How cool are cool roofs?
This article will explore one aspect of high-performance buildings — cool roofs
- Understand what a cool roof is and its design and cost implications.
- Learn where cool roofs are most effective.
- Know about the important reasons for using cool roofs.
Cool roofs are an integral part of high-performance buildings and mitigating urban heat islands and climate change. High-performance buildings are those that have significantly lower energy consumption when compared with buildings that meet minimum code requirements, create a comfortable environment for occupants and are cost effective to operate.
Many factors can be attributed to higher performance buildings such as efficient heating, ventilation and air conditioning systems; sophisticated building controls for mechanical, electrical and plumbing systems; efficient envelope systems using materials with high thermal resistance or R-values for walls, windows and roofs; and ongoing monitoring and optimization, among other factors.
What is a cool roof?
A cool roof, as defined by the Berkeley Lab, is a roofing system that has the ability to reflect ultraviolet, visible and infrared wavelengths of light, thereby reducing the heat transfer to buildings. Cool roofs also have the ability to radiate absorbed or nonreflected light when compared to a standard roofing system design. They also can be known as a reflective roof membrane.
Cool roofs have a high solar reflectance or albedo that allows it to reflect ultraviolet, visible and infrared wavelengths of light, thereby reducing the heat transfer to buildings. In addition, cool roofs also have a high thermal emittance that allow them to radiate absorbed or nonreflected light when compared to a standard roofing system design. High solar reflectance and thermal emittance properties allow cool roofs to absorb less heat and help it remain 50°F to 60°F cooler than their conventional counterparts during the peak summer months.
The majority of roofs in the Unites States are dark colored where the temperature of the roof in the summer months can be as much as 100°F higher than ambient temperature. This heat is then transferred to the building, thereby increasing cooling demand and increased energy costs. In some cases, this heat transfer can also lead to thermal discomfort for the occupants if the HVAC systems were not designed to meet the cooling load.
Excessive heat and temperature variances can also damage the roofing materials and reduce their life, leading to increased renovation costs during the life span of the building. Consumers that reside in cities where a demand charge is included in their utility structure may see higher bills owing to the increased cooling demand as well as potential effects on resiliency due to additional strain on the power grid during peak cooling months. (The Global Cool Cities Alliance has an informative guide, A Practical Guide to Cool Roofs and Cool Pavements, that was developed to help speed a transition to cool roofs and pavements.)
One of the most common impacts of urban heat islands is increase in the energy consumption of a building by raising its temperature during the summer months, thus leading to an increased cooling demand. Darker colored roofs have been found to accelerate the problem of urban heat island effect because hotter roofs warm the air around it, thus increasing the localized ambient temperature around the building.
This has led to urban areas warming faster than the wild areas, leading to an earlier onset of spring, thus altering the cycles of budding flowers and release of pollen. This may have adverse impacts on birds, butterflies and other wildlife that rely on such eco-cycles for their survival. The urban heat island effect has also led to increased surface runoff water temperatures, which have been found to alter the temperature of streams and rivers that this water washes into, thus negatively impacting the aquatic species.
How do cool roofs affect energy consumption, costs?
Some studies have shown that darker roofs can lower heating demand in winter but increase cooling demand in the summer months. On the other hand, other projects also have shown that cool roofs can increase heating demand in winter, but lower cooling demand in the summer months. These studies have also noted that the energy savings in summer from cool roofs exceed the energy consumption increase in winters, thereby turning out to be more energy-effective throughout the year when compared to standard roofing designs.
However, the climate, heating energy source and demand charges also play a crucial role when determining the energy cost savings from cool roofs. Studies have found that cool roofs typically reduced energy costs for buildings, however this can increase energy costs for buildings in heating dominated climates that use electric heating with lower demand rates. Demand charges are fees — in addition to the actual consumption charges that utility companies charge — based on the peak usage, and rates can vary based on the time of the day.
