There Is Evidence Cool Roofs Provide Benefits to Buildings in Climate Zones 4 through 8

FIGURE 1: Reflective roof requirements in ASHRAE 90.1 and IECC only apply in Climate Zones 1 through 3, shown here on the ASHRAE Climate Zone Map. SOURCE: U.S. Department of Energy

FIGURE 1: Reflective roof requirements in ASHRAE 90.1 and IECC only apply in Climate Zones 1 through 3, shown here on the ASHRAE Climate Zone Map. SOURCE: U.S. Department of Energy

Reflective roofs are a tried and true way to improve building energy efficiency and comfort, generate net energy savings and help mitigate summer urban heat islands. Reflective roofs work by reflecting solar energy off the roof surface, rather than absorbing the energy as heat that can be transmitted into the building and surrounding community.

The simple act of switching from a dark to a light-colored roof surface has a number of benefits. Buildings protected by these types of roofs require less energy to cool and help building owners and residents save money. Cool roofs on buildings without air conditioning can save lives during heat waves by lowering indoor temperatures. Cooler city air is safer to breathe and less polluted, which makes cities more livable and less vulnerable during heat waves. Increasing the reflectivity of urban surfaces can also offset the warming effect of green- house gases already in the atmosphere and help us address the challenges of climate change. Taken together, these benefits are worth billions of dollars to the growing number of people that live and work in U.S. cities.

The energy-savings case for cool roofs in warm climates is clear. Widely adopted model building-code systems, ASHRAE and the IECC, address roof reflectivity. ASHRAE 90.1-1999 added a credit for highly reflective roofs with IECC allowing compliance via ASHRAE in 2003. ASHRAE 90.1-2010 added reflectivity requirements for new and replacement commercial roofs in Climate Zones 1 through 3. IECC added the same requirements in its 2012 version. (Figure 1 shows the ASHRAE climate zone map for the U.S.)

There is, however, an ongoing debate about whether cool roofs deliver net energy benefits in northern climates that experience cold winters and warm to hot summers (Climate Zones 4 through 8). Do reflective roofs remain beneficial as the cold weather season kicks in? The same properties that allow reflective roofs to keep buildings cooler in the summer may also cause them to make buildings colder in the winter. Theoretically, buildings with cool roofs could require more energy to reach a comfortable temperature in winter—a consequence known as the “winter heating penalty.” Furthermore, building codes tend to require more roof insulation in colder climates than warmer climates, potentially reducing the energy-efficiency benefits of roof surface reflectivity.

FIGURE 2A: Annual energy-cost savings ($1 per 100 square meters) from cool roofs on newly constructed, code-compliant buildings with all-electric HVAC. SOURCE: Energy and Buildings

FIGURE 2A: Annual energy-cost savings ($1 per 100 square meters) from cool roofs on newly constructed, code-compliant buildings with all-electric HVAC.
SOURCE: Energy and Buildings

The “winter heating penalty” and the impact of insulation are considerations when installing reflective roofs in some cold climates, but their negative effects are often greatly exaggerated. The sun is generally at a lower angle and days are shorter in winter months than summer months. In fact, in northern locations winter solar irradiance is only 20 to 35 percent of what is experienced in summer months, which means the sun has a reduced impact on roof surface temperature during the winter. Heating loads and expenditures are typically more pronounced in evenings, whereas the benefit of a darker roof in winter is mostly realized during daylight hours. Many commercial buildings require space cooling all year because of human activity or equipment usage, thereby negating the little—if any—heating benefit achieved by a dark roof.

Two new studies, along with decades of real-world examples from the marketplace, indicate that reflective roofs are an effective net energy (and money) saver even in our coldest cities.

SNOW’S IMPACT

In a study recently published in Energy and Buildings, researchers from Concordia University in Montreal evaluated the energy-consumption impact of adding cool roofs to a number of retail and commercial buildings in Anchorage, Alaska; Milwaukee; Montreal; and Toronto. The researchers looked at older, less insulated building prototypes, as well as newer buildings built with code-compliant levels of insulation. Unlike earlier work evaluating the impact of roof reflectivity on building energy consumption in cold climates, this new analysis also accounted for the impact of snow on the roof during winter months.

