Hot and dry climatic regions are synonym with high solar radiation levels, high ambient temperatures and relatively low specific humidity. Dr. Ajay Singh Jethoo, Associate Professor, Department of Civil Engineering, Malaviya National Institute of Technology, Jaipur explains the various approaches to redirect the solar radiations from the roof by using different roof coating materials and their relative benefits in terms of energy as well as cost savings.
The roof surface of the building is directly exposed to the intense perpendicular solar heat radiations and is a significant mode of heat transfer inside the building. The solar radiation heats the roof which in turn reradiates the heat onto the ceiling and finally on to the room occupants.
At higher roof temperatures, the deterioration of roofing materials starts and leads to increased roof maintenance costs and high levels of roofing waste directed to landfills. The best way to reduce this is with the help of radiant barriers such as light coloured metallic or non-metallic surfaces which have high solar reflectance and a high emittance to redirect the radiations back to the environment. By blocking the solar radiations, the building air-conditioning is largely mitigated as it usually uses largest portion of energy in buildings.
The heat stored in the roofing material is transmitted slowly through the roof into the rooms. Due to large thermal mass of roof, this phenomenon of heat transfer continues even after the sunset. With the use of roof coating, the room will remain cooler during summer and warmer in winter as compared to outside. To maximize cooling energy savings, high-albedo roof coatings should have high solar reflectance (both in the visible and near-infrared bands) and high infrared emittance and maintaining these properties over the service life of the coating.
Roof coatings such as acrylic and metal-based, protect the roof from moisture and add an insulating barrier between the roof and interior of a building. To rate the level of reflectiveness of different products, manufacturers have come up with a classifying system. The two basic characteristics that determine the performance of a cool roof are solar reflectance and thermal emittance. Both properties are rated on a scale from 0 to 1, where 1 is the most reflective (Draft of USAID ECO III, 2007). Although, there is no industry-wide definition of a coated roof, the EPA’s Energy Star Roof Products Program has established a minimum standard, requiring that good coating roof products have an initial reflectance of at least 0.65, and a reflectance of at least 0.5 after three years of weathering. By contrast, conventional asphalt roofs have a reflectivity of between 0.06 and 0.26, resulting in large amounts of heat transfer into the building’s interior.
The Lawrence Berkeley Laboratory in California estimates that reflective roof coatings can save a minimum of 25% in energy usage and the ultraviolet blocking pigments of the coatings can increase the life of the roof by almost 15 years. This is especially true in buildings with low slope or flat roofs, which tend to absorb sun energy at a higher rate than slanted roofs. Since roof maintenance accounts for more than 80% of a building’s total maintenance cost, any product that can extend roof life will result in substantial money savings.
Roof Coating Materials
Roof coatings can be divided into two categories – field-applied and factory-applied. Field-applied coatings are applied directly onto the roof surface, either on a new roof assembly or over an existing roof surface. Factory-applied coatings are applied during the manufacturing process. Examples of factory-applied coatings include coatings applied to metal and glazes that are applied to tiles.
Roof coatings fall into two general categories – elastomeric-based and bituminous-based products. The base materials for elastomeric-based coatings include acrylics, hypalon, neoprene, silicone and urethane and hybrid materials. These coatings are compatible with most types of roofing systems but appear to be used most widely on single-ply, spray-applied polyurethane foam, and metal roofing systems. Bituminous-based coatings are used as a cost-efficient material to prolong the life of metal roofing.
Elastomeric-based Coatings: Elastomeric-based coatings is suitable for all types of roofs including metal, modified bitumen, single ply, and polyurethane foam and can reflect up to 90% of total thermal radiations. Due to effective reflective properties, cooling load in building is reduced significantly. Elastomeric can also be used on most built-up and modified bitumen systems. It reduces daily thermal expansion and contraction of metal roofing due to lower temperature. The coating contains very strong rust inhibitive pigments and fungicides and good adhesive properties on substrates. It has excellent reflective properties which help to reduce cooling costs. This is due to the fact that the majority of heat reflective metal roof paint coverings will also act as insulators. The coating prevents premature degradation, has superior resistance to pick-up dirt and maintains its reflectivity. Elastomeric-based coatings are non-toxic and VOC compliant.