This can be owed to the fact that gas prices, in general, are lower than electricity prices in the U.S. Buildings that use electric heating can see their cost savings diminish with cool roofs during the winter months because their electricity usage would increase with the additional heating load. However, if these same building had gas heating, the summer electricity cooling savings would outweigh the increased gas heating in the winter.
Are cool roofs addressed by codes?
Yes — cool roofs are addressed by local and international codes.
The International Energy Conservation Code and ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings prescriptively require buildings in climate zones 1, 2 and 3 have a minimum three-year aged solar reflectance of 0.55 and thermal emittance of 0.75 or solar reflectance index of 64. ASHRAE 90.1 also prescriptively requires increased roof insulation if the building does not meet these criteria. The International Green Construction Code also provides some definition for low- and steep-sloped cool roof performance requirements.
ASHRAE Advanced Energy Design Guides provide some recommendations (including the use of cool roofs) for increasing energy savings when compared to ASHRAE Standard 90.1.
California Title 24: Building Energy Efficiency Standards also include certain cool-roof credits and requirements. Los Angeles adopted an ordinance in 2013 that came into effect in 2015 requiring all new residential roofing material to have a cool roof rating.
Green building rating systems
The U.S. Green Building Council LEED rating system provides incentives to projects to use lighter color roofs under the Heat Island Reduction credit. Projects are incentivized to use lighter color roofs and hardscapes to minimize the heat island effect.
In addition, projects can increase the annual energy savings, which can increase the points earned under the Optimize Energy Performance credit where project teams are required to create an energy model for their building and show energy cost savings when compared to a baseline building. Project teams can earn additional points by showing incremental increases in energy cost savings. The energy model considers the location of the project, building envelope and MEP attributes and compares the proposed design to a minimum-code baseline building in accordance with ASHRAE 90.1 Appendix G. Therefore, using lighter color roofs can yield energy cost savings for the project, thus helping the project earn more points under the rating system.
Similar to the LEED rating system incentives for cool roofs, projects pursuing the Green Globes building certification also have an incentive to install cool roofs to gain points under the heat island reduction measure.
Using energy models
If project teams are unsure of what type of roof to install on their buildings to maximize energy and cost savings, project teams can consider creating an energy model and comparing different roof options to help inform their decision. Smith Seckman Reid’s sustainability team conducted a study to understand the performance of cool roofs in different climates and project settings, the results of which are summarized in Table 1.
For this study, an energy model was created in Trane Trace 700 for a typical office building with the following input assumptions.
This model was analyzed in eight cities located in climate zones (as defined by ASHRAE) 2A, 3A, 3B, 4A, 4C, 5A and 6A. These climate zones represent different climate settings ranging from hot-humid, hot-dry, mixed-dry, mixed-humid, marine-cold, cold and very cold weathers.
In each of these climate zones, roof colors, solar reflectance and thermal emissivity values were analyzed to understand the impact of cool roofs in each of the different climate zones.
|Roof color||Solar reflectance||Thermal emissivity|
To determine cost savings, Table 2 shows 2019 U.S. Energy Information Administration blended state average commercial utility rates were used to calculate energy costs. A blended rate is calculated using total annual cost (including consumption, demand and fees) divided by the total annual consumption.
Based on the results of the project team’s annual energy simulations, offices in every climate zone realized some savings with cool roofs when compared to the black roof option (see Table 3).
Recommendations for future studies
The project team’s analysis is limited to a typical office building located in different climate zones within the U.S. It is highly recommended that building professionals conduct a similar study for different types of buildings in different regions of the country to better understand how cool roofs may affect energy consumption and costs. For example, hospitals have a high internal load and are less affected by externalities such as building envelope, however, offices may be significantly impacted by building envelope characteristics, owing to plenum returns.
Therefore, different building types bring with them varied operating factors that may affect their energy consumption in conjunction with cool roofs. In addition, energy savings may differ with gas or electric heating sources in the different climate zones; savings are also affected by the cost of natural gas/electricity.