Cool roofs 101: Planning and specification considerations

by brittney_cutler | July 4, 2022 4:00 pm

[1]

By Kurt Shickman

Rising temperatures are quickly becoming a key challenge for cities and urbanized areas. The effects of heat have negative outcomes for health and well-being, social equity, energy use and peak demand, resilience of health, transportation and electrical infrastructure, crime, education, and productivity[2]. A study of nearly 1700 cities found these combined effects would cost a city which does not address heat approximately 1.7 percent of its annual economic output by 2050 and 5.6 percent by 2100[3]. This could be considered a tax for inaction on rising temperatures measured in the billions of dollars.

When it comes to high temperatures, there is a lot stacked against cities. Cities tend to be hotter than surrounding less-developed areas because they absorb more solar radiation, have less vegetation, release built-up heat more slowly, and generate more waste heat from vehicles and mechanical (or active) cooling. These factors contribute to a phenomenon called the urban heat island (UHI), wherein the annual mean air temperature of a city with 1 million people or more can be two to five degrees warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 22 degrees. This rise in average ambient temperatures is affecting cities in all climates (Figure 1) and increases the frequency, duration, and intensity of extreme heat waves. Since the 1980s, the number of days of extreme heat in the U.S. has nearly doubled (Figure 2).

[4]

The intensity and speed of these changes to urban environments will benefit from immediate action, particularly in built environments where people live, learn, and work. Transforming building design and material choices to focus on improving the ability of building occupants and communities, and to mitigate and manage the effects of rising temperature—often referred to as heat resilience—has started to take place in some urban environments. However, it has not reached anywhere near the scale needed to offset the effects of UHIs.

Solutions to improve heat resilience within building enclosures

There are solutions building experts can adopt today to minimize heat gain and reduce temperatures in buildings, in cities, and on the planet. These solutions can be considered in three basic categories.

Passive, non-mechanical cooling solutions, such as:

• Roof, wall, and pavement materials which reflect a greater portion of solar radiation than traditional materials (highly reflective surfaces).
• Expanded vegetated areas and tree canopies (shading, green roofs/walls, and permeable/pervious surfaces).

Heat-resilience planning, such as:

• Natural and human-made water features including lakes, rivers, and fountains (water infrastructure).
• Urban planning which minimizes heat buildup and retention (urban design).
• Passive cooling designs for buildings, like increased thermal insulation (passive building design).

Energy-efficient cooling solutions, such as:

• Energy-efficient cooling technologies and climate-friendly centralized cooling applications, including district cooling[5].
• Technologies designed to reduce waste heat exhaust, including electric vehicles.

[6]

Each of these solutions is an important part of an integrated strategy to build heat resilience. Cool roof and wall materials are widely available in North America and can cover nearly every type of roof and wall structure. Cool roof and wall solutions can be applied at various stages, including when a building is retrofitted, when a roof is being repaired or replaced, or simply over an existing functional roof or wall.

Importantly, for most roof and wall types, there is a cost-comparable cool version which can be specified with minimal maintenance costs. While cool walls are a part of any complete cooling strategy, this article will focus mainly on cool roofs.

How cool surfaces function

Cool surfaces work by reflecting solar energy rather than absorbing it. The reflected solar energy mostly passes out of the earth’s atmosphere and into space, creating a net cooling effect at the building scale and, when deployed widely, at community, city, and even global scales. Figure 3 summarizes the impact of shifting to a light-colored cool roof. At the building scale, solar-reflective roofs can reduce cooling energy demand by 10 to 40 percent. In winter, the heating penalty may range between five and 10 percent as a function of local climate and building characteristics (e.g. the amount of roof and wall insulation, window-to-wall ratio, etc.). In unconditioned structures, cool roofs result in five- to six-degree reductions in temperatures on the floor below the roof[7]. Cool walls provide similar benefits, at about 80 percent of the level generated by cool roofs.[8] At the building scale, cool roofs and cool walls can contribute to cool air temperature. A comprehensive literature review has found a 0.1 increase in solar reflectance results in a 0.5-degree reduction in average outdoor air temperatures and a 1.5-degree reduction in peak temperatures. Solar reflectance is measured on a scale from 0 to 1, so a 0.1 increase is similar to shifting from a dark- to a medium-grey color.

[9]

These potential effects on air temperature can influence the formation of ozone. A decrease in air temperatures tends to correlate with a reduction in the amount of ozone formed, and thus an overall improvement in air quality.[10] The relationship between air temperature and air quality is a complex one. Some of the air-quality improvements from reduced ozone formation may be offset because reduced air temperatures near the urban surface may slow wind speeds and vertical mixing of air with higher air levels, leaving some pollutants near the ground. That said, the amount of air-temperature reduction needed to trigger this effect at a significant level is not practically achievable by simply adopting cool roofs.

A recent analysis of the potential benefits of passive cooling in Los Angeles, California, highlights just how important even small changes in average air temperature can be. Researchers studying the potential impact of passive cooling on mortality during historic heat waves found the indoor and outdoor cooling resulting from highly reflective roofs and vegetation areas could have saved one out of four lives lost during these heat waves and would delay climate change-induced warming by 25 to 60 years[11].

