Cool roofing is a hot topic these days, having gained popularity as a green technology over the last couple of decades because of its promise of reducing building cooling energy costs by allowing HVAC units to operate more efficiently while also lowering the overall cost of building ownership.
A cool roof limits heat absorption by reflecting the sun’s heat and then releasing it back into the atmosphere. Cool roofs rely on a reflective roof surface to accomplish this, allowing for a more comfortable and controlled indoor environment in warm weather. They can also reduce local air temperatures (urban heat island effect).
Reflective roof substances are measured using the Solar Reflectance Index (SRI), which is important to understand when selecting the right roof coating. The SRI is a calculation method using the values of solar reflectance (fraction of solar energy reflected by the roof) and thermal emittance (relative ability of a roof to radiate absorbed heat) to calculate the roof's overall capacity to turn away solar heat. SRI values range on a scale from 0 to 100, with a higher SRI value indicating the coating has a greater ability to remain cooler in sunshine. For example, a clean black roof could have an SRI of 0, while a clean white roof could have an SRI of 100. Dark roofs usually have an SRI less than 20.
Effective White Coatings
Roof coatings are an important part of a building and probably one of the most “forgotten”. Often, roofs are overlooked until leaking or other problems occur.
There are many types of white roof coatings available in the market. A liquid-applied coating based on acrylic elastomeric polymer technology with white reflective pigments is commonly used. White membranes or other roofing systems can also be used for cool roofing and energy conservation, but acrylic elastomeric-based coatings with white or reflective pigments tend to have an optimum cost-performance ratio when it comes to labor and material. In low-slope roofs, the annual growth rate of coatings has been steadily increasing while membrane growth has been flat or declining over the last decade.
Protection and reflectiveness over the life of a roof coating are properties determined by the selection of binders, pigments and additives. Most white reflective water-borne liquid-applied roof coatings show good values of Solar Reflectance Index (SRI) when they are newly applied. Due to weather conditions, dust and other external factors, the white surface over time can become soiled, reducing drastically the initial reflectiveness and energy cost savings.
The selection of high performing water-borne binders and dispersants ensure a long-term reflectiveness and durability of the roof coating.
The proper dispersant improves dispersion and stability of the white pigments and fillers used in roof coating formulation. The benefits are translated into more cost-effective formulations by reducing the amount of white pigments and by using less expensive filler pigments. Solsperse™ W100 Hyperdispersant from Lubrizol is one such dispersant. It is a 40% active APE-free polymeric dispersant in water, which will improve dispersion and stability in aqueous coatings. Due to its excellent hydrophobicity, water sensitivity of roof coating formulations is improved. For occupied buildings sensitive to noxious odor, roof coatings with Solsperse W100 with the low VOC and low odor content is a good option. Solsperse W100 is a hyperdispersant that will enable robust roof coating formulations, reducing final cost and making them easier to apply and maintain.
Cool roof guidelines/definitions can vary by location to satisfy local building codes or meet rebate program requirements, but on a national level, the U.S. Department of Energy qualifies cool roofs in one of two ways (values are for low-sloped roofs):
- Meets or exceeds both the minimum solar reflectance (0.55) and thermal emittance (0.75) values.
- Meets or exceeds the minimum SRI requirement (64). This allows some roofs that have a low thermal emittance and a high solar reflectance (or vice versa) to still qualify as a cool roof.
Complying with LEED Standards
The SRI can be used in cool roof calculations to demonstrate compliance with certain standards, such as LEED (Leadership in Energy and Environmental Design)—which uses a minimum SRI value of 78 for low-sloped roofs. A number of local utility companies also offer rebates when approved roof coatings are installed.
While a reflective roof coating can certainly lower building cooling costs, it is important to factor in the building locale regarding the roof coating color that is selected. In some regions, cool roofs are required by legislation, but cool roofs don’t perform equally well everywhere. They are typically more suited to warmer climates where cooling costs tend to be higher.
The cost savings that cool roofs generally bring has encouraged the US Department of Energy (DOE) to move faster on other areas of the commercial building structure (building envelope), such as air filtration with air barriers to prevent unwanted air infiltration and exfiltration with the wall cavity. According to the DOE, air leakage accounts for 10% of the energy that buildings consume.
Whether it’s a reflective roof or other areas of the building, polymers and performance additives play an important role in creating an effective building envelope and reducing the amount of energy required to heat, cool and ventilate a building.
Contact us to learn more about a range of additives that can be incorporated into roof coatings to withstand harsh elements and promote reflectivity to deliver energy savings.
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