September 26, 2016
by Russell M. Sanders, AIA
With New York City’s 80×50 initiative to reduce greenhouse gas (GHG) emissions 80 percent by 2050, the stakes are high for the city to adopt progressively more stringent energy codes. Similarly, the Sustainable DC Plan professes the lofty goal of making the nation’s capital “the greenest, healthiest, and most livable city in the nation.” Other states and cities are following suit, as building owners, managers, and the design/ construction industry race to keep up with rapidly evolving codes, energy analysis requirements, and documentation standards.
For their part, manufacturers are responding with a flood of new products and technologies to meet the stricter energy efficiency requirements. However, while there are numerous options addressing the insulation, reflectance, durability, and moisture management properties stipulated by the new laws, choosing the right option for a specific project can be daunting.
This may seem like unprecedented upheaval in the building industry, but reroofing an existing building has always posed similar challenges. Even if the energy codes bring new terminology and processes, the roofing industry has always been a moving target, with product innovations rapidly making even a five-year-old low-slope roof seem eons behind its newer counterparts. Fortunately, the process of selecting and designing a code-compliant roof replacement remains much the same as it always has:
Far from a passing trend, the sustainability movement has made lasting changes to expectations for new roof assemblies. Although these changes can take some getting used to, the focus on ecological roof technology provides building owners and managers with new options for reroofing that can reduce heating and cooling demands, improve indoor comfort, and even increase the assembly’s projected lifespan. Becoming familiar with the pros and cons of different types of sustainable roofing takes the guesswork out of choosing a system that meets performance and energy standards, and creates a positive image for the building.
Understanding vegetated roofs
The poster child for green building, vegetated roofs are subject to all the same enthusiasm, scorn, hype, and derision that has followed the push for sustainable design since the creation of the Leadership in Energy and Environmental Design (LEED) rating system in the late 1990s. Early missteps resulted in fields of scorched, brown sticks, but as government projects and private industry experimented with new systems, these assemblies have improved, with scientific papers documenting best practices for everything from plant selection to waterproofing details.
After nearly 15 years since the nation’s first municipal green roof was installed on Chicago’s City Hall, many more examples of both thriving and failing vegetated assemblies have helped further refine approaches to creating a living roof. Today’s options have the benefit of longer in-service evaluation, running the gamut from economical prefabricated tray systems to custom landscaped terraces.
Although vegetated assemblies cost more than traditional roofs, they have the potential to add value to an existing building by replacing utilitarian surfaces and setbacks with something eye-catching.
Roof gardens and terraces
Rooftop oases where people amble along a plaza stocked with foliage and furniture—such as the widely publicized Terminal 5 Rooftop at John F. Kennedy International Airport—are examples of ‘intensive’ green roof. They are often labor-intensive, both in terms of initial installation and ongoing irrigation and maintenance.
Intensive green roofs can act as an extension of the building, adding attractive usable space where there might otherwise be a blank stretch of featureless roof membrane. As an amenity, intensive green roofs can capitalize on unused portions of an existing building; they have the added benefit of drawing positive attention to building envelope retrofits by showcasing the building owner’s commitment to environmentalism, sustainability, and well-being of tenants and building users.
To accommodate trees and other large plantings, growing media for intensive roof gardens tends to be deep (i.e. 300 mm [12 in.] or more, depending on the desired vegetation), with soil dense in organic material and fully saturated weights of 3.8 to 5.75 kPa (80 to 120 psf). This does not include the added weight of pavers, fixtures, furnishings, and decorative elements, along with the live load of visitors and maintenance personnel and equipment. (This means significant structural considerations for an existing building—for more, see “Vegetated Roofs: A Green Light for Your Building?”)
Extensive landscaped roofs
A lighter-weight, lower-maintenance option is an extensive green roof. Although it does not provide added usable space, an extensive assembly offers a lower structural load, minimal upkeep, and a greatly reduced cost over intensive systems.
To keep loading to a minimum, extensive systems use shallow growing media with high inorganic content that tends to have a much lower saturated weight (roughly 0.7 to 2.4 kPa [15 to 50 psf]) than that of an intensive green roof. Sedum, native grasses, and other hardy plants that are drought- and heat-tolerant allow extensive green roofs to flourish without supplementary irrigation. Although the weight of an extensive vegetated assembly is still more than a traditional built-up (BUR) or single-ply roof, these shallow, self-sustaining systems are often readily manageable as a retrofit option for existing buildings, as they tend to require little, if any, structural modifications.
Even pitched roofs can be fitted with extensive vegetated assemblies—some prefabricated tray systems are designed to accommodate slopes of up to 30 degrees, and even steeper grades can be managed using grids or lathes to secure the trays in place. However, if waiting for a roof garden to germinate and grow is not feasible, if maintaining plantings on a 48th-floor setback is impractical, or if the structure cannot tolerate additional load, then it is worthwhile considering systems more like a traditional low-slope roof, but with energy-saving perks.
|VEGETATED ROOFS: A GREEN LIGHT FOR YOUR BUILDING?|
Beyond the obvious aesthetic appeal of rooftop greenery, vegetated roofs offer benefits to the building owner and the larger community.