FIGURE 2B: Annual energy-cost savings ($1 per 100 square meters) from cool roofs installed on older buildings with all- electric HVAC. SOURCE: Energy and Buildings

FIGURE 2B: Annual energy-cost savings ($1 per 100 square meters) from cool roofs installed on older buildings with all- electric HVAC.
SOURCE: Energy and Buildings

Snow has two impacts on the roof that 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, snow is white and, therefore, reflective. At a thickness of about 4 inches, snow will turn even a dark roof into a highly reflective surface (approximately 0.6 to 0.9 solar reflectance).

When snow is factored in, the benefits of cool roofs in cold climates be- come much clearer. Figure 2a shows the net energy savings and peak electricity reduction with and without snow for cool roofs installed on newly constructed, code-compliant buildings, assuming all-electric HVAC. Figure 2b shows savings from cool roofs installed on existing, older vintage buildings. The paper, available from the journal Energy and Buildings also includes results with gas HVAC systems.

INSULATION’S EFFECTS

Another argument often heard against reflective roofing in cold climates is that buildings in northern climates tend to have higher levels of roof insulation that reduce or negate the energy-savings impact of roof surface color. A new field study and model analysis of black and white roof membranes over various levels of insulation by the City University of New York and Princeton University and Princeton Plasma Physics Lab, the latter two of Princeton, N.J., clearly rebuts the “insulation versus reflectivity” tradeoff.

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Never Stop Learning

This year as I watch my friends and family send their little ones off to school, I, too, am starting a new educational journey. I’m taking piano lessons. I’ve wanted to play since I was a child but never had the opportunity. My husband heard me talk about wanting to play a few times, so he suggested giving me lessons and a piano as a gift for our first wedding anniversary.

I literally thought about it for a full day. I was completely touched that my husband wanted to help me accomplish a lifelong dream. However, did I really want to commit myself to something completely out of the ordinary? I learned to play the trumpet in middle school and played through high school, so I can read music—treble clef. I’ve never had to learn bass clef or how to make my left hand and right hand play different music at the same time. Could I do it? What if I’m the worst adult student my teacher has ever had?

I came to the realization that the accomplishments of which I’m most proud pushed me out of my comfort zone. Plus, how could I possibly say no to my husband when his gesture was so sweet? I’ve had one lesson so far and the idea of being able to coordinate my hands still seems a little like being able to rub my stomach while patting my head. However, I’m excited about the future and am hoping I’ll be playing well by the holidays!

Every issue of Roofing has an educational bent, but this issue may push you out of your comfort zone. For example, cool roofs have been a hot topic for many years. Conventional wisdom states cool roofs are not appropriate for northern climates. Kurt Shickman, executive director of the Washington, D.C.-based Global Cool Cities Alliance, will challenge that notion in “Cool Roofing”. He presents new evidence from several scientific studies that demonstrate cool roofs provide benefits to buildings in Climate Zones 4 through 8.

Meanwhile, Thomas W. Hutchinson, AIA, FRCI, RRC, CSI, RRP, principal of Hutchinson Design Group Ltd., Barrington, Ill., and a member of Roofing’s editorial advisory board, shares his in-the-field experiences regularly. He notes in “From the Hutchinson Files” that code-mandated insulation thicknesses are forcing designers to take roof access door and clerestory sill details seriously. Hutch’s goal with his article is to give designers some confidence to create appropriate design and detailing solutions.

These articles may challenge what you’ve always done but they’re worth considering and discussing. In fact, I’d really like to hear what you think about them. In return, I’ll keep you updated on whether I’m becoming the next Chopin!