Bituminous-based Coatings: Bituminous-based coatings are manufactured to be compatible with either asphalt or coal-tar built-up roofing systems or with modified bitumen membranes. Leaks in modified bitumen roofs are difficult to isolate and a seamless roof coating system is a cost effective solution. A reflective coating applied within five years of installation can extend the service life of the membrane by 10 or more years.
Procedure of Applying Roof Coating
Inspection: Before applying any coating, inspect the roof to look for signs of wear, cracks, uneven bumps and debris. If the roof is old or cracked, address these issues before applying the coating. Older roofs in need of replacement should not be coated because the coating will cause difficulty when replacing the roof.
Cleaning: A roof coating is basically a type of paint, which means that the surface will need to be cleaned before the coating will be able to stick to the roof. A pressure washer is the best way to get rid of dirt and other contaminants on the roof. Pressure washers should not be used on gravel roofs because it will cause stones to come loose and ruin the surface of the roof. Gravel roofs should be cleaned with brushes and a cleaning detergent that will not damage the adhesive.
Preparation: Post inspection and cleaning, any visible cracks or damages should be repaired before applying the roof coating. This is usually done with roofing putty or tar. The putty will provide a drier surface to paint over. If tar is used, it will bubble during hot days which can damage the roof coating. Any loose shingles or paper can be nailed back into place. Fix any leaks in the roof before proceeding to the coating stage. Allow any repairs to dry for 24 hours in the warm sun before proceeding.
Coating: The easiest way to apply roof coating to a flat roof is with a roller and to shingled or rocky roofs is with a paint sprayer. Allow each roller applied coat to dry for four to six hours before applying a second coat. Generally, two to three coats will be enough to add effective coverage to the roof. In the sprayer method, only apply the paint on a low wind day. Tape off areas of the roof that you do not want to paint. It is necessary to apply four to five coats with a sprayer.
Major Benefits of Roof Coating
Increased lifespan of roofing materials: Coated roofs do not experience thermal cyclic loading as much as conventional roofs. Thermal cyclic loads can deteriorate the bond between shingles and asphalt, potentially causing premature failure.
Energy savings: According to the US Department of Energy, some reflective roof products can lower roof surface temperature even up to 100 ˚C and can reduce peak cooling demand by as much as 15%. However, coated roofs may result in increased heating costs during the winter, but this increase is greatly outweighed in hot and dry climatic areas where major energy savings achieved during summer and winter is very short and mild.
Reduction of the ‘heat island’ effect: This is the tendency for metropolitan highly populated areas to be warmer than their surroundings due to the large amount of building materials that retain heat. Studies have shown that this phenomenon increases the risk of death during heat waves in summer and decreases air quality by increasing the production of pollutants at higher temperatures, such as ozone and NO2. Cooler air in metropolitan cities slows down the formation of smog, beyond reducing cooling energy needs and emission production.
Lower peak electrical demand: The peak energy load an electric utility experiences in late afternoons around 3‘O clock in summer as businesses and residences turn up their air-conditioners instantaneously, is reduced by the implementation of reflective roofs. Roof coatings reduce the roofing materials produced: If cool coatings are used to increase roof life, this waste can be greatly reduced and the energy and materials needed to produce new roofing can be conserved.
Evidently, coatings that reflect the sun’s UV rays and infrared radiation reduce absorbed heat and prolong roofing life, they also increase energy efficiency of air-conditioning ducts that run through the plenum, more so if the plenum space is un-insulated. Roof coatings have noticeably provided energy savings of up to 70% in hot and dry conditions of western Rajasthan and 20% to 50% in moderate climatic regions.