Longer waterproofing service life
Habitat and environment
Energy codes and performance requirements for cool roofs
Also known as reflective or white roofs, cool roofs have a high solar reflectance or albedo, which means they reflect sunlight much better than do traditional roofs. By radiating energy back into the atmosphere, cool roofs’ high thermal emittance allows them to reduce solar heat loads on the building. Like vegetated assemblies, cool roofs help reduce the trapped heat in urban areas known as the ‘heat island effect,’ which, in turn, cuts levels of air pollutants contributing to smog.
In response to concerns that reflective roof surfaces in northern climates actually increase energy consumption by requiring additional heating in the winter, American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, was revised as of the 2010 edition to specify cool roofs solely for warm climates; in previous editions, reflective roof assemblies were the standard, irrespective of geographic region. However, there is some contradictory evidence the assemblies reduce energy consumption across all North American climate zones.
According to a recent study by the Oak Ridge National Laboratory (ORNL), even in colder climates, cool roofs can significantly reduce peak energy demand. (For more, see T.W. Petrie et al’s 2004 paper, “Effect of Solar Radiation Control on Electricity Demand Costs: An Addition to the DOE Cool Roof Calculator,” from the ASHRAE Performance of Exterior Envelopes of Whole Buildings IX International Conference Proceedings.) When installed with appropriate insulation, cool roofs were shown to cut peak energy use enough to compensate for increases in winter heating caused by the roof reflectivity, resulting in an overall net energy savings. However, this energy savings may depend on other factors, including whole building envelope thermal performance and type of heating fuel used.
To guide building owners, managers, and project teams in determining potential energy savings, the Department of Energy (DOE) provides an online tool, the “Cool Roof Peak Calculator,” for low-slope commercial and institutional roof assemblies.
Energy use calculations aside, the decision to install a cool roof on a building may be mandated by code. The 2014 New York City Building Code requires new roof coverings on all low-slope roofs (i.e. less than 17 percent grade) meet minimum reflectance requirements, with the exception of:
The New York City Energy Conservation Code, adopted in January 2015, also incorporates requirements for roof reflectivity, with tables for acceptable solar reflectance and thermal emittance values.
Similarly, the Washington D.C. Energy Conservation Code, based on the 2012 International Energy Conservation Code (IECC), includes stipulations for reflective roofs. According to the Cool Roof Rating Council (CRRC), dozens of other states and cities have adopted ASHRAE or IECC standards that include provisions mandating cool roofs. (Examples include Connecticut, Delaware, Massachusetts, Maryland, New Jersey, Virginia, Pennsylvania, New Hampshire, and Rhode Island.)
High-albedo roof assemblies
In response to these codes and standards, and to the increased demand for energy-saving roofing options, roofing manufacturers have increased their product lines for reflective roof assemblies. To meet baseline requirements for current national standards, a cool roof must have a minimum initial solar reflectance (i.e. fraction of incident solar energy reflected by the surface) of 0.70 and thermal emittance (i.e. measure of a material’s ability to release absorbed heat) of 0.75. When aged three years, solar reflectance may be reduced to no less than 0.55, and thermal emittance must remain at 0.70 or greater. Values for solar reflectance and thermal emittance are calculated on a scale of 0 to 1, with 0 being a black roof that absorbs 100 percent of solar energy, and 1 being a perfectly reflective roof.
The Solar Reflectance Index (SRI) is a combined measure of solar reflectance and thermal emittance, which rates surfaces from 0 (a standard black surface) to 100 (a reflective white roof). IECC and ASHRAE 90.1-2010 stipulate a minimum SRI of 82 at initial installation and 64 for a three-year-aged roof. (SRI is a measure of a roof’s ability to reject solar heat, and it may be used in addition to—or in lieu of—solar reflectance and thermal emittance when specifying roof assemblies. For example, ASHRAE 90.1 defines a cool roof as having a minimum solar reflectance of 0.70, a minimum thermal emittance of 0.75, or a minimum SRI of 82.)
The roofing types that best suit these qualifications are:
Of these, the single-ply systems tend to offer the higher reflectance values, as the membranes are uniformly light in color, with lightweight material that does not hold heat. However, single-ply systems lack redundancy and can be prone to seam failure if not correctly installed, so they tend to be less durable and resilient than their multiple-ply counterparts. Although MBR systems do use dark-colored, heat-absorbing bituminous material as the base layers, the granular reflective cap sheet disperses and reflects the majority of solar radiation, preventing heat energy from reaching the layers below.
Single-ply systems tend to be less expensive than MBR assemblies, but installation may necessitate contractors with special training and equipment, which can add to the total cost. For modified-bitumen roofs with reflective cap sheets, the material and installation costs tend to be comparable to those of standard modified bitumen assemblies. (Given that some degree of specialized training and equipment is required for all types of roof installations, single-ply systems in general tend to be less expensive than MBR assemblies even when installation costs are taken into consideration. That said, each system and situation is different. Although the initial cost of a single-ply assembly may be less than that of an MBR system, the redundancy, self-healing properties, longevity, and lower maintenance demands of modified bitumen assemblies tend to make them the more cost-effective option in the long term.) Fluid-applied assemblies are monolithic, which eliminates joints and laps in the membrane and so minimizes potential water entry points. However, of the three types of roofing considered above, fluid-applied systems are typically the most expensive.
