heat iSlanD eFFeCt—rooF - LEED Reference Guide for Green Building Design and Construction 2009

nc scHooLs cs

credit ss credit 7.2 ss credit 7.2 ss credit 7.2

Points 1 point 1 point 1 point

intent

To reduce heat islands¹ to minimize impacts on microclimates and human and wildlife habitats.

requirements

nc, scHooLs & cs oPtion 1

Use roofing materials with a solar reflectance index² (SRI)equal to or greater than the values in the table below for a minimum of 75% of the roof surface.

Roofing materials having a lower SRI value than those listed below may be used if the weighted rooftop SRI average meets the following criteria:

area Roof Meeting Minimum sRi

————————————————

total Roof area

x

sRi of installed Roof

——————————

Required sRi

≥

^75%

Roof Type Slope SRI

Low-sloped roof ≤ 2:12 78

steep-sloped roof > 2:12 29

oR oPtion 2

Install a vegetated roof that covers at least 50% of the roof area.

oPtion 3

Install high-albedo and vegetated roof surfaces that, in combination, meet the following criteria:

area Roof Meeting Minimum sRi

————————————————

0.75

+

area of Vegetated Roof

——————————

0.5

≥

total Roof area

Roof Type Slope SRI

Low-sloped roof ≤ 2:12 78

steep-sloped roof > 2:12 29

SS CreDit 7.2

Exemplary performance option 2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

1. Benefits and issues to Consider Environmental issues

The use of dark, nonreflective roofing surfaces contributes to the heat island effect by absorbing the sun’s warmth, which then radiates into the surroundings. As a result, ambient temperatures in urban areas are artificially elevated, increasing cooling loads, electricity consumption, and emissions of greenhouse gases and pollution. Heat island effect is also detrimental to site habitat, wildlife, and animal migration corridors. Plants and animals are sensitive to large fluctuations in daytime and nighttime temperatures and may not thrive in areas affected by heat islands. Projects that earn SS Credit 7.2 by providing vegetated roofs contribute to increased habitat areas for birds, insects, and other wildlife.

Economic issues

The energy used to cool a building represents a substantial portion of the operating budget over its lifetime. Vegetated roofs and roof surfaces with high SRIs can reduce costs associated with HVAC equipment. Vegetated roofs typically require additional investment; cool roofs that effectively reflect the sun’s energy could cost the same as more conventional roofing systems. However, any upfront investment is likely to result in energy cost savings throughout the life cycle of the project.

According to EPA researchers who monitored 10 buildings in California and Florida, cool roofs save residents and building owners 20% to 70% in annual cooling energy costs.¹⁵ An increasing number of jurisdictions are beginning to require the use of cool roofs on new building projects.

scHooLs For schools, a vegetated roof can function as a living classroom, providing an added educational amenity.

2. related Credits

Vegetated roofs help capture and treat stormwater while also providing habitat and open space, providing synergies with the following credits:

n SS Credit 5.1: Site Development—Protect or Restore Habitat

n SS Credit 5.2: Site Development—Maximize Open Space

n SS Credit 6.1: Stormwater Design—Quantity Control

n SS Credit 6.2: Stormwater Design—Quality Control

Vegetated roofs also reduce the availability of rainwater that may be harvested for nonpotable purposes, making it more challenging to achieve this water-efficiency credit:

n WE Credit 3: Water Use Reduction

Using highly reflective roofing materials and/or vegetated roofs can decrease cooling loads and help projects with earning this credit:

n EA Credit 1: Optimize Energy Performance 3. Summary of referenced Standards ASTM Standards

http://www.astm.org

ASTM E1980–01, Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces

This standard describes how surface reflectivity and emissivity are combined to calculate a solar reflectance index (SRI) for a roofing material or other surface. The standard also describes a laboratory and field testing protocol that can be used to determine SRI.

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

ASTM E408–71(1996)e1, Standard Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques

This standard describes how to measure total normal emittance of surfaces using a portable inspection-meter instrument. The test methods are intended for large surfaces where nondestructive testing is required. See the standard for testing steps and a discussion of thermal emittance theory.