A real-world case of the effect of large-scale deployment of cool surfaces comes from Almeria, Spain (Figure 4, page 26), which has a unique tradition of whitewashing its greenhouses in preparation for summer weather. Almeria has more than 27,113 ha (67,000 acres) of land area covered by greenhouses, making it one of the largest concentrations of greenhouses in the world. The region reflects substantially more sunlight than neighboring regions with fewer whitewashed greenhouses. A 20-year longitudinal study comparing weather-station data in Almeria to similar surrounding climatic regions found average air temperatures in Almeria have cooled 0.7 degrees, compared to an air temperature increase of 0.5 degrees in the surrounding regions lacking whitewashed greenhouses—a 1.2-degree difference.

Cooling buildings and cities directly addresses the many negative effects associated with rising temperatures. Valuing those many benefits shows every dollar invested in cool surfaces generates $12 in net economic gains. Beyond the economics, the negative effects of heat are overwhelmingly borne by low-income communities of color. Thus, a concerted effort to deploy cool roofs[12] (and other passive measures) will meaningfully contribute to efforts to improve social and racial equity.

Key factors to consider when planning to install a cool roof

While cool roofs are applicable and deliver benefits in all but the coldest climates, there are a few issues—all relatively minor—to consider when determining which type of roof to install.

Winter heating penalty

[13]

While a cool roof reduces cooling energy demand in the summer, it can increase heating energy demand in the winter when installed in some cold climates. This is known as the “winter heating penalty,” and though it is an issue to be factored into energy savings determinations, its effects are often greatly exaggerated for a number of reasons. In the winter, the sun is generally at a lower angle and days are shorter than in the summer months. In fact, in northern locations, winter solar irradiance is only 20 to 35 percent of what is experienced in the summer months, which means the sun has a reduced impact on roof surface temperature during the winter.[14] Heating loads and expenditures are typically more pronounced in the evenings, whereas the benefit of a darker roof in the winter is mostly realized during daylight hours. Further, many commercial buildings require space cooling all year, due to human activity or equipment usage, thereby negating what little, if any, heating benefit would be achieved by a dark roof.

Snow cover on roofs also affects the winter heating penalty. Snow has two impacts on the roof which are relevant to understanding the true impact of roof surface reflectivity on energy consumption. First, snow helps insulate the roof. As a porous medium with high air content, snow conducts less heat than soil. This effect generally increases with snow density and thickness. Second, at a thickness of about 101.6 mm (4 in.), snow will transform any roof into a highly reflective surface (approximately 0.6 to 0.9 solar reflectance). Researchers evaluated the impact of reflective roofs[15] on new and older-vintage commercial buildings in Anchorage, Alaska; Milwaukee, Wisconsin; Montreal, Quebec; and Toronto, Ontario, when snow cover is factored in[16]. The study finds “cool roofs for the simulated buildings resulted in annual energy expenditure savings in all cold climates.” The study also identified peak energy savings in addition to the base energy-efficiency gains.

Insulation

[17]

Another argument often heard against reflective roofing in cold climates is buildings in northern locations tend to have higher levels of roof insulation which reduce or negate the energy-saving impact of roof surface color. A field study and model analysis of black and white roof membranes over various levels of insulation by City University of New York, Princeton University, and Princeton Plasma Physics Lab showed the relationship between roof reflectivity and insulation is symbiotic, not a trade-off.[18] The Princeton papers highlight the interconnected role of reflectivity and insulation in roofing systems, and find reflectivity is the variable that minimizes heat flux during the summer, while insulation levels are the driving variable during the winter. In other words, to have a high-performance roofing system which minimizes heat gain in the summer and heat loss in the winter, buildings need both insulation and a highly reflective roof surface.

The researchers deployed high-resolution heat-flux sensors over and through various roofs on buildings inside the Princeton University campus. The buildings were very similar in design and usage, with the exception of different roof membranes (black or white) and thicknesses of polymer-based insulation (resistance values [R-values] from R-21 to R-48).

This study, which gathered measurements for one year, is one of the first to directly observe heat fluxes entering and leaving buildings. It is the first study to pair a field test with an evaluation of the same buildings using a finely resolved model to validate and more deeply understand the interaction between insulation levels and surface reflectivity.

Figure 5 shows the net heat entering (positive values) or leaving (negative values) the building by month for each roof. The findings are better observed on the two buildings with R-24 insulation levels but different membrane colors (LSBb is black and LSBw is white). These two roofs were installed at the same time on buildings which are close to one another. During the cold months, both the white and black roofs allow roughly the same amount of heat to escape from the building. In the summer months, however, the white roof has substantially less heat gain than the black roof and even maintains a negative heat flux in August and September. The benefits of cool roofs are still evident in the building with R-48 insulation (ADMw). There is a net reduction in heat flux in the roof with the highly reflective surface.

