The impact that roofs have on energy 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 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 dark, absorbing solar energy and the infrared portion of sunlight. Roof temperatures from noon to late afternoon are often 60-100F higher than the ambient temperature. While some heat is radiated back to the atmosphere, much is conducted to the conditioned space below.
Sustainable Roofing Principles
The concept of sustainability is changing how facility executives look at roofing. Roofs designed with sustainability in mind go beyond cost considerations, incorporating energy efficiency, the impact that the roof will have on the environment, and maintenance and renewal requirements over the life of the building.
Energy savings are one of the most attractive features of sustainable roofing. 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.
The roof has another impact on the environment: waste products. All roofs have a finite service life and 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.
Reflective Roof Surfaces
One of the most cost-effective steps to improve the energy efficiency of the roof is to apply a reflective coating like those from Conklin to the roof surface. An uncoated, black roof absorbs 70-80% of solar energy. When a white or light-colored coating is applied, the solar absorption rate decreases to 20-30%. Surface temperatures will be only 15-20F higher than the ambient temperature, far less than the 60-100F temperature rise found with uncoated roofs.
On average, reflective coatings reduce heat gain through the roof by 50% 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 10-15%.
Reflective coatings do not boost winter heating costs significantly.The heat loss through the roof small relative to the heat gain. For example, when it is 30F outside and 70F 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 70F 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 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 also help to reduce urban heat islands, which occur when a concentration of dark surfaces (e.g., 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. While coatings are durable, they do wear and their reflectivity decreases with age. Most must be reapplied every 5-10 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% 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%.
NOTE: Roof insulation and green roofs are other forms of sustainable roofing systems, which we will discuss in future blog posts.
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