Episode 56 - Cool Roofs, Warm Results: How Metal Roofing Saves Energy

In this episode of Building with Metal, McElroy Metal unpacks the science behind cool metal roofing and its real-world energy savings. Learn how reflectivity, emissivity, and advanced coatings reduce heat absorption, lower cooling costs, and support sustainable building design. Discover why “cool roofs” are making a hot impact on both performance and the planet.

  • 35 min

Episode 56: Cool Roofs, Warm Results: How Metal Roofing Saves Energy

The Building with Metal Podcast – Presented by McElroy Metal

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Notable Quotes

• "A dark colored surface, say a dark brown or a black, would have a solar reflectance of about ten percent, which means that ninety percent of that energy is absorbed." — Andre Desjarlais, Oak Ridge National Laboratory

• "In the south, the reduction in demand and the reduction in the cost that the building owner has to pay for demand is almost equal to the energy savings that the cool roof supplies. So it's not like a ten percent number. It's half." — Andre Desjarlais, Oak Ridge National Laboratory

• "What now seems to be the biggest energy consumer for buildings is how airtight it is. The payback for improving air tightness is typically a couple of years." — Andre Desjarlais, Oak Ridge National Laboratory

What You'll Learn

• How solar reflectance is measured and what the numbers actually mean for your building
• Why roof color choices differ between hot southern climates and cold northern ones
• How much money a reflective roof can realistically save — for both residential and commercial buildings
• Why commercial buildings are held to lower insulation standards than residential buildings
• How cool roofs reduce peak electricity demand charges, not just total energy use
• What the urban heat island effect is and how widespread cool roof adoption could reduce city temperatures
• How solar reflectance changes over time due to dirt, biological growth, and coating degradation
• Why PVDF coatings hold their reflectivity better than shorter-service-life roof coatings
• When it makes economic sense to clean a roof — and when to just wait for rain
• Why air tightness may now matter more than insulation in well-built modern buildings
• Where to find free ORNL calculators for estimating energy savings from cool roofs and air tightness improvements

Key Timestamps

[0:00] Introduction to the episode and guest Andre Desjarlais, Program Manager for the Building Envelope and Materials Research Program at Oak Ridge National Laboratory

[2:30] Andre's background — 35 years at ORNL, 15 years as an energy consultant, and early work with the aerospace industry during the space program

[5:00] Overview of Oak Ridge National Laboratory — history, size, mission, and its role within the 17 DOE national laboratories

[10:00] How ORNL tests building energy performance — from material-level samples to full-scale buildings — and why combining modeling with physical testing produces more reliable results

[15:30] How roof color and solar reflectance work — the science behind why darker roofs absorb more energy and what that means for heating and cooling costs

[20:00] Real dollar estimates for residential and commercial buildings — what cool roofs can save in southern versus northern climates

[27:00] How cool roofs affect peak electricity demand charges — and why this benefit extends even into northern climates

[34:00] Urban heat island effect — how widespread cool roof adoption could lower air temperatures in cities and reduce air pollution

[38:00] How reflectivity degrades over time — dirt, biological growth, coating chemistry, and why PVDF outperforms shorter-life coatings

[46:00] Air tightness as a top energy priority — why recladding projects are the right time to add air barriers and how to use ORNL's free savings calculators

Mentioned Resources

• Cool Roof Rating Council — database of solar reflectance ratings for roof systems, including aged (3-year) performance data: coolroofs.org
• ORNL Air Tightness Savings Calculator — free tool for estimating energy savings from improved air tightness: ornl.gov/content/air-tight-savings-calculator
• ORNL Cool Roof Calculator — free tool for estimating energy benefits of installing a cool roof: coolroof.ornl.gov
• Andre Desjarlais, ORNL — available for direct questions via email: desjarlais@ornl.gov
• McElroy Metal Website
• Connect with the host Kathi Miller on LinkedIn

Episode Deep Dive

Most building owners think about roof color as an aesthetic decision. Andre Desjarlais would like to change that.

Desjarlais is the Program Manager for the Building Envelope and Materials Research Program at Oak Ridge National Laboratory, the Department of Energy facility outside Knoxville, Tennessee, where roughly 250 of its 7,000 employees focus on buildings. He spent 35 years at ORNL after a career as an energy consultant — work that started in aerospace during the space program and shifted to buildings after the energy crisis of the late 1970s. On a recent episode of the Building with Metal podcast, he broke down what the research really shows about roof reflectivity and the numbers that are worth paying attention to.

How Does Roof Color Affect Solar Reflectance and Energy Absorption?

Solar reflectance is a number from zero to 100 percent. A surface with zero reflectance absorbs every bit of solar energy that hits it. A surface at 100 reflects all of it. Nothing in the real world hits either extreme, but the difference between a dark roof and a light one is striking.

"A dark colored surface, say a dark brown or a black, would have a solar reflectance of about ten percent, which means that ninety percent of that energy is absorbed," Desjarlais explained. A bright white surface, by contrast, reflects 80 to 90 percent of solar energy before it can enter the building.

