The impact that roofs have on energy is often overlooked, the impact of which can be significant. In winter, insufficient or damaged roof insulation allows heat to readily escape. In summer, heat gained through the roof increases not only the cooling load, but also the peak cooling load and — for buildings where the primary cooling system is electrically driven — the peak electrical demand. A flat or low-slope roof gives maximum exposure to the sun’s rays when the building’s cooling loads and costs are the highest.
Most flat and low-slope roof materials are black. These materials readily absorb solar energy, including the infrared portion of sunlight. Roof temperatures from noon to late afternoon are often 60 to 100 F higher than the ambient temperature. While some heat is radiated back to the atmosphere, much is conducted to the conditioned space below.
The impact the roof has on energy use depends on the climate, the orientation of the roof, the thickness and quality of insulation, the reflectivity of the roof’s surface, and how well the roof has been maintained.
Good roof maintenance should be a given. Leaks can allow water to penetrate the surface and saturate the insulation, destroying its thermal resistance. Wet insulation is almost the equivalent of no insulation. Wet insulation also accelerates the deterioration of other roofing components. It is essential for both energy efficiency and the integrity of the roof that roofs be properly maintained.
Just as the growing use of life-cycle costing has changed how building system and component options are evaluated, the concept of sustainability is changing how facility executives look at roofing. Roofs designed with sustainability in mind go beyond first cost considerations and incorporate three other key factors: energy efficiency, the impact that the roof will have on the environment, and maintenance and renewal requirements over the life of the building.
One of the most attractive features of a sustainable approach is energy savings. By limiting the heat loss from the building during the heating season and by reducing the heat gain during the air-conditioning season, facility executives can reduce energy use and the environmental impact from energy use. Although a sustainable roof is more expensive than conventional roofs, energy savings recover those costs in a relatively short time, typically three to five years.
In addition to energy, the roof has another impact on the environment: waste products. All roofs have a finite service life. At the end of their service life, they have to be removed, replaced, recovered or renewed. A sustainable approach minimizes the waste produced from the roof over the life of the building by minimizing the number of times that the entire roof must be fully removed from the building. Instead of requiring removal and replacement, the roof is built from materials that can be recoated or renewed.
One of the most cost-effective steps to improve the energy efficiency of the roof is to apply a reflective coating to the roof’s surface. An uncoated, black roof absorbs between 70 and 80 percent of the solar energy that strikes it. When a white or light-colored coating is applied, the solar absorption rate decreases to 20 to 30 percent. Surface temperatures will be only 15 to 20 F higher than the ambient temperature, far less than the 60 to 100 F temperature rise found with uncoated roofs.
On average, reflective coatings reduce heat gain through the roof by 50 percent for buildings in warm climates. Because most air conditioning systems are electrically driven, the reduction in cooling load cuts demand for electricity when the building is setting its peak use; peak reduction is typically between 10 and 15 percent.
Moreover, reflective coatings don’t boost winter heating costs significantly. One reason is that the heat loss through the roof is often small relative to the heat gain. For example, when it is 30 F outside and 70 F inside, the temperature differential is only 40 degrees — probably less because the sun will heat up even a reflective surface somewhat. During summer, the temperature differential is 70 F or more. The cost of air conditioning energy is also higher than the cost of heating energy. Finally, in winter, flat roofs in cold climates may well be covered with snow, which reflects solar energy.
Reflective roof coatings can also extend roof life. Ultraviolet light from the sun breaks down roofing materials, causing them to weaken or become brittle. Reflective roof coatings shield roofing materials from ultraviolet light.
High surface temperatures also accelerate the breakdown of roofing materials by increasing the rate at which chemical reactions take place, reactions that weaken materials or reduce their flexibility.
Uncoated roofs are also subjected to wide temperature swings during the day. Reflective coatings greatly decrease the thermal stress generated by temperature swings, which can lead to early component failures.
Reflective roofs can also help to reduce an environmental impact that all dark-surfaced roofs contribute to: urban heat islands. Urban heat islands occur when a concentration of dark surfaces, such as roofs and asphalt pavement, absorb enough solar radiation to elevate the surrounding temperature by several degrees. These higher local temperatures increase cooling loads and energy use.
Reflective coatings must be matched to the type of roof installed, as not all coatings are compatible with all roofing materials. Most coatings can be applied for less than $1 per square foot. While coatings are durable, they do wear and their reflectivity decreases with age. Most must be reapplied every 5 to 6 years to maintain reflectivity and protective properties.
When considering reflective coatings, look for one that carries the Energy Star® label. These products have been certified to reflect at least 65 percent of the solar radiation that strikes the roof, without reducing the quality or the performance of the roof. Some products that carry the certification have reflectivities as high as 85 percent.
Depending on the type of roof and the structure on which it is installed, a range of roofing insulation products can be used, including fiberboard, fiberglass, perlite, cellular glass, corkboard, polyurethane and polystyrene. Most come in flat boards ranging from 1 to 4 inches thick. Boards are available with a built-in taper of 1&Mac218;8 to 1&Mac218;4 inch per foot for roofs where the insulation is used to increase slope for roof drainage. Not all insulation types are compatible with all membranes.
The most obvious function of insulation is to limit heat transfer to or from the building interior through the roof. However, more insulation is not always better. While additional insulation may decrease the rate of heat transfer into or out of the building through the roof, it can also stress the roof membrane by increasing thermal shock. Thermal shock occurs when large and rapid swings in temperature cause the membrane to move relative to the roof’s deck. The thicker the roof insulation, the greater the potential for thermal shock. If the membrane moves relative to the roof deck, failed flashings, splits in the membrane seams and cracks around roof penetrations could occur. Therefore, it is essential that manufacturer’s guidelines be followed when determining type and thickness of roof insulation.
One type of roof that, while not new, is receiving considerable attention is the so-called green roof, a system that includes vegetation as part of the roof. The vegetation is planted in a layer of dirt or an artificial growing medium as the top layer of the roofing system. The bottom layers consist of a conventional roof applied to the roof deck. Between the two layers is a drainage system to carry away excess water and a filter system to prevent the growing medium from entering and clogging the drainage system.
The green roof offers several advantages. The vegetation and growing medium shield the other roofing components from ultraviolet rays. The green roof also adds another layer of insulation between the building and the outside air. Solar heat gain is nearly eliminated. Evaporation of water from the growing medium and the loss of water from the leaves of the plants provides evaporative cooling for the building. Major temperature swings in the roofing membrane are practically eliminated. Like reflective roofs, green roofs help reduce the urban heat island effect. The roofs can also help control storm-water runoff by temporarily storing and then slowly releasing rainwater.
One concern with green roofs is the difficulty of finding leaks.
Before a green roof can be installed, the building’s roof structure must be studied to determine that it can carry the additional weight of the growing medium and retained water. Vegetation must be carefully selected based on the local climate, including range of temperatures, rainfall, wind conditions and sun exposure. It is very important to have someone with a good deal of experience with green roofs design the roof and oversee the installation.
What is the net economic effect of a sustainable approach to roofing? Facility executives have found that their cooling loads decrease, some by as much as 40 percent, without a significant increase in winter heating loads. By lowering the temperature of the roof’s surface, roofing materials have an extended service life. Reduced thermal stresses on components cuts maintenance requirements and extends the service life of the roof. The additional first cost of a sustainably designed roof is typically recovered in three to five years.
James Piper has more than 25 years of experience in the facilities field. He is a contributing editor to Building Operating Management.