ASTM E903–96, Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres

Referenced in the ENERGY STAR roofing standard, this test method uses spectrophotometers and need only be applied for initial reflectance measurement. It specifies methods of computing solar-weighted properties using the measured spectral values. This test method is applicable to materials having both specular and diffuse optical properties. Except for transmitting sheet materials that are heterogeneous, patterned, or corrugated, this test method is preferred over Test Method E1084. The ENERGY STAR roofing standard also allows the use of reflectometers to measure roofing materials’

solar reflectance. See the roofing standard for more details.

ASTM E1918–97, Standard Test Method for Measuring Solar Reflectance of Horizontal and Low-Sloped Surfaces in the Field

This test method covers the solar reflectance measurements, using a pyranometer, of various horizontal and low-sloped surfaces and materials. The test method is intended for use when the angle from a surface to the sun is less than 45 degrees.

ASTM C1371–04, Standard Test Method for Determination of Emittance of Materials Near Room Temperature Using Portable Emissometers

This test method covers a technique for determination of the emittance of typical materials using a portable differential thermopile emissometer. The purpose of the test method is to provide a comparative means of quantifying the emittance of opaque, highly thermally conductive materials near room temperature as a parameter in evaluating temperatures, heat flow, and derived thermal resistances of materials.

ASTM C1549–04, Standard Test Method for Determination of Solar Reflectance Near Ambient Temperature Using a Portable Solar Reflectometer

This test method covers a technique for determining the solar reflectance of flat, opaque materials in a laboratory or in the field using a commercial, portable solar reflectometer. The purpose of the test method is to provide the solar reflectance data required to evaluate temperature and heat flows across surfaces exposed to solar radiation.

4. implementation

The choice of roofing can maximize energy savings and minimize the heat island effect. This credit can be achieved through high-reflectance surfaces, vegetated roofing, or a combination.

High-Reflectance Materials

The solar reflectance index (SRI) of a roofing material is calculated from emissivity and solar reflectance values. Multiple testing methods are available for measuring emissivity and solar reflectance; check the manufacturer’s literature carefully to ensure the use of appropriate data. For example, some manufacturers measure visible reflectance, which differs from the solar reflectance measurement used in this credit. Visible reflectance correlates to solar reflectance, but the 2

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

for specific information, including the SRI value of a material. Table 1 provides examples of SRI values for typical roof surfaces. These values are for reference only, not for use as substitutes for actual manufacturer’s data. Reflectance and emittance data for manufacturers are available from the Cool Roof Rating Council website (see the Resources section). Note that the infrared emittance of aggregates and cementitious materials is always 0.9.

table 1. solar Reflectance index (sRi) for typical Roofing Materials

SRI Values for Solar Infrared

Temperatures Solar Reflectance Infrared Emittance Temperature Rise SRI

Gray EPDM 0.23 0.87 68°f 21

Gray asphalt shingle 0.22 0.91 67°f 22

unpainted cement tile 0.25 0.9 65°f 25

White granular surface bitumen 0.26 0.92 63°f 28

Red clay tile 0.33 0.9 58°f 36

Light gravel on built-up roof 0.34 0.9 57°f 37

aluminum coating 0.61 0.25 48°f 50

White-coated gravel on built-up roof 0.65 0.9 28°f 79

White coating on metal roof 0.67 0.85 28°f 82

White EPDM 0.69 0.87 25f 84

White cement tile 0.73 0.9 21f 90

White coating, 1 coat, 8 mils 0.8 0.91 14f 100

PVc white 0.83 0.92 11f 104

White coating, 2 coats, 20 mils 0.85 0.91 9f 107

source: Lawrence Berkeley national Laboratory cool Roofing Materials Database. these values are for reference only and are not for use as substi-tutes for actual manufacturer data.

Vegetated Roofs

A vegetated roof is a layered system that consists of vegetation, growing medium, filter fabric, drainage, and a waterproof membrane atop a conventional roof (Figure 1). A vegetated roof can be installed as a complete system or as a modular system consisting of interlocking trays. Potted plants do not usually count as a vegetated roof because they do not offer the same magnitude of environmental benefits, and so do not contribute to meeting the credit requirements.

Vegetated roofs can be very beneficial. They can reduce the heat island effect by replacing heat-absorbing surfaces with herbaceous plants, shrubs, and small trees to cool the air through evapotranspiration. They also retain stormwater, provide insulating benefits, are aesthetically appealing, have longer lifetimes than conventional roofs, and often require less maintenance than conventional roofs. Some vegetated roofs are actual gardens and require significant plant care;

others have grasses and other plants that require no maintenance or irrigation. All types of vegetated roofs require semiannual inspection.