For building owners in the South, that absorbed energy is a direct driver of cooling costs. In northern climates, the logic flips — absorbing heat in winter can offset heating bills. The commercial building energy code reflects this geography, granting solar reflectance credits in climate zones one through three, which roughly corresponds to everything south of the Georgia-Tennessee line. Northern building codes do not currently credit roof color.

What Are the Real Dollar Savings from a High-Reflectance Roof?

Desjarlais runs the math on a two-thousand square foot two-story home in a hot southern climate like Houston or Tampa. The savings from switching to a high-reflectance roof run to a couple hundred dollars per year. For a twenty-thousand square foot commercial strip mall in the same market, that figure climbs to around two thousand dollars annually — and the gap is partly explained by insulation requirements.

Commercial buildings are required to have less insulation than residential buildings because the type of insulation needed for commercial construction (foam plastic) costs significantly more per R-value than the loose fiberglass used in residential attics. Because insulation is cheaper in homes, codes require more of it, which in turn reduces the marginal benefit of roof color. Owners of older residential buildings — built before today's insulation standards — see even larger savings from cool roofing because the envelope has more room to improve.

How Does Cool Roofing Reduce Peak Electricity Demand Charges?

Energy cost savings are only part of the story. Desjarlais pointed to more recent research looking at how cool roofing affects peak electricity demand — the highest draw a building places on the grid during any fifteen-minute window in a billing period. Commercial electricity customers are increasingly billed for that peak demand separately from total usage, and the charges are substantial.

"In the south, the reduction in demand and the reduction in the cost that the building owner has to pay for demand is almost equal to the energy savings that the cool roof supplies. So it's not like a ten percent number. It's half," he said.

What surprised Desjarlais was what the research found further north. Even in Minneapolis, where cool roofs have traditionally been dismissed as irrelevant or counterproductive, peak demand charge reductions showed up. Building owners in northern climates may not save energy with a high-reflectance roof, but they can reduce the cost they pay for peak demand — a meaningful distinction when half the electric bill may be demand-driven.

What Impact Do Cool Roofs Have on Urban Heat Islands?

One building switching to a cool roof has a negligible effect on city temperatures. But at scale, the math changes. Research suggests that if half the buildings in a dense urban area adopted cool surfaces, measurable reductions in ambient air temperature would follow. Lower air temperatures reduce cooling loads across the entire city, slow smog formation, and improve air quality — benefits that are real but hard to assign a dollar value to.

How Does Roof Reflectivity Change Over Time — and What Coating Lasts Longest?

New products and aged products are not the same, and Desjarlais emphasized that buyers focused on reflectivity should understand what happens after year one. Two forces degrade solar reflectance over time. The first is surface fouling — dirt, dust, and biological growth. In hot, humid climates, biological growth can cause a significant and rapid drop in reflectance. In drier environments, the effect is slower and rain often recovers much of the lost performance on smooth surfaces.

"On a smooth surface, like a painted metal, rainfall can remove a lot of the dirt. It won't remove the biological growth, but it will remove a lot of the dirt," he noted.

The second factor is the coating chemistry itself. Many commercial roof coatings are designed for a seven-to-ten year service life. As they age and degrade, their reflectance falls. PVDF coatings — used by manufacturers including McElroy Metal on painted panels — have much longer service lives and show far less degradation over the same time period. Their smooth surface also sheds dirt more readily, which helps maintain reflectance between rain events.

The Cool Roof Rating Council maintains a database of solar reflectance values for roof products, including measurements taken after three years of field aging. For any buyer prioritizing long-term reflectivity, those aged ratings are the ones that matter.

Does Cleaning a Cool Roof Improve Energy Performance?

Desjarlais has studied rooftop cleaning economics directly, and his finding is counterintuitive: for most applications, the cost of cleaning a roof exceeds the energy benefit of cleaning it. The exception is hot and humid climates where biological growth causes heavy reflectance loss. In those cases, cleaning is worth considering. For dust and general dirt, waiting for rain is the more economical strategy.

Why Is Building Air Tightness Now a Bigger Energy Factor Than Insulation?

As insulation requirements in building codes have risen over the past fifteen years, U.S. construction has largely caught up to European energy standards. But Desjarlais pointed to a factor that now matters more than insulation level in well-built buildings.

"What now seems to be the biggest energy consumer for buildings is how airtight it is. The payback for improving air tightness is typically a couple of years," he said.

For any project that involves recladding — when the wall assembly is already open — adding an air barrier and a water-resistant barrier is a logical and cost-effective upgrade. ORNL offers a free online calculator at ornl.gov that allows building owners to input their building size, location, and energy costs to see projected dollar savings from air tightness improvements. A separate cool roof calculator is available at coolroof.ornl.gov.

Both tools are free, and Desjarlais is reachable directly at desjarlais@ornl.gov for questions — though he notes a hundred emails a day means some patience is required.