Lighter is better when it comes to roofing … right? That’s not always the case, as it turns out. Cool pigments can provide benefits across the roof color spectrum.
If you’re looking for a cool metal roof, you normally look for light colors that are as close to white as possible. "Traditionally, the lighter and closer to white that a roof color is, the higher the solar reflectance of that particular roof," says Nick Allen, owner of Manor Metal Roofs.
These light colors reflect the sun’s rays; therefore, less heat is conducted into the building. When less heat is conducted into the building, the building’s energy consumption is reduced because of the decreased cooling load.
Cool, But Not White
While you may be interested in the beneficial qualities that a light-colored roof would bring to your building, what if you’re not interested in having a light-colored roof?
All roof colors can receive the benefits of these traditionally cool roofs due to cool pigments. "Cool pigments boost the solar reflectance abilities of all metal roof colors – even the darker tones," says Allen. Because almost half of the sun’s radiation reaches us in the form of infrared (IR) energy, "these cool pigments have been formulated to reflect much of that IR energy, and have proven to be very effective with darker colors as well."
A dark metal roof will perform very differently than it would if it were coated with a "cool" pigment, even though the two look the same. "We have colors in a non-cool version and a cool version. They look identical, but perform very differently," says Thomas McKay, product manager at PPG Industries. "The cooler versions have a higher solar reflectance and are ‘cooler.’ "
Before cool coatings were available, McKay explains that metal roof manufacturers had standard color palettes of anywhere from 15 to 20 colors. "When cool roof coatings were introduced, we had to match their existing colors, but with higher reflectance in the IR spectrum of the sun’s energy. Metal roofs can offer the full range of colors currently available, and offer them as cool roofs."
White and other light colors are already highly reflective in the visible range, so the IR reflective pigments provide the most benefit to colors in the medium and darker color range.
Heat Island (Not in the Caribbean)
Cities have a higher temperature than suburbs as a result of rooftops and roadways absorbing the heat of the day and slowly releasing it at night (the heat island effect). Cool roofing can have a positive impact on reducing this effect. "Roofs that are cooler don’t get as hot as non-cool roofs (they absorb less energy/heat); thus, they don’t heat the surrounding microclimate as much," explains McKay. "Cool roofs reflect energy vs. non-cool roofs that absorb more and re-radiate it to their surroundings."
Another perk of the decreased cooling load is reduction of greenhouse gases, says Montreal-based Concordia University’s Hydro-Quebec Industrial Research Chair Professor Hashem Akbari. He explains that, if you have a cooler pigment that reflects solar radiation, the reflected solar radiation escapes the atmosphere and doesn’t contribute to global warming; however, if there’s a dark pigment that absorbs solar radiation, it becomes heat. When that heat escapes the earth’s atmosphere, it causes global warming.
"The initial cost of these coatings is minimal, and most metal roof manufacturers have adopted the technology across their full color lines, so you don’t have any limitations during color selection," says Allen. He also emphasizes that there aren’t any additional maintenance requirements beyond what’s typically necessary for non-cool pigment finishes.
Kylie Wroblaski (firstname.lastname@example.org) is associate editor for BUILDINGS magazine.