Other options for roof coverings with a high solar reflectance index include inverted roof membrane assemblies (IRMA) in which the roof membrane is covered with insulation and reflective pavers, or metal panels with high-SRI finishes. An architect or engineer should advise on the compatibility and effectiveness of any rehabilitation options, as not all products are appropriate for all buildings.
|ROOF INSULATION REQUIREMENTS|
In the drive toward net-zero energy buildings and ever-greater energy efficiency, expectations for building performance have rapidly progressed over the past several years. With each successive iteration of the generally accepted national energy standards (i.e. American Society of Heating, Refrigeration, and Air-conditioning Engineers [ASHRAE] 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and the International Energy Conservation Code [IECC]), requirements for building envelope thermal performance have become increasingly stringent. For this reason, insulation considerations are an essential part of any reroofing project.
In 2006, IECC recommended a roof insulation R-value of 15 for commercial buildings in most North American climate zones. Just six years later, in the 2012 edition, that R-value jumped to 20 for southern states and to 25 for most of the rest of the country, with the far northern states having insulation requirements as high as 30 to 35. R-value is a measure of a materials’ resistance to heat transfer and is the reciprocal of U-factor, which describes how well an assembly conducts heat. The higher the R-value of a building element, the better it is at protecting against heat loss.
For reroofing projects on existing buildings, meeting increasingly rigorous insulation requirements can be challenging. Even where the project calls for a thin cool roof membrane and not a dense bed of planting media, the depth of insulation necessary to achieve the requisite R-value can impact door thresholds, flashings, walking surface elevations, guardrail heights, edge conditions, and other details. Considering all these implications during the design phase helps minimize problems in the field, when discovering increased insulation height prevents a bulkhead door from opening becomes a costly last-minute modification.
Reflected solar radiation and glare
Before opting for a cool roof, it is worth preventing future problems by considering not only its impact on the energy use and sustainability of the building on which it will be installed, but also of the surrounding buildings. For roof setbacks or roofs surrounded by taller buildings, the assembly needs to be designed so the reflected sunlight from the roof surface does not create problems with undesirable glare and heat redirected into windows above.
Without appropriate design considerations, reflected heat may negate some of the energy benefits of a cool roof. Some studies have found temperatures above a reflective roof may be higher than those over a traditional darker-colored roof, which may impact rooftop equipment, conduits, wiring, piping, and other materials subjected to the reflected heat. In some regions, widespread use of cool roofs may have even broader climate implications, as the heat redirected back into the atmosphere may adversely impact rainfall, necessitating appropriate tradeoff measures. As such, owners and design professionals should analyze geographically dependent variables when designing and detailing a cool roof.
Controlling glare and reflected heat continues to be a concern for cool roofs, but this should not deter building owners and project teams from considering reflective roof materials for reroofing projects. Where code requirements mandate cool roofs, excessive heat gain is unlikely to be a widespread problem, and glare may be managed through low-emissivity (low-e) window coatings, daylighting controls, baffles, and other architectural elements. Specifying a combination of vegetated and cool roof systems for different roof areas can offer a customized solution balancing comfort with performance.
A step ahead
Even where building codes do not mandate increased insulation, vegetated or cool roofs, or other energy-saving assemblies, it is still good practice to opt for assemblies meeting national standards for energy performance. The list of states and municipalities that have newly adopted codes based on ASHRAE or IECC standards is constantly growing, with some cities, such as New York, enacting energy and building codes even more stringent than those at the state level. Rather than chase after evolving requirements, design/construction professionals should stay at the forefront of energy efficiency policy by choosing assemblies meeting or, better, exceeding national standards.
In general, building codes for energy performance are the lowest end of what is acceptable, with plenty of room for improved efficiency beyond what is mandated by law. Increasing insulation levels from 2006 IECC-required values to those mandated in the 2012 edition of the code, for example, has been shown to have a significant impact on electricity consumption; installing a cool roof covering over this increased insulation further cuts electric bills by reducing peak demand.
As building performance standards change, so too do the product offerings from manufacturers, which may mean a straightforward replacement of an existing roof with the same or similar assembly is no longer an option. Rather than an item to check off the to-do list, reroofing presents the opportunity to consider possibilities for increased efficiency, better indoor comfort, improved building user experience, and enhanced aesthetics. Although some new technologies, particularly vegetated roof systems, present a greater up-front investment, they can return benefits of long-term durability and create a sustainable building amenity.
Russell M. Sanders, AIA, is executive vice president and senior director of technical services at Hoffmann Architects, an architecture/engineering firm specializing in the rehabilitation of building exteriors. As one of Hoffmann Architects’ first employees, he has been with the firm for 38 years and has expertise in the technical details and design considerations for roof replacement. He can be reached via e-mail at email@example.com.
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