Evaluation of Glass Bubbles for Solar Heat Reflection in Waterborne Acrylic Elastometric Roof Coatings

By Kevin Rink , Andrea Charif Rodriguez

The expanding residential building market has placed a higher demand on electricity for cooling in warmer climate regions. This demand has created opportunities for coatings companies to investigate solar heat reflective coatings as a means to combat the increasing energy costs. Two popular areas being investigated in the architectural arena are roof and wall coatings. Typically, solar heat reflective coatings are characterized by a high solar reflectance and high emittance values in the thermal infrared region.

Waterborne white elastomeric and aluminum pigmented asphalt are two types of coatings used for this evolving market. There are many binder types within the elastomeric class. This study focuses on utilizing a 100% acrylic-based binder in a white pigmented formulation. The spectral distribution for solar irradiance is divided into three regions: UV (200–400 nm—5% of sunlight energy), Visible (400–700nm—45% of sunlight energy) and Near-IR (700–2500 nm—49% of sunlight energy and felt as heat). Approximately 96% of the sunlight’s radiation falls in the 400–2500 nm range, so analysis of the data in this region is of particular interest. Solar reflectance values are typically >80% for coatings formulated specifically as “cool” roof paints, which means they absorb and/or transfer <20% of the incident energy.

The thermal emittance is a measure of how easily a surface will give up heat, and a high emittance surface will give off heat more readily and thus reach equilibrium at a lower temperature. This makes it desirable to also have a high emittance value for exterior coatings. The total solar reflectance (TSR), is a weighted average of how well a material reflects energy at each specific solar energy wavelength. ASTM standard C1483 defines an RCC (radiation control coating) as a liquid applied coating having a solar reflectance of 0.8 and an ambient temperature infrared emittance of at least 0.8. Emittance and total solar reflectance properties are used together to calculate a solar reflectance index (SRI), which is typically zero for a black surface and 100 for a white standard.

SRI values can exceed 100 by definition in the calculations for cool materials. The SRI values can be entered into standard energy calculator cost modules to calculate overall potential energy savings. Other factors have to be taken into consideration when using these calculators, such as insulation values, geographical region, and current energy costs, to name a few.