Occupants’ use of roof gardens may be incompatible with high-reflectivity roofing materials. An area-weighted SRI equivalent may help allow for low-glare pavers where people congregate. The planted portions and unoccupied areas of the roof will offset a lower SRI.

Vegetated roofs may unwittingly cause bird collisions with adjacent structures, such as glazed buildings or penthouses. Measures to prevent or reduce bird collision include using exterior shading devices, introducing etched or frit patterns in the glass, and creating “visual markers” in sufficient locations. Visual markers are differentiated planes, materials, textures, colors, opacity, or other visually contrasting features that help fragment window reflections and reduce overall transparency and reflectivity. See strategies and references under SS Credits 5.1 and 5.2.

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

When designing green roofs, select native or adapted plant species to reduce or eliminate the need for irrigation. Where irrigation is required, consider using graywater or harvested stormwater to reduce potable water use.

Figure 1. typical Vegetated Roof

Roof Structure Roof Membrane Root Barrier Insulation Drainage &

Water Storage Filter Fabric Growing Medium Vegetation

5. timeline and team

Early in the design process, the project team should engage the architect, landscape architect, civil and mechanical engineers, and an ecologist in evaluating cool roof systems. Architects should specify roofing materials with a high SRI value in the construction documents, and the general contractor should work with vendors to procure manufacturers’ data with compliant SRI values.

When designing vegetated roofs, involve landscape architects and civil engineers to determine appropriate soil types, average rainfall, and regional plant species that provide wildlife habitat.

6. Calculations

Obtain the roofing material’s SRI value from the manufacturer and complete the following steps.

stEP 1

Determine the total roof surface area of the project building (square feet).

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

stEP 3

Determine whether the areas of qualifying reflective and vegetated roofing are adequate to meet the credit requirements, using Equation 1. If more than 1 type of low-slope or steep-slope material is used, determine the weighted rooftop SRI average and verify that 75% or more of the roof area complies with the credit requirements.

Equation 1 through the following measures. Refer to LEED-Online for the complete descriptions of all required documentation.

n Prepare roof drawings that show total roof area and the areas of reflective materials or vegetated roof systems.

n List the roofing products, and their emittance percentages, reflectance percentages, SRI values, and slopes. Retain product specifications that verify product characteristics.

8. examples

A new office building has a 10,000-square-foot low-slope roof and is designed to have both highly reflective roofing materials and a vegetated roof system. The vegetated roof makes up 35% of the roof area. White EPDM roofing with a SRI of 85 covers 60% of the roof area, and the remaining 5% is covered by rooftop mechanical equipment. Table 2 summarizes the roofing types.

table 2. Roofing area summary, by type

Roofing Type Area (sf)

Vegetated roof area 3,500

White EPDM roof area (sRi-85), low slope 6,000

Mechanical equipment 500

In this example, the white EPDM roofing plus the vegetated roofing meets the requirements of this credit, so the project earns 1 point.

9. exemplary performance

Projects may earn an Innovation in Design credit for exemplary performance by demonstrating that 100% of the project’s roof area (excluding any mechanical equipment, photovoltaic panels, and skylights) consists of a vegetated roof system.

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

10. regional Variations

Heat island intensities depend on an area’s weather and climate, proximity to water bodies, and topography. Buildings in very cold climates or at high latitudes may not experience the same rise of surface and ambient temperatures. Buildings in urban areas and those located in climate zones 1, 2, and 3 (as defined by ASHRAE 90.1–2007) are most affected by heat islands and are likely to benefit from measures to decrease cooling loads. Consider climate and rainfall at the site when determining the feasibility of a vegetated roof for the project building. In sunny climates, window shading devices will help mitigate glare from reflective roofing into the building.

11. operations and Maintenance Considerations

Materials with high reflectivity must be periodically cleaned to maintain their heat island reduction properties. These surfaces should be cleaned at least every 2 years to maintain good reflectance.

Building operators should obtain the necessary information to maintain any vegetated roofing system. Extensive green roof systems with low-growing plants are generally easier to maintain than intensive systems with deeper soil and larger plants. The design team should select plantings that are easy to maintain and tolerant of extreme conditions, and avoid plants that produce airborne seeds that will litter the surroundings. The project team should establish an operations plan for inspecting the roof membrane and plantings, as well as maintaining drainage paths. Until plants are fully established, watering and fertilization might be necessary. Properly designed green roofs do not require mowing or cutting, but occasional weeding may be required.