White Paper Identifies Appropriate Mean Reference Temperature Ranges and R-values of Polyiso Roof Insulation within this Range

A number of recent articles have explored the relationship between temperature and R-value with an emphasis on the apparent reduction in R-value demonstrated by polyisocyanurate (or polyiso) roof insulation at cold temperatures. The science behind this apparent R-value decrease is relatively simple: All polyiso foam contains a blowing agent, which is a major component of the insulation performance provided by the polyiso foam. As temperatures decrease, all blowing agents will start to condense, and at some point this will result in a marginally reduced R-value. The point at which this occurs will vary to some extent for different polyiso foam products.

a mean reference temperature of 40 F is based on the average between a hot-side temperature of 60 F and a cold-side temperature of 20 F.

A mean reference temperature of 40 F is based on the average between a hot-side temperature of 60 F and a cold-side temperature of 20 F.

Because of this phenomenon, building researchers have attempted to determine whether the nominal R-value of polyiso insulation should be reduced in colder climates. Because of the obvious relationship between temperature and blowing-agent condensation, this certainly is a reasonable area of inquiry. However, before determining nominal R-value for polyiso in colder climates, it is critical to establish the appropriate temperature at which R-value testing should be conducted.

TO DETERMINE the appropriate temperature for R-value testing of polyiso, it is important to review how R-value is tested and measured. Figure 1 provides a simplified illustration of a “hot box” apparatus used to test and measure the R-value of almost all thermal-insulating materials. The insulation sample is placed within the box, and a temperature differential is maintained on opposing sides of the box. To generate accurate R-value information, the temperature differential between the opposing sides of the box must be relatively large—typically no less than 40 F according to current ASTM standards. The results of this type of test are then reported based on the average between these two temperature extremes, which is referred to as mean reference temperature. As shown in Figure 1, a mean reference temperature of 40 F is based on the average between a hot-side temperature of 60 F and a cold-side temperature of 20 F. In a similar manner, a mean reference temperature of 20 F is based on a hot-side temperature of 40 F and a cold-side temperature of 0 F.

NOW THAT we’ve had an opportunity to discuss the details of R-value testing, let’s apply the principles of the laboratory to the real-world situation of an actual building. Just like our laboratory hot box, buildings also have warm and cold sides. In cold climates, the warm side is located on the interior and the cold side is located on the exterior. If we assume that the interior is being heated to 68 F during the winter, what outdoor temperature will be required to obtain a mean reference temperature of 40 F or 20 F? Figure 2 provides a schematic analysis of the appropriate mean reference temperature.

As illustrated in Figure 2, the necessary outdoor temperature needed to attain a 40 F mean reference temperature would be 12 F while an outdoor temperature as low as -28 F would be needed to obtain a 20 F mean reference temperature. And herein lies a glaring problem with many of the articles published so far about the relationship between temperature and R-value. Although a 20 F or 40 F “reference temperature” may sound reasonable for measuring R-value, average real-world conditions required to obtain this reference temperature are only available in the most extreme cold climates in the world. With the exception of the northernmost parts of Canada and the Arctic, few locations experience an average winter temperature lower than 20 F.

schematic analysis of the appropriate mean reference temperature.

A Schematic analysis of the appropriate mean reference temperature.

To help illustrate the reality of average winter temperature in North America, a recent white paper published by the Bethesda, Md.-based Polyisocyanurate Insulation Manufacturers Association (PIMA), “Thermal Resistance and Temperature: A Report for Building Design Professionals”, which is available at Polyiso.org, identifies these average winter temperatures by climate zone using information from NOAA Historical Climatology studies. As shown in Table 1, page 2, the PIMA white paper identifies that actual average winter temperature varies from a low of 22 F in the coldest North American climate zone (ASHRAE Zone 7) to a high of 71 F in the warmest climate zone (ASHRAE Zone 1).

In addition to identifying a realistic winter outdoor average temperature for all major North American climate zones, Table 1 also identifies the appropriate mean reference temperature for each zone when a 68 F indoor design temperature is assumed. Rather than being as low as 40 F or even 20 F as sometimes inferred in previous articles, this mean winter reference temperature varies from a low of no less than 45 F in the coldest climate zone to above 50 F in the middle climate zones in North America.

