By James Piper - May 2003 - Roofing
The building envelope gives facility executives a range of opportunities to improve the energy efficiency of facilities. Using high-efficiency building-envelope components not only reduces energy use, but also allows smaller heating and cooling systems to be installed, generating additional savings. While the opportunities can be found almost anywhere in the building envelope, the most significant savings can be found in roofs and windows.
Traditionally, most low-slope roof surfaces have been installed with dark surfaces. These dark surfaces are very effective absorbers of solar energy, typically soaking up between 80 and 90 percent of the solar energy striking them. This solar energy raises the temperature of the roof, increasing the heat gain to the conditioned space. On a clear, windless day, it is not uncommon to find that surface temperatures for dark roofs reach 180 to 200 degrees.
The heat gain through dark roofs contributes to building energy costs in two ways. First, the heat gain increases the load on the building’s cooling system. Every Btu gained through the roof must be removed by the cooling system. Second, the heat gain from the roof closely tracks the energy use profile for the facility. A dark roof contributes to the building cooling load and energy use the most when the rates for electricity are the highest.
A white roof surface reduces the load imposed on the building’s cooling system by reflecting much of the solar energy striking it. In contrast to a dark roof surface, a white roof surface will reflect between 70 and 80 percent of the incoming solar energy. In most cases, the temperature of a reflective roof will remain between 20 and 25 degrees higher than the ambient temperature. But because the surface temperature of the reflective roof is lower than it would be with a dark roof, the heat gain from the roof will be reduced, cutting air-conditioning requirements and energy costs. A side benefit of the reduced roof temperatures is a longer service life for the roof as cooler temperatures slow the breakdown of many roofing materials.
There are several ways to make the roof more reflective. Single-ply roof membranes are available in white finishes. Built-up roofs can be installed with white marble chips to increase reflectivity to about 70 percent. Reflective coatings can be applied to single-ply, built-up and modified bitumen roofs, generally increasing reflectivity to between 50 and 65 percent. Metal roofs can have white coatings applied that will reflect roughly 60 to 70 percent of the solar energy.
Not all reflective coatings are compatible with all roofing materials, however. Therefore it is essential that care be taken in matching coatings with roof materials.
While reflective roof coatings and finishes can reduce energy costs, they do require ongoing maintenance. Weather, dirt and exposure to the sun’s UV light will degrade the reflectance of most coatings and finishes, frequently by as much as ten percent per year. Also, many coatings have a service life of only three to five years and will require reapplication. If roof reflectivity values are to be maintained, roofs will have to be inspected on a regular basis, and surfaces cleaned or recoated as necessary.
When roofs are installed, facility executives can increase the thermal efficiency of the building envelope by carefully considering the type of insulation. Roof insulation comes in many different types; in general, a particular type of insulation is required for the type of roof being installed. Similarly, layers of roof insulation cannot simply be piled upon other layers of insulation. Roofs are designed to function with a specific thickness of insulation. Therefore, it is important to consider the thermal efficiency of different roof and insulation combinations when evaluating roofing options.
Equally important is how well the roof is maintained. Water destroys the effectiveness of roof insulation, reducing its insulating ability to near zero. Splits in seams, punctures in the membrane, failed flashings and open blisters all allow water to soak into the roof’s insulation. Even relatively minor defects will allow water to damage large areas of insulation.
All roofs should be inspected twice each year for damage that could compromise the integrity of the membrane. Damaged areas must be quickly sealed using approved materials and procedures for that type of roof. If there is reason to suspect that a large area of the roof’s insulation has been damaged by water, then additional testing, including core sample and infrared scanning, should be performed to quantify the extent of the damage and to identify the area requiring replacement.
New glazings offer opportunities to improve efficiency and comfort. Facility executives can select from tinted, reflective, spectrally selective, and low-emissivity (low-e) glazings. In addition, metalized films can be added to existing windows.
The key to the success of these glazings is their ability to control solar heat gain while allowing a specified amount of visible light to pass through. Tinted glazings are designed to absorb a portion of the solar energy striking the window, including both visible light and infrared heat. They are available in a variety of hues, ranging from bronze to gray; the darker the tint the greater the reduction in light passing through the window.
Reflective coatings produce the mirror-like appearance used on many high-rise buildings. The reflective coatings are designed to reduce both visible light and infrared heat passing through the glazing by as much as 70 percent. They are most effectively used in applications having large areas of glass that are located in warm climates.
Unlike both tinted glazings and reflective coatings, spectrally selective glazings allow visible light to pass through the glazing while absorbing most of the infrared heat energy. Rather than having a distinct color or a reflective appearance, spectrally selective glazings are very similar in appearance to clear glazings.
Low-e glazings offer a different approach. These glazings use a thin coating that allows visible light to pass through while reflecting — not absorbing — long-wave radiation.
