The Big Bounce
When the sunlight bounces off the roof, where does it go?
Have you ever wondered what happens to solar radiation when it bounces off a “cool” reflective white roof? Many people in roofing design and construction have recently been asking that very question.
What answers have they found so far? Samir Ibrahim, director of Design Services at Carlisle Construction Materials, presented a paper at the 28th RCI International Convention and Trade Show in Orlando, Florida, provocatively entitled “Where Does the Heat Go?”. This paper showed the results of a study that Ibrahim conducted in Carlisle, Pennsylvania. He found that air temperatures above a white roof are in most cases hotter than those above a black roof, contrary to what many researchers had intuitively expected.
Matt Dupuis, PE, principal of Structural Research, Incorporated, published a study testing air temperatures above multiple roof surfaces in Chicago and discovered that black roofs make air hotter than white roofs below a height of 1 foot, but from 1 to 8 feet above the roof, there is less than 1ºF of difference on average.
A third study, conducted by T. C. Lindsey, W. T. Black, and P. A. Sharpe in Las Vegas, found that between the heights of 1 inch and 36 inches, white roofs make the interior of metal conduits hotter than do black roofs. This finding is important because electrical wires running through those conduits are less efficient at hotter temperatures.
A fourth study, out of Stanford, by M. Z. Jacobson and J. E. Ten Hoeve addressed the issue on a larger scale. The authors performed a simulation where black roofs were broadly converted to white roofs worldwide, and concluded that the new white-roof scenario would provide local cooling but would actually increase the overall global temperature. These four studies fueled the already raging debate in the roofing community about the appropriate use of reflective roofs.
My Experimental Study
This is where I entered the picture. As an academic type in a temperate climate, and with no horse in the race, I was ideally suited to study these effects with a dispassionate eye. I received a grant from the RCI Foundation to conduct an experimental study.
Thanks to the magic of Google Earth, I surveyed the rooftops of my campus and found a building that would satisfy my needs. It had to be big enough to ensure that my students and I would never be near a roof edge while we were working, and it also had to have south- or west-facing opaque and glazed walls so we could look at the effect of reflected light on these surfaces.
Our sprawling Virginia-Maryland College of Veterinary Medicine complex afforded the perfect study location. Along with my research assistant Kenneth Black, a small crew of graduate and undergraduate students, and a certified roofing installer, I affixed overlays of thermoplastic polyolefin (TPO) and ethylene propylene diene monomer (EPDM) membrane atop the existing EPDM membrane. Then we installed a weather station, and put 102 sensors in every imaginable location.
So why should the average designer care about all this? One answer is because white roofs have been actively promoted as one solution to the heat island effect. Designers have been encouraged to specify white roofs so that their projects can earn credits in green-building certification programs like Leadership in Energy and Environmental Design (LEED).
Thanks to jurisdictions and organizations around the country insisting on buildings achieving LEED certification, cool roofs have become commonplace. And while green roofs also fill the bill, they are often too expensive to implement, and white roofs become the default choice.
We measured the roof surface temperature. We measured the air temperature at four heights above the membrane. We measured the temperature of electrical metallic tubing (EMT) at those same four heights. At the opaque and glazed walls we also measured temperatures at four heights.
All these measurements were replicated three times, and I hired a statistician, Stephen Werre, to ensure that the findings were statistically significant.
We collected data on two sunny days with calm winds, one in May and one in August, when we could have the mechanical systems in the building shut off to avoid any interference with the study.
What did we find? Here are the highlights:
- Air temperatures were 3–4°F higher above EPDM versus TPO up to 5.5 inches.
- There was no significant difference in air temperatures above 5.5 inches.
- Temperatures were 3–4°F higher at EMT above TPO versus EPDM.
- A precast concrete panel wall was 6–9°F warmer adjacent to TPO versus EPDM.
- Exterior glazing surface temperatures were 3°F warmer adjacent to TPO versus EPDM.
Dupuis, M. "Air Temperature on Reflective Roofs." Midwest Roofer, December 2013 pp. 16–17.
Ibrahim, S. "Where Does the Heat Go? A Look into Energy Performance of Reflective Membranes." Paper presented at the 28th RCI International Convention and Trade Show, Orlando, Florida, 2013.
Jacobson, M. Z., and Ten Hoeve, J. E. "Effects of Urban Surfaces and White Roofs on Global and Regional Climate." Journal of Climate 25. no 3 (2012): 1028–44.
Lindsey, T. C., Black, W. T., and Sharpe, P. A. "Effect of Rooftop Exposure on Ambient Temperatures Inside Conduits." IAEI NEWS (2006): 32–41.
What the Findings Mean
What are the implications of these findings? Essentially, we discovered the same thing as Dupuis—that black roofs heat air in a zone a short distance above a roof, but beyond that zone, the color of the roof doesn’t matter. And like Lindsey et al., we found that roof reflectivity does matter when light reflected off roof surfaces heats the objects around it, such as metal conduits and adjacent walls. We didn’t do heat flow calculations through those opaque and glazed walls, or look at thermal effects over time, but those would be logical next steps.
I will be presenting these results in more detail on March 21 at the 32nd RCI International Convention and Trade Show, in Anaheim, California, and a paper scintillatingly entitled “The Influence of Roof Reflectivity on Adjacent Air and Surface Temperatures” will be published in the March 2017 issue of Architectural Science Review.
The take-home message from both the presentation and the paper will be this: Roof selection is not as simple as gaining a LEED point. Architects and roof consultants need to be aware of potentially adverse thermal effects when choosing roof membranes, particularly in areas where mechanical equipment is located on rooftops, and where roof surfaces are adjacent to walls that can be affected by bounced light.
We do need to think about where the heat goes.
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