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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NRCA’s Roof Calculator Has Been Updated to Include ICC’s IECC and IgCC, ASHRAE Standard 90.1, and More

NRCA’s EnergyWise Roof Calculator Online has been updated to include information from the 2015 versions of the International Code Council’s IECC and IgCC, as well as the 2013 version of ASHRAE Standard 90.1. Revised minimum long-term thermal resistance values and NRCA’s latest recommendations for minimum R-values for polyisocyanurate insulation have been included in the application. The application also will determine the temperature gradient through a roof assembly and present the information graphically on a report.

Users will find this beneficial when evaluating the effectiveness of a vapor retarder. The EnergyWise Roof Calculator Online is available for free on NRCA’s EnergyWise Roof Calculator page.

‘The International Energy Conservation Code as Applied to Commercial Roofing’ Brochure Is Released

A new energy code brochure, “The International Energy Conservation Code as Applied to Commercial Roofing”, has been released explaining reroofing clarifications in the 2015 International Energy Conservation Code (IECC). The reroofing clarifications make it very clear that almost every commercial reroofing project involving the removal and replacement of the existing roof covering must be upgraded to the current IECC R-value levels.

The Institute for Market Transformation (IMT), with the assistance of the Center for Environmental Innovation in Roofing (the Center) and the Polyisocyanurate Insulation Manufacturers Associations (PIMA), developed and released the new energy code brochure.

“Billions of square feet of low-slope of commercial roofs (roofs with insulation above the deck) are replaced every year in the United States,” said Jared Blum, President, PIMA. “The clarification in the IECC means that whenever an existing low-slope roofing membrane is removed before a new roofing membrane is installed, the underlying roof insulation must be brought up to current code-mandated R-value levels.”

The new code clarification establishes specific definitions for each major type of roofing activity that may occur on a commercial building:

    Reroofing. The process of recovering or replacing an existing roof covering. See Roof Recover and Roof Replacement.
    Roof Recover. The process of installing an additional roof covering over a prepared existing roof covering without removing the existing roof covering.
    Roof Replacement. The process of removing an existing roof covering, repairing any damaged substrate and installing a new roof covering.
    Roof Repair. Reconstruction or renewal of any part of an existing roof for the purposes of its maintenance.

The new brochure, similar in format to many other IMT brochures, contains:

  • A detailed listing of the key definitions and energy regulations that apply to commercial roofing.
  • Illustrations of typical roofing conditions.
  • A decision tree to determine the specific compliance path for any roofing application.

“Because it is considered a clarification rather than a new addition to the code, officials can start enforcing the update now and don’t have to wait until the 2015 version of the IECC is adopted in their jurisdiction. This brochure is succinct, easy to follow and clearly explains how to comply with the clarification,” added Blum.

“The International Energy Conservation Code as Applied to Commercial Roofing” brochure will help local code officials better understand the energy efficiency requirements for all types of commercial roofing projects and also serve as a useful guide to explain the code requirements to roofing contractors seeking construction permits, design professionals (architects, engineers, roof consultants) involved in roofing selection and specification, as well as building owners as the ultimate end-user of the code.

“The brochure is a part of a comprehensive effort by PIMA to inform members of the design community about their legal obligations to comply with the reroofing energy upgrade requirement,” added Blum.

In addition to advocating for increased building energy efficiency via improved building codes, IMT also works to increase compliance with energy codes by developing and distributing informational materials suitable for use in local code jurisdictions, not only for code officials but also for owners, designers, and contractors.

Insulation and Roof Replacements

When existing roofs (that are part of the building’s thermal envelope) are removed and replaced and when the roof assembly includes above-deck insulation, the energy code now requires that the insulation levels comply with the requirements for new construction, according to a proposal approved by International Code Council at public comment hearings held in October 2013.

This high-performance roof system was recently installed on a high school north of Chicago. It features two layers of 3-inch 25-psi, double-coated fiberglass-faced polyisocyanurate insulation set in bead-foam adhesive at 4 inches on center, weighted with five 5-gallon pails of adhesive per 4- by 4-foot board to ensure a positive bond into the bead foam until set. PHOTO: Hutchinson Design Group LLC

This high-performance roof system was recently installed on a high school north of Chicago. It features two layers of 3-inch 25-psi, double-coated fiberglass-faced polyisocyanurate insulation set in bead-foam adhesive at 4 inches on center, weighted with five 5-gallon pails of adhesive per 4- by 4-foot board to ensure a positive bond into the bead foam until set. PHOTO: Hutchinson Design Group LLC

As a result of this proposal approval, the 2015 International Energy Conservation Code (IECC) provides new language that provides clear unambiguous direction on how the energy code provisions apply to roof repair, roof recover and roof replacement.

Until this update there was a great deal of confusion given the various terms—such as reroofing, roof repair, roof recover and roof replacement—used to describe roofing projects on existing buildings in the International Building Code and the IECC. The clarification will help to mitigate this confusion.

Numerous studies have demonstrated the energy savings provided by a well-insulated roofing system. It is critical to minimize energy losses and upgrade insulation levels when roofs are replaced to comply with code requirements for new construction.

Each year about 2.5 billion square feet of roof coverings are installed on existing buildings and the opportunity to upgrade the insulation levels on these roof systems occurs just once in several decades when the roof is replaced or even longer when existing roofs are “recovered”. Until recently this requirement was prescribed using vague and confusing language, as noted.

Moving forward the IECC will use the same definitions found in the International Building code:

  • Reroofing: The process of recovering or replacing an existing roof covering. See “Roof Recover” and “Roof Replacement”.
  • Roof Recover: The process of installing an additional roof covering over a prepared existing roof covering without removing the existing roof covering.
  • Roof Replacement: The process of removing the existing roof covering, repairing any damaged substrate and installing a new roof covering.
  • Roof Repair: Reconstruction or renewal of any part of an existing roof for the purposes of its maintenance.

A survey of building departments in many states and regions in the U.S. found that online roofing permit application forms rarely included any information on the energy code and required insulation levels. With the changes to the 2015 IECC, it will be easier for building departments to correlate the building code and energy code requirements for roof replacements.

The clarification to the 2015 IECC makes the code easier to interpret and enforce. Along the way, it will help ensure that the opportunity to save energy when replacing roofs is not lost.

Another benefit of this update is that the exemption for roof repair is now clearly defined making it easier for building owners and roofing contractors to perform routine maintenance without triggering energy-efficiency upgrades, which would add costs.