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Vapor Retarders

The need for, use and design of a vapor retarder in the design of a roof system used to be a hotly debated topic. It appears now—when vapor retarders are needed more than ever—the design community seems to have lost interest, which is not good, considering how codes and standards (altered through concerns for energy savings) have changed how buildings are designed, constructed and operated. Most notably, positive building pressures are changing the game.

If not controlled, constructiongenerated moisture can have deleterious effects on new roof systems.

PHOTO 1: If not controlled, construction-generated
moisture can have
deleterious effects on new
roof systems.

A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system, where it can condense. Everyone knows that water in roof systems is never a positive. Typically, a vapor retarder has to have a perm rating of 1.0 or less to be successful. Through my recent observations, the lack of or poorly constructed vapor retarders contribute to ice under the membrane, soaked insulation facers, destabilized insulation, rusting roof decks, dripping water down screw-fastener threads, compromised fiber board and perlite integrity, mold on organic facers and loss of adhesion on adhered systems, just to name a few. Oh, and did I fail to mention the litigation that follows?

The codes’ “air-barrier requirements” have confused roof system designers. Codes and standards are being driven by the need for energy savings and, as a consequence, buildings are becoming tighter and tighter, as well as more sophisticated. This article will discuss preventing air and vapor transport of interior conditioned air into the roof system and the need for a vapor retarder. The responsibility of incorporating a vapor retarder or air retarder into a roof system is that of the licensed design professional and not that of the contractor or roof system material supplier.

It should be noted that all vapor retarders are air barriers but not all air barriers are vapor retarders. In so much that the roof membrane can often serve as an air barrier, it does nothing to prevent this interior air transport.

WHEN TO USE A VAPOR RETARDER

So the question arises: “When is it prudent to use a vapor retarder?” This is not a simple question and has been complicated by codes, standards, costs and building construction, changing roof membranes and confusion about air barriers. Then, there is the difference in new-construction design and roof removal and replacement design. Historically, it was said that a vapor retarder should be used if the interior use of the building was “wet”, such as a pool room, kitchen, locker shower rooms, etc.; outside temperature in the winter was 40 F or below; or when in doubt, leave it out. In my experience, changes in the building and construction industry have now made the determination criteria more complex.

I find there are typically three primary scenarios that suggest a vapor barrier is prudent. The first is the interior use of the building. The second is consideration for the control of construction-generated moisture, so that the roof can make it to the building’s intended use (see photo 1). The third consideration is the sequence of construction. In all three situations I like to specify a robust vapor retarder that “dries in” the building so that interior work and construction work above the vapor retarder can take place without compromising the finished roof. Consider the following:

BUILDING USE

This characteristic is often the most determinant. If the interior use of the building requires conditioned air and has relative-humidity percentages great enough to condense if the exterior temperatures get cold enough, a vapor retarder is needed to prevent the movement of this conditioned air into the roof system where it can condense and become problematic.

Most designers consider building use only in their design thinking, and it is often in error as the roof system can be compromised during construction and commissioning (through interior building flushing, which can drive moist air into the roof system) before occupancy.

To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap sheet, eliminating the glaze coat.

PHOTO 2: To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the
inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap
sheet, eliminating the glaze coat.

CONTROL OF CONSTRUCTION-GENERATED MOISTURE

I have seen roof systems on office buildings severely compromised by construction- generated moisture caused by concrete pours, combustion heaters, block laying, fireproofing, drywall taping and painting. Thus, a simple vapor retarder should be considered in these situations to control rising moisture vapor during construction, which includes the flushing of the building if required for commissioning.

CONSTRUCTION SEQUENCING AND MATERIALS

Building construction takes place year round. It is unfortunate decision makers in the roofing industry who are pushing low-VOC and/or water-based adhesives do not understand this; problems with their decisions are for another article. If the roof is to be installed in late fall (in the Midwest) and interior concrete work and/or large amounts of moisture-producing construction, such as concrete-block laying, plastering, drywall taping or painting, are to take place, a vapor retarder should be considered.

How will the building, especially the façades, be constructed? Will they be installed after the finished roof? This creates a scenario for a damaged “completed” roof system.