For existing buildings, the new generation of window films offers facility executives the option of improved glazing performance without having to replace existing windows. Costing between $3 and $6 per square foot, these films offer excellent performance and scratch-resistance. Manufacturers control the properties of the films by making small changes in the film’s metalized layer. As a result, films are available that allow between 40 and 70 percent of the visible light to pass through them while blocking between 50 and 75 percent of the heat-producing infrared radiation. When spectrally selective materials are used in the film’s construction, the film can transmit as much as 80 percent of visible light while still blocking 75 percent or more of infrared radiation.
Before making any changes in a building’s glazings, a test window should be installed. New window glazings or the addition of film to existing windows will alter the appearance of the building. But changes go beyond aesthetics. New glazings might change the ability of people to see inside the building during daylight hours, or outside the building during the night. These factors must be considered before selecting a particular window glazing.
Contributing editor James Piper is a writer and consultant with more than 25 years of experience in facilities management.
Innovative energy-efficient exteriors are worth considering
A variety of innovative techniques can be used to improve the performance of the building envelope. Not all of these strategies work in every environment, but they are worth considering on a case-by-case basis.
Double-Skin Facades: Double-skin facades, long popular in Europe and gaining popularity in the United States, provide both energy savings and thermal comfort. The two facades, separated by air, create a thermal pad of sorts between an interior wall and an exterior glass wall.
Rain Screens: Already common in Europe, some new U.S. buildings are using rain screens. The screens provide a pressure-equalized thermal buffer between the interior and exterior walls, improving thermal performance and moisture control.
The Donald Danforth Plant Science Center in St. Louis County has a rain screen cladding system that reduces energy use. Terra cotta tiles suspended 4 inches in front of the exterior wall act as a break for sun, wind and rain. The 4-inch gap between the plates and the insulation prevents direct sunlight from striking the Plant Science Center. There is no thermal bridging from the exterior to the interior.
Transpired Collectors: Transpired collectors capture solar gain in a thermal cavity behind a dark-colored, perforated surface, where the air is warmed. That creates a preheated air supply for the mechanical system, improving its efficiency.
Roofs: In addition to light-colored roofs, so-called “green roofs,” in which vegetation is planted over the entire roof surface, reduce heat gain and the heat island effect. Roofs with vegetation provide thicker levels of insulation, which helps keep buildings cooler in the summer and warmer in the winter. In addition, green roofs provide habitats for for birds and insects. The roofs are planted with rugged species that are native to the region and can survive there year-round.
Photovoltaics: Photovoltaic panels can be placed on rooftops, parking lots or buildings. After the initial investment, the building owner has minimal maintenance costs.
Building-integrated photovoltaics place photovoltaic material directly in a common building-envelope system, such as the roof, skylights, curtain walls or sunshades. This eliminates the cost of the framework that supports the photovoltaics. Various incentives exist for these systems.
Landscaping: Using plant material to shade a building often is less expensive than other envelope systems.
— Bill Odell is sustainable design principal with HOK.
What the envelope is and isn’t must be tied to design
The building envelope of the 11-story, 370,000-square-foot expansion for the Alfred A. Arraj U. S. Courthouse in Denver incorporates light shelves to provide daylight for most of the office space; and a high-performance curtain-wall system that is triple-glazed, with two 1&Mac218;2-inch air spaces between and two low-e coatings, eliminated the need for radiant heating along the wall. The building is crowned by a series of photovoltaic cells.
The southeast facade of the building was originally designed with a double-wall system that was to operate as a thermal flue, providing preheated supply air to the mechanical system while functioning as a plenum to exhaust air from the mechanical system. It also was to superinsulate the facade in the winter.
Energy modeling and analysis, how-ever, revealed that the system was not cost-justifiable. A direct-indirect evaporative-cooling system is so efficient in the dry climate that the cost of cooling was extremely low. The amount of energy saved by a double-curtain wall was minimal.
— Bill Odell
Doing daylighting right
Although daylight can decrease the amount of electric light used, heat gain can offset any savings from reduced lighting loads. When buildings are internally load-dominated, meaning the building must be cooled for most of the year, it’s important to fine-tune the glazing system to harvest daylight without too much heat gain.
Spectrally selective coatings can be used to block most of the infrared and ultraviolet radiation while allowing the majority of the visible light spectrum through the glass. Even with high-performance glass, however, light energy still enters the building. This light energy then hits solid surfaces and is absorbed and reradiated in the space as heat.
With careful design, heat gain related to daylighting can be minimized. For example, in the Alfred A. Arraj U.S. Courthouse expansion in Denver, clear glass was used above the light shelf and slightly tinted glass below the shelves. The tint improves the shading coefficient, provides visual comfort and reduces heat gain.
In an expansion and renovation of the Missouri Historical Society Museum, high-performance glazing was used in combination with horizontal shades on the south elevation and vertical shades on the east-west elevations. Custom ceramic frits on the building improved glass performance.
— Bill Odell