What a Cool Roof Is, and What It Does on an Upstate Building

Across the Upstate, the roof takes full sun from late spring into September. If you are replacing a roof anyway, the surface you choose decides how hot the attic gets for as long as that roof is on the house. That is the moment to ask about a cool roof. The Department of Energy's consumer guide adds a caution worth keeping: it is rarely economical to replace a mechanically sound roof just to increase its solar reflectance.

The site's post on why metal roofs suit Anderson heat makes the case for light colors. This guide adds the ratings behind that case, the difference between bare and painted metal, and the trade-offs the post does not cover.

Anderson Metal Roofing and Shingles offers residential metal roofs in a wide range of colors, and its commercial metal page lists wall panels in various finishes, colors and protective coatings.

The Cool Roof Rating Council (CRRC) states it plainly: the two basic characteristics that determine the coolness of a roof are solar reflectance and thermal emittance, and both are measured on a scale from 0 to 1, where 1 is 100 percent reflective or emissive.

Solar reflectance is the fraction of sunlight the surface bounces away. The DOE's cool roof guidelines say most dark roof materials reflect 5 to 20 percent of incoming sunlight, while light-colored roof materials typically reflect 55 to 90 percent. Reflectance has the biggest effect on roof temperature.

Thermal emittance is how efficiently the surface cools itself by radiating the heat it has absorbed. The DOE notes that nearly all nonmetallic surfaces have high emittance, usually between 0.80 and 0.95, while bare, shiny metal has low emittance and stays warmer. A bare metal surface that reflects as much sunlight as a white surface will still run warmer in the sun because it emits less heat.

Some programs combine the two into a Solar Reflectance Index (SRI); in the DOE's example, a clean black roof scores near 0 and a clean white roof near 100.

Metal is the interesting case because the two numbers pull in different directions. The DOE guidelines put it this way: unpainted metals are usually good solar reflectors but poor thermal emitters, so they rarely satisfy low-slope cool roof requirements, though some may still have a high enough SRI to count as cool on a steep roof.

Paint changes that. The same DOE document says paint applied at the factory or in the field can increase a metal roof's solar reflectance and thermal emittance, allowing it to achieve cool roof status, and that metal roofs coated with fluoropolymer- or silicone-polyester paints come in many colors that reach cool roof performance. The Metal Roofing Alliance describes the mechanism: cool pigments in the paint raise solar reflectance and lower the surface temperature of the roof.

A cool roof does not have to be white. The CRRC explains that many cool color products use darker pigments that are highly reflective in the near infrared, the part of sunlight you cannot see. So a dark bronze or charcoal metal roof can be a cool roof if the pigment is right, and a bare silver panel may not be. The painted vs bare Galvalume guide goes further into that choice.

The CRRC runs the rating program that product labels and most codes rely on. A few rules make the numbers useful instead of confusing.

• Initial and aged values. Products are tested new and again after three years of outdoor exposure, because dirt and weathering lower reflectance. The DOE says to use the aged values when estimating savings, since most roofs are not washed often.
• Listed is not the same as cool. The DOE guidelines note that any product tested by a CRRC-accredited lab can be listed, and being listed does not imply that a product is cool. You have to read the numbers against whatever code or program you care about.
• The CRRC does not pick products. Its own FAQ says it does not recommend specific products or product types. It publishes the Rated Roof Products Directory and leaves the choice to you, your contractor or a roof consultant.
• Ask for the exact color. Ratings are by product and color. Ask your installer for the CRRC listing of the panel or shingle in the color you want, and check both the aged reflectance and the emittance.
• Codes use different thresholds. Requirements vary by slope and program, and are less strict for steep roofs (DOE). Ask the Anderson County or City of Anderson building department whether any cool roof requirement applies to your project.

Older brochures and websites still mention ENERGY STAR roofing. That program is over. The ENERGY STAR site states that its specification for Roof Products was sunset on June 1, 2022, and EPA's notices say certification of new roof products ended on June 1, 2021 and brand owners had to drop the ENERGY STAR name and mark from roof products as of the sunset date.

The DOE guidelines also note that the old ENERGY STAR criteria considered only reflectance, not emittance.

What that means for you: a product described as ENERGY STAR rated is being sold on an expired label. The current references are the CRRC directory for the measured numbers and DOE guidance for whether they matter in your climate.

A cool surface is one layer of a system. The DOE guidelines are blunt: even though cool roofs reduce solar heat gains, they are never a substitute for sufficient thermal insulation, and a well-insulated roof will benefit less from a cool roof than a poorly insulated one. Attic ventilation does related work by moving hot air out before it soaks into the ceiling. The site's post on whether your roof can help you save on energy covers insulation, vents and radiant barriers.

Savings are real but climate-dependent. The DOE consumer guide says installing a cool color roof on a home can shave at least 5 to 20 percent from the cooling costs of your energy bill, that savings vary and are greatest in hot and warm climates, and that a cool roof may increase the need for heating energy in winter by reflecting sunlight that would otherwise warm the building. It adds that even in mixed climates, winter penalties can be smaller than summer savings. The CRRC's FAQ calls the winter penalty usually insignificant compared with summer savings, because winter sun is lower and days are shorter.

The Upstate has both a long cooling season and real winter heating, so neither number is the whole story for your house. If sustainability is the question, the site's post on whether metal roofing is environmentally friendly covers recycling and waste; this guide stays with heat.

A cool roof adds little if the roof is shaded most of the day, or if the attic is already well insulated and vented. The DOE notes that cool roofs add only marginal savings to buildings that are already highly efficient. Appearance rules matter too: the DOE lists ordinances that restrict roof color among the questions to settle first, and HOA covenants can limit colors too, so check yours before you pick one.

If your roof is sound, do not replace it for reflectance alone. Keep the idea for the next replacement. If you are already comparing materials, the metal roof vs asphalt shingles guide puts heat alongside weight, slope, noise and repairs. For a low-slope commercial roof, the question is usually a coating or recover rather than new panels, and when to replace a commercial roof covers that decision.

When you ask for a quote, ask for the CRRC-rated colors available for the panel or shingle on offer. Anderson Metal Roofing and Shingles provides free quotes and verifies measurements on site; start with the quote form.