RAMCONRoofing.com or call us at (877) 726-2661.