Moisture/condensation

[19]

Another consideration when installing highly reflective roofs in cold climates is the potential for moisture damage from condensation. It is important to note a properly installed code-compliant roof system should not have any issues with moisture damage. Most of the cases of damage described in the literature can be explained by installation errors and do not indicate a systemic problem with the use of cool roofs.

Over the course of a year in seasonal climates, moisture can build up in roof systems during cool periods and dry out during warm periods.[20] This process occurs regardless of roof color. Research indicates that although cool roofs may take a little longer to dry out than dark roofs, they also fully dry out, resulting in no net moisture buildup over yearly weather cycles.[21] As the U.S. Department of Energy has noted in reference to the potential which has been observed in both cool and dark roofs in cold climates, “the authors are not aware of any data that clearly demonstrates [sic] a higher occurrence in cool roofs.” This finding is borne out by field tests of a large number of buildings as well as through model-based research.

A roofing manufacturer and a prominent retail store chain undertook a field performance evaluation of single-ply white roofs in service for 10 to 14 years on 26 stores located in cold climates.[22] Two test cuts were made on each roof (and three test cuts on one roof) for a total of 53 samples, and moisture was present in two of them. One was determined to be the result of leakage from a nearby HVAC unit, and the other showed no signs of staining, mold, or deterioration which would indicate a long-term moisture problem.

In another case, researchers simulated the performance of several roofing systems—including typical, smart, and self-drying roofs for residential and commercial buildings in very cold climate regions—and found office buildings did not experience moisture accumulation problems during the simulation period (five years) using WUFI software modeling. The “smart roof” features related to venting and vapor retardation are covered in International Green Construction Code/American Society of Heating, Refrigerating, and Air-Conditioning Engineers (IgCC/ASHRAE) 189.1, Standard for the Design of High-Performance Green Buildings Except Low-Rise Residential Buildings. In a paper presented at the 2011 National Roofing Contractors Association (NRCA) International Roofing Symposium, the Single Ply Roofing Industry (SPRI) reported on a field survey and modeling studies to verify whether cool roofs were, in fact, susceptible to condensation issues[23]. The study was designed to achieve the greatest likelihood of observing condensation within the roofs. The roofs studied all consisted of a white roof membrane (aged two to 12 years) mechanically fastened over a single layer of insulation on a steel deck without a vapor retarder. The roofs were surveyed during February and March 2010 and were located in ASHRAE Climate Zone 5. Two test cuts were done on each of the roofs. All cuts were done in the morning, to minimize the impact of any heating of the roof surface which might have occurred under the afternoon sun. In seven of the roofs, there was no evidence of any moisture in the assembly. Though moisture was observed on the top face of the insulation and/or the underside of the membrane on three roofs, researchers noted no detrimental effects due to moisture in any of the roofs. WUFI modeling was performed for the 10 roofs included in the study, with simulations conducted for both a black and a white surface in each case. Although the modeling results showed all of the roofs would be subjected to condensation in the winter months, it predicted higher levels of condensation below a cool white membrane than below a black sheet. However, in all cases, for both white and black membranes, the modeling showed the resulting moisture would dry out completely in the summer months.

Cool roofs for the future

Despite ample bad news and dire predictions about the present and future climate, there are solutions available to meet these challenges. An abundance of cool roofing materials is readily available on the market, and at little to no incremental cost, depending on the product type. Building enclosure consultants, architects, specifiers, contractors, and builders can use the Cool Roof Rating Council’s (CRRC’s) online Products Directory to identify roofing products which meet a variety of needs, including:

• Complying with local energy code requirements and/or green building certifications.
• Qualifying for financial incentives (e.g. rebates, tax incentives, property assessed clean energy [PACE] efficiency financing).
• Decreasing indoor air temperature and increasing occupant comfort.
• Lowering the building’s cooling demand and utility bills.
• Reducing energy peak demand.
• Helping to mitigate the UHI effect by lowering ambient and surface temperatures.

Each of the products featured on the directory has a CRRC rating which indicates the radiative performance (solar reflectance, thermal emittance, and solar reflective index [SRI]) of the roofing product. Knowing the radiative performance of roofing products is important for understanding the roof’s impact on building energy use, occupant comfort, and the surrounding environment.

Author

Kurt Shickman is the former executive director of the Global Cool Cities Alliance, an organization that partners with Chemical Fabrics and Film Association (CFFA) to share the latest information on urban heat islands and cool roofing research and policy. The vinyl roofing division of the CFFA was created to educate architects, specifiers, building owners, and roofing contractors on the attributes of polyvinyl chloride (PVC) as a durable, reflective, heat-weldable material for single-ply roofing systems. Representing all the leading manufacturers of thermoplastic PVC (vinyl) roofing systems in North America, the division is committed to making available sound, scientifically backed information on the environmental and functional performance of energy-efficient PVC (vinyl) roofing membranes.

Source URL: https://www.constructionspecifier.com/cool-roofs-101-planning-and-specification-considerations/

---