PHOTOS: Hutchinson Design Group Ltd.

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Energy-efficient Cool-roof Legislation: Creating Jobs and Reducing Energy Costs

Building on two roofing trends—higher thermal performance and cooler roofs in hotter climates—that have policymakers and architects seeing eye to eye, energy-efficient cool-roof legislation offers a significant opportunity to increase building energy efficiency and create jobs. Known in the last Congress in the Senate as S. 1575, the Energy-Efficient Cool Roof Jobs Act, and in the House of Representatives as H.R. 2962, the Roofing Efficiency Jobs Act, the legislation is scheduled to be reintroduced this spring.

The intent of the legislation is to encourage improvement in the thermal performance of existing roofs and, where appropriate in the designer’s judgment, encourage the use of a white or reflective roof surface in hotter climates. This is a clear win-win for the environment and building owners in terms of reduced energy costs and reduced pollution associated with energy consumption.

energy efficiency

Click to view larger

SIGNIFICANT SAVINGS lie within the commercial roofing sector, where more than 50 billion square feet of flat roofs are currently available for retrofit, 4 billion of which are typically retrofitted each year. The legislation would provide a 20-year depreciation period (instead of the current 39 years) for commercial roofs that meet minimum R-values that are significantly higher (requiring more insulation) than those required under state and local building codes and that have a white or other highly reflective surface. This change would correct an inequity in the current depreciation system (the average life span of a low-slope roof is only 17 years). By providing this incentive, the federal government would allow building owners and architects to decide whether the combination of thermal insulation and reflective roofs are appropriate for a given climate.

The required R-values under the proposed legislation are identical to the prescriptive requirements found under ASHRAE 189.1-2011, “Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential Buildings”. This legislation would be limited to retrofits of existing low-slope roofs and would not be available to new buildings. The cool roof requirement would only apply to buildings in ASHRAE Climate Zones 1 through 5, which covers approximately the area of the country from Chicago and Boston south. Roofs may qualify for the depreciation in zones 6, 7 and 8 but would not need a cool surface. View a map of the ASHRAE Climate Zones.

According to the U.S. Department of Energy’s Annual Energy Review, 2011, buildings account for 19 percent of the nation’s total energy usage and 34 percent of its electricity usage. Policies directed at commercial buildings are important to improving the economy, reducing pollution and strengthening energy efficiency. Although the country has over time maintained a steady pace in improving energy efficiency, a huge potential still exists, especially for commercial buildings. A wide range of credible estimates are available that point to this potential for cost-effective energy-efficiency improvements (see the graph).

THIS PROPOSED legislation complements the approaches taken in more comprehensive energy-efficiency proposals by focusing on the roof, which is the only building-envelope component that is regularly replaced but rarely upgraded to address energy and other environmental impacts.

Most buildings were constructed before building energy codes were first developed in the mid-1970s, or buildings were constructed under relatively weak codes, so these older, under-insulated roofs offer an important opportunity for increased energy savings. During the next 17 to 20 years, most of the weatherproof membranes on all commercial roofs will be replaced or recovered, which is the most cost-effective time to add needed insulation.

By accelerating demand for energy-efficient commercial roofs, the proposed legislation would:

    ▪▪ Create nearly 40,000 new jobs among roofing contractors and manufacturers.
    ▪▪ Add $1 billion in taxable annual revenue to the construction sector.
    ▪▪ Save $86 million in energy costs in the first year.
    ▪▪ Eliminate and offset carbon emissions by 1.2 million metric tons (equal to emissions of 229,000 cars).

THE LEGISLATION has the support of the Polyisocyanurate Insulation Manufacturers Association; National Roofing Contractors Association; Alliance to Save Energy; American Council for an Energy-Efficient Economy; Associated Buildings & Contractors Inc.; Building Owners and Managers Association International; United Union of Roofers, Waterproofers and Allied Workers; and several more construction industry associations.