In the roof design, consider including habitat for the animal and insect populations needed to pollinate the installed plantings, such as beehives and wild bee shelters. The project team should request maintenance information from product manufacturers and installers, and then make sure this information is given to the operations team.

12. resources

Please see USGBC’s LEED Registered Project Tools (http://www.usgbc.org/projecttools) for additional resources and technical information.

Websites

American Bird Conservancy http://www.abcbirds.org

ABC is a national leader in reducing human effects on birds and wildlife. ABC’s bird collision program supports national efforts to reduce bird mortality through education and advocacy.

Birds and Buildings Forum http://www.birdsandbuildings.org

This Chicago-based non-profit supports bird-friendly design through education and advocacy.

Their website maintains lists of organizations and resources.

Cool Roof Rating Council http://www.coolroofs.org

This nonprofit organization is dedicated to implementing and communicating fair, accurate, and credible radiative energy performance rating systems for roof surfaces; supporting research into energy-related radiative properties of roofing surfaces, including durability; and providing education

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

Whole Building Design Guide, Extensive Green Roofs http://www.wbdg.org/resources/greenroofs.php

This article by Charlie Miller, PE, details the features and benefits of constructing green roofs.

Fatal Light Awareness Program http://www.flap.org

Initiated the Bird-Friendly Building Development Program for the City of Toronto, FLAP monitors and promotes bird-friendly design.

Green Roofs for Healthy Cities http://www.greenroofs.com

This nonprofit industry association consists of individuals and public and private organizations committed to developing a market for green roof infrastructure products and services across North America.

Lawrence Berkeley National Laboratory, Heat Island Group, Cool Roofs http://eetd.lbl.gov/HeatIsland/CoolRoofs/

This site offers a wealth of information about cool roof research and technology, including links to the cool roofing materials database.

New York City Audubon http://www.nycaudubon.org

This Audubon chapter takes a leadership role in reducing bird collisions with buildings. The chapter publishes Bird-Safe Building Guidelines, conducts monitoring, and through its Project Safe Flight, promotes bird-friendly design.

Penn State University, Center for Green Roof Research http://web.me.com/rdberghage/Centerforgreenroof/Home.html

The Center has the mission of demonstrating and promoting green roof research, education, and technology transfer in the Northeastern United States.

13. Definitions

An area-weighted SRI is a weighted average calculation that may be performed for buildings with multiple roof surfaces to demonstrate that the total roof area has an average solar reflectance index equal to or greater than that of a theoretical roof 75% of whose surfaces have an SRI of 78 and 25%

have an SRI of 30.

Emissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a black body at the same temperature.

Greenhouse gases are relatively transparent to the higher-energy sunlight but trap lower-energy infrared radiation (e.g., carbon dioxide, methane, and CFCs).

Hardscape consists of the inanimate elements of the building landscaping. Examples include pavement, roadways, stone walls, concrete paths and sidewalks, and concrete, brick, and tile patios.

Infrared emittance, or thermal emittance, is a parameter between 0 and 1 (or 0% and 100%) that indicates the ability of a material to shed infrared radiation (heat). The wavelength range for this radiant energy is roughly 4 to 40 micrometers. Most building materials (including glass) are opaque in this part of the spectrum, and have an emittance of roughly 0.9. Materials such as clean, bare metals are the most important exceptions to the 0.9 rule. Thus clean, untarnished galvanized steel has low emittance, and aluminum roof coatings have intermediate emittance levels.

Solar reflectance, or albedo, is a measure of the ability of a surface material to reflect sunlight—

visible, infrared, and ultraviolet wavelengths—on a scale of 0 to 1. Black paint has a solar reflectance of 0; white paint (titanium dioxide) has a solar reflectance of 1.

nc credit 7.2

scHooLs credit 7.2

cs credit 7.2

For personal use only and subject to the sales order agreement terms by (BP 10303644). May NOT be copied or distributed.

The solar reflectance index (SRI) is a measure of a material’s ability to reject solar heat, as shown by a small temperature rise. Standard black (reflectance 0.05, emittance 0.90) is 0 and standard

In document LEED Reference Guide for Green Building Design and Construction 2009 (Page 146-156)