Just painting a roof can cut surface temperatures by 15 degrees — here's how heat-reflective coatings work

Can simply painting a rooftop lower surface temperatures in the middle of summer? As heat waves persist across the country, heat-reflective coatings — paints designed to repel sunlight and keep buildings from overheating — are drawing growing attention. The technology works on two main principles: boosting the ability to reflect sunlight in the first place, and quickly releasing any heat that has already been absorbed. Researchers are also developing a newer category called "radiative cooling coatings."

The key performance indicators for heat-reflective coatings are solar reflectance and thermal emittance, according to the paint industry. The coatings are engineered to reflect more sunlight than conventional paint — reducing the energy a building surface takes in — while also allowing heat that remains in the paint film to escape outward. The effect is similar to wearing a white shirt instead of a black one on a hot summer day.

When sunlight strikes an exterior wall or roof, some of it bounces off while the rest is absorbed by the surface. The absorbed solar energy converts to heat, raising the temperature of the roof and walls — which in turn drives up air-conditioning costs inside during summer.

Heat-reflective coatings reflect light more effectively than conventional paints because they use different materials. The coatings incorporate pigments that reflect near-infrared radiation and fine particles that scatter light, reducing the share of sunlight absorbed by the paint film. Typical ingredients include highly reflective pigments such as titanium oxide and ceramic or inorganic fillers.

Heat-reflective coatings tend to come in lighter colors, but white is not a requirement. Darker versions use specialty pigments that absorb some visible light to produce the desired color while still reflecting a relatively high proportion of near-infrared radiation. That is why two surfaces painted the same shade of gray or dark color can register different temperatures depending on whether a standard or heat-reflective coating was used.

The second principle is heat emission. Objects release energy in the form of infrared radiation, and heat-reflective coatings use resins and inorganic particles to enhance the paint film's ability to discharge residual heat as mid- to long-wave infrared radiation. The higher the thermal emittance, the more efficiently the surface sheds the heat it has retained.

Near-infrared radiation from the sun arrives at relatively short wavelengths — roughly 0.7 to 2.5 micrometers — while the thermal radiation a heated building surface emits back outward has longer wavelengths. "Combining acrylic silicone polymers allows the coating to release energy efficiently in the mid- to long-wave infrared range," an industry official said.

The operating principle of heat-reflective coatings can be summed up as reducing incoming heat and expelling the heat that does get in. Higher solar reflectance means the building absorbs less energy from the outset, while higher thermal emittance makes it easier to discharge whatever energy is absorbed. Together, the two mechanisms allow the building surface to reach thermal equilibrium at a lower temperature.

Sustained heat waves have sharpened market interest in the coatings. KCC said an analysis of inquiries received at its customer centers and sales teams nationwide from June through Wednesday found that questions about heat-reflective coatings had risen more than fivefold compared with the same period a year earlier. Sales through Wednesday were also up more than 60 percent year on year.

KCC said measurements taken as part of its "Cool Roof" support project showed a roughly 15-degree difference in exterior surface temperature before and after the coating was applied.

"Heat-reflective coatings are formulated with pigments that do not absorb near-infrared radiation and a particle structure that scatters light to send sunlight back outward, while exploiting the molecular vibration characteristics of resins and inorganic particles to release residual heat in the paint film as long-wave infrared radiation," an industry official said.