When Sens. Cardin and Crapo reintroduce the Energy-Efficient Cool Roof Jobs Act, they hope it will influence the future debate about tax and energy policy. Although consideration of tax reform has stalled for the moment, when Congress returns to this issue it will be a golden opportunity to consider ideas for reforming cost-recovery periods and removing the disincentives that overly long depreciation schedules currently place on building energy-efficiency improvements.

The Cool-roof Bandwagon: Is It Headed To Your City?

Spring is here, and summer is on the horizon. But for millions of Americans, it will take more than a few days of sunshine to thaw the memories of the winter of 2013-14. The National Weather Service is still compiling the statistics to let us know just how bad the winter really was. In the meantime, most of us have a more immediate way to measure the impact of the polar vortex on our lives: One look at our heating bills and we know that this past winter deserves its reputation as one of the most brutal on record.

On the West Coast, as 2014 dawned, very different climate issues were front and center. The city of Los Angeles was being praised for its mandate requiring all new and renovated domestic housing to install “cool”, or reflective, roofing. The L.A. City Council passed the requirement as one of its last acts of 2013, and the new ordinance became part of California’s Title 24, which already required “cool” roofs in new and remodeled commercial construction.

THE NEWS media hailed Los Angeles as the “first major city to require cool roofs”, implying other urban areas will inevitably follow its lead. However, the winter of 2013-14 did a good job of reminding us that the climatic conditions of Southern California are dramatically different from the Midwest, Northeast and Mid-Atlantic regions of the U.S. This simple fact needs to be underscored as the bandwagon to require cool roofs travels somewhat erratically to major Eastern cities.

Last June, the mayor of Pittsburgh initiated a lukewarm cool roofs program by calling for volunteers to help paint the roofs of 10 city buildings white. Two-thirds of the Pittsburgh effort—$56,000—was funded by the Bloomberg Philanthropies, a project of former New York City Mayor Michael Bloomberg. The tagline of Bloomberg Philanthropies is “Good Intentions, Great Results.” I applaud the mayor’s good intentions in supporting projects that are designed to save energy. As for achieving “great results” by painting the roofs of 10 Pittsburgh buildings white? Don’t bet your next heating bill on it.

While Bloomberg was mayor of New York, the city launched the “NYC °Cool-Roofs” initiative, encouraging building owners to cool their rooftops by applying a reflective white coating as part of the city’s overall plan to reduce greenhouse- gas emissions 30 percent by 2030.

In Baltimore, the talk about cool roofs was fueled by a report issued last October by the Abell Foundation, a non-profit dedicated to enhancing quality of life in Baltimore and Maryland. The report, which is primarily an overview of previously published research, recommended increased use of cool roofs in Baltimore.

While these cities institute varied programs to support cool roofs, several major facts are ignored:

    ▪▪ Energy costs are closely related to climate. A solution that works in a warm and temperate climate to curb energy costs will not necessarily work in a colder climate.
    ▪▪ It’s vitally important to consider the source of information about cool roofing. Unbiased, up-to-date scientific studies can provide the data you need to make an independent judgment. Likewise, the manufacturers of roofing membranes have a vested interest in ensuring their products are used correctly and have in-depth knowledge of how roofing systems will perform in a wide variety of conditions.
    ▪▪ Choosing and installing a roof that will contain energy costs is a complex business. It requires understanding the interaction between building design, climate, insulation and all the other factors that impact the efficiency of a roofing system. A one-size-fits-all approach will only delay the discovery of workable, cost-effective, energy-efficient solutions.

IN FACT, a study conducted by Arizona State University published this past winter in the Proceedings of the National Academy of Sciences underscores the pitfalls of disregarding climate differences in roofing decisions. “What works over one geographical area may not be optimal for another,” says sustainability scientist Matei Georgescu, who led the research.

Although the headlines are touting Los Angeles’ cool roof requirements, I’d like to see headlines that read, “Energy Savings Achieved by Roofs Designed to meet Midwest and Northeast Climate Challenges”. Before anyone thinks about driving that cool-roofing bandwagon from Los Angeles to New York, you might want to equip it with snow tires.