Introduction
In reference to operational and energy saving opportunities, commercial building property managers and facility managers have often been told that they are sitting on a gold mine. This adage may be true, but property managers and facility managers astute enough to assess the possibilities of their roof, are realizing that they may be sitting under a gold mine as well.
Roofs, often overlooked by building owners, are starting to be viewed in a new light: as an asset to be optimized. Not only can new roofing technologies reduce cooling costs and energy consumption of a building, roofs can play an important role in reducing stormwater runoff and improve air quality. Several different technologies have emerged in recent years that can be easily incorporated into new construction projects or installed on existing buildings.
Typical roofs are non-white, non-porous surfaces that absorb heat from the sun’s rays. Heat gain increases the load on HVAC systems causing them to run more often to keep occupants comfortable. This increases energy use, often during expensive peak times in the summer. In the clustered urban settings of most major cities, roofs also contribute to the heat island effect. This phenomenon occurs in urban (and suburban) areas where highly absorbent materials such as pavement, asphalt, and roofs cause local temperatures to increase 2 to 10°F compared to rural areas. Elevated temperatures increase peak energy demand and air conditioning costs resulting in increased power plant emissions. To combat rising energy costs and negative environmental impacts, building owners have several options when considering how to use their roofs effectively.
Green Roofs
Also called vegetative roofs, green roofs are thin layers of living vegetation planted over a waterproofing membrane. Additional layers, such as a root barrier and drainage and irrigation systems may also be included. The benefits of green roofs are numerous. On hot summer days, the surface temperature of a green rooftop can be cooler than the air temperature, lowering indoor temperatures and reducing the need to cool the interior space as frequently. Green roofs reduce stormwater runoff by absorbing large amounts of rainwater, and depending on local construction codes, it may be possible to do without storm water infrastructure investments. Another factor to consider is the life of a roof, which can be extended by installing a green roof. This is because fewer intensive solar rays reach the rooftop directly, limiting damage from UV radiation as well as daily temperature fluctuations, which cause repeated contraction and expansion in typical roofing materials. In addition, green roofs offer an attractive urban garden for building occupants to enjoy.
Green roofs are divided into two main categories: extensive and intensive roofs. Extensive roofs consist of little-to-no-maintenance landscaping in shallow soil depths (3 in. or less). Appropriate plant varieties for this medium are typically mosses, grasses and some wildflowers. Intensive roofs have thicker depths (6 in. or more) and are multi-layer constructions with elaborate irrigation and drainage systems requiring intensive maintenance. The advantage of intensive roofs is their ability to incorporate a wider variety of plant types, including shrubs and small trees, and they are designed to be accessible for use as parks or building amenities.
The savings associated with green roofs can be significant. According to a December 2000 Environmental News Network article, “a 3 to 7-degree temperature drop translates to a 10% reduction in air conditioning requirements. For a one-story structure with a green rooftop, cooling costs can be cut by 20 to 30%.” In addition to saving energy and money, green roofs can increase the asset value of the property. It is well-known that reductions in operational costs increase asset value. With typical market capitalization rates of 10%, every $1 saved in operational costs translates to $10 in increased asset value. Changing an otherwise unappealing area of your building to an open-space green roof could increase asset value further while providing tenants and occupants pleasant space to use.
Reflective Roofs
Sometimes called cool roofs, reflective roofs have emerged in recent years to combat the heat-trapping effects of dark-colored roofing materials. According to the U.S. Environmental Protection Agency (EPA), over 90% of roofs in the United States have dark-colored surfaces. Because of their low-reflectance, they can reach temperatures of 150 to 190°F on hot summer days. In addition to higher energy costs, the intense heat absorbed by dark-colored roofs accelerates the deterioration of roofing materials creating increases in maintenance costs. By contrast, buildings that use reflective roofing materials stay up to 70°F cooler than traditional roofs during peak summer weather.
Reflective roofs incorporate both high solar reflectance, or albedo, and high thermal emittance. Solar reflectance is a measure of the percentage of solar radiation that is reflected when it strikes the roof surface. Thermal emittance, or emissivity, is the percentage of energy a roof can radiate away after it is absorbed.
Warm climates generally experience the most significant cost savings from using reflective roofing materials, but cooler climates can also benefit. Even when a building uses heat systems more than air conditioning, the EPA and the Department of Energy’s Oak Ridge National Laboratory have found that even in cold climates, there is a yearly net savings for using cool roofs. In the summer, the days are longer and the sun is directly overhead meaning more intense heat gain. Air conditioning, which uses electricity, is much more expensive to run than many heating fuels such as natural gas or oil. Finally, in the wintertime, the ratio of cloudy days to sunny days decreases and roofs have more snowcover making the reflectivity of the roof unimportant.
While actual energy saving depends on many variables, the use of high albedo roofing with appropriate insulation can result in significant savings. According to the EPA cool roofs can reduce peak cooling demand by up to 15%. In one example, an 1970’s vintage, 83,000 sq.ft. elementary school in Alexandria, VA installed a high albedo roof increasing the reflectivity of the roof from less than 20% to more than 78%. The result was an annual energy cost savings of $31,000, or just over 25% of the average annual energy bill, and substantially increased comfort for the teachers and students.
Closing
Is a green or reflective roof right for your building? The answer, as you would expect, depends. What can be said is that the cost savings, increased asset value, and increased comfort found in those buildings which have replaced their heat-trapping, dark-colored roofs with green or reflective roofs has been substantial. Instead of sitting under a potential gold mine, find out if green or reflective roofs are right for you.
Resources
There are many businesses that specialize in installing green or reflective roofs. It is important to consider the individual project and it’s location to find the best option.
The ENERGY STAR® Roof Products Program (sponsored by the Environmental Protection Agency) provides guidance for consumers in selecting a roofing product that suits their needs, as well as a list of products that meet the qualifications established for reflective roofing products.
The U.S. Green Building Council offers credits toward achieving their Leadership in Energy and Environmental Design (LEED) certification for using vegetative roofs and reflective roofs.
