Reflective roofs incur a winter heating penalty in mixed climates, but it’s typically smaller than the summer cooling savings because roofs in northern U.S. latitudes get about 3-5X as much daily sun in summer than in winter. This makes annual heating, ventilation, and air conditioning (HVAC) energy savings positive for cool roofs and cool walls in most buildings in U.S. climate zones 1 – 5 (Figure 7, Rosado et al. 2019). Snow on roofs (not considered by Rosado et al. 2019) can substantially decrease the winter heating penalty because all roofs become white when covered with snow (Hosseini et al. 2016).  The carbon offset of 7 kg CO2/m² per 0.01 albedo increase is a one-time benefit rather than an annual benefit stream. The Shine On Initiative’s technical working group is evaluating how to best annualize this offset.

High UV reflectance (UVR) can accelerate smog formation. However, the study by Epstein et al. (2017) cited in the Earth.com story mistakenly assumed that cool surface materials have high UV reflectance (UVR). UVR correlates much more with material than with solar reflectance. For example, anything colored with the ubiquitous white pigment titanium dioxide rutile has low UVR. Cementitious coatings, whether light (based on white cement) or dark (based on gray cement), have UVRs (300 – 350 nm) that are on average about 0.20 higher than those of other high-slope roofing products, and can be up to about 0.40 higher. Ultrawhite barium sulfate pigments can dramatically increase UVR (Li et al. 2021) and should not be used in smog-prone regions. The real opportunity to reduce UVR-stimulated formation of ozone by roofing is to avoid the use of cementitious color coatings on concrete roofing tiles (or the use of bare concrete roof tiles, though those are uncommon). Concrete tiles can be finished with polymeric color coatings instead to mitigate this issue. The same approach applies to pavement coatings.

Local cooling is usually gauged by decreases in building envelope temperature, pavement surface temperature, indoor air temperature (in an unconditoned space), building energy use (in a conditioned space), and near-ground outside air temperature (say, at 2 m above ground level). Local cooling of outside air results mostly from reductions in the convection of heat from building envelope surfaces and pavement surfaces, while atmospheric cooling results derive mostly from reductions in the emission of long-wave (thermal infrared) radiation from these surfaces. Please see figure “The Albedo Effect” on p. 11 of Global Cool Cities Alliance (2012).

There are a variety of solar reflectances in pavement products (e.g. concretes with more reflective mortar cement, supplemental cementitious materials (SCMs), and aggregates) that have been used routinely in northern climates. Tokyo has done longitudinal studies of various cool pavement interventions over 10 years, but they do not have the temp variation of NYC.  In addition, some cool pavement coating products have been studied in freeze-thaw climates.

The work to improve the efficiency and climate-friendliness of active cooling falls in this category. See Clean Cooling Collaborative. We have decades of studies, models, and field examples of how passive cooling reduces energy demand and thus greenhouse gas impact. In addition, cementitious concretes, some of which can be made quite reflective, as noted in the previous answer, have the benefit of slowly absorbing CO2.  However, the CO2 uptake is a slow process occurring over decades, and the conventional production of portland cement is a significant contributor to GHG generation, so care must be taken to consider the use of less carbon-intensive components (for example, IMPACT | Blue Planet Systems).

The materials used in producing many cool surface products are quite similar to those used in conventional surface products, for example, often differing only in the pigments employed or the design.  Properly selected and designed, they should have similar risk profiles to those of the materials they replace. As always, however, we recommend consulting the manufacturer’s Safety Data Sheet for product-specific safety information.

Forever chemicals such as per- and polyfluoroalkyl substances (PFAS) would not be expected to enhance the reflective properties of a material. However, PFAS materials are surfactants that can alter surface properties such as soiling, and they can also be used as processing additives.  As Safety Data Sheets do not disclose all formulation information (e.g. for components comprising less than 1% of a product’s formulation), you should check with a specific product’s manufacturer to validate that their product does not contain PFAS materials.

Yes, and depending on the local climate, this can provide a significant energy-saving benefit as well. There are various products, such as films, that can be applied to the interior or exterior of glass to reflect portions of the solar spectrum, such as damaging UV radiation and invisible IR radiation while allowing much of the visible portion to transmit through the glass. The National Fenestration Rating Council (NFRC) is an independent non-profit organization that establishes objective window, door, and skylight energy performance ratings to help compare products and make informed purchase decisions. (Also see Development and Start of Shipping of Solar Control Window Film Albeedo with Improved Installability and Retro-Reflectivity | Dexerials)

We have done a disservice by separating mitigation from resilience/adaptation investments. Why it happened makes sense: We can unitize and then monetize a mitigation investment much easier than we do with adaptation. Ideally, we’d have separate regimes, but the reality is that we need funding now to protect people, and mitigation financing (95% of all climate finance) is leading the path forward.