The Science of Cool Roofs: What Roof Membrane Reflectivity Does for Energy Savings

Imagine it’s the peak of summer, and the temperatures outside are above 90° Fahrenheit. While your building has air conditioning, employees and guests still feel the brunt of the heat. Work slows down, and your HVAC units run nonstop but can’t keep up. What if your building’s dark roof is the culprit behind the extreme heat in your indoor areas?

On a hot summer day, a standard dark roof can reach 150° Fahrenheit or higher. This heat doesn’t just stay on the roof, but it transfers to the building and creates a “radiator effect”.

The good news is that with cool roofs — roofing that reflects sunlight and absorbs less heat — your buildings stay cooler, and you save energy by not running your HVAC units at maximum power. In this guide, we will explore the science of roof membrane reflectivity and how it powers an energy-efficient roofing system.

What Are Roof Membrane Reflectivity, Solar Reflectance Index and Thermal Emittance?

Before getting into the benefits of cool roofs, let’s unpack the science behind roof membrane reflectivity, the Solar Reflectance Index (SRI), and thermal emittance, and how they work together to make cool roofs work.

Roof Membrane Reflectivity and SRI

Roof membrane reflectivity, or solar reflectance, is a roof’s ability to reflect a fraction of solar energy — UV, visible and infrared — and other radiation away from a building. The Solar Reflectance Index (SRI) scale, ranging from 0 to 100, measures reflectivity. The higher the SRI, the better your roof will be at bouncing sunlight off and cooling your building. Standard black roofs typically have an SRI of 0, meaning they absorb most of the solar energy, while standard white roofs have an SRI of 100 or higher.

Though there is no definitive standard or rules for cool roofs at the federal level, a roof can be considered a ‘cool roof’ if it has an SRI high enough to reflect sunlight and heat effectively, resulting in lower energy use. The minimum SRI value varies by roof slope and building standards, but according to the U.S. Green Building Council, Inc., low-sloped roofs require an SRI of 82, while steep-sloped roofs require an SRI of 39.

Thermal Emittance

Thermal emittance is the roof’s ability to radiate the heat it has absorbed back into the atmosphere. Think of it like wearing the same color of a breathable fabric and a rubber suit. Even if they are the same color, the breathable suit will release heat more quickly due to its high thermal emittance.

The Thermal Emittance Scale ranges from 0 to 1. The higher the number, the better your roof will be at releasing absorbed heat and cooling itself. Cool roofs also require high thermal emittance, with various organizations recommending 0.75 as a general minimum. However, this may vary depending on roof surface, slope type and other factors.

Solar Reflectance vs. Thermal Emittance

Solar reflectance and thermal emittance may seem similar at first, but they serve distinct functions that make a cool roof work. Think of solar reflectance as a roof’s ability to stop heat from getting in, while thermal emittance is a roof’s ability to get heat out. You need a high combination of both to achieve excellent commercial flat roof cooling.

To better understand the importance of both, let’s look at how different combinations of reflectivity and emittance perform in the real world:

  • Low SRI plus low emittance: This is the worst-case scenario for your building, as it absorbs most of the sunlight and heat without letting them return to the atmosphere. This often results in the highest possible surface temperature and, in turn, an extremely hot indoor temperature.
  • High SRI plus low emittance: These roofs reflect sunlight like a mirror but can’t shed heat efficiently. This can keep the indoors uncomfortably hot. One common example is aluminum roofing.
  • Low SRI plus high emittance: These roofs can retain heat, but because the sun adds heat faster than they can release it, they stay hot and can keep buildings warm.
  • High SRI plus high emittance: For a truly efficient and functioning cool roof, you need both SRI and thermal emittance at high values so they can work together to keep heat out of your buildings and return any heat stored in your roof to the atmosphere.

How High Reflectivity Reduces Cooling Loads

The science behind a roof with high reflectivity or SRI and your energy bills is simple — as heat is reflected away from your roof and your building, your HVAC system doesn’t have to keep up with the high temperatures. This translates to a cooler, more energy-efficient roofing system that’ll also promote better comfort and safety for your people.

Another added benefit of reflective roofing is reducing the urban heat island effect, which is a phenomenon that can raise ambient temperatures in your community when clustered buildings absorb heat. By bouncing heat back into the atmosphere, it won’t be trapped in your building. You can contribute to a cooler, more sustainable environment for you and your surrounding community.

Long-Term Membrane Durability Benefits

While a cool roof delivers substantial improvements in your building’s cooling and energy use, it also benefits the roof itself. With a sustainable roof and a combination of lower surface temperatures and UV radiation avoidance, you invest in your asset’s value and longevity while also saving on your monthly utility costs.

Lower maintenance and upkeep are also advantages of cool roof membranes. A standard black roof experiences wide temperature swings during a hot summer day and a breezy night, which can stress the material, cause damage and increase the risk of leaks. A cool roof stays much closer to ambient air temperature and, in turn, experiences much less expansion and contraction. This chemical stability contributes to a longer roof lifespan and less need for maintenance.

Start Saving Energy and Extending Roof Life With a Cool Roof Installation

A cool roof goes beyond saving on energy and cooling your building. It’s also a long-term investment that provides your employees and customers with a safe environment while enhancing your roof’s lifespan and the value of your entire asset.

Ready to take the proactive step toward a cooler and more energy-efficient building? Turn to DDP Roofing Services, Inc. for your cool roof installation. We offer extensive roof services, from expert removal and replacement to biannual preventive maintenance to ensure your investment stays efficient and durable for the long haul. Contact us for a consultation or call us at (888) 552-6220 for a quote today!

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