In this article, my endeavour is to explore an effectual, eco friendly, roofing system that delivers long years of seepage-free performance with near zero maintenance along with enhanced building energy efficiency. Over the past few decades, rooftops have become larger contributors to extreme heat issues, as most of our roofs are poorly designed with non-reflective, heat-absorbing materials. As we know that dark-coloured substrates absorb more heat from the sun and so, staying comfy under dark-colored roofs means more air conditioning and higher energy bills. Further, they heat the air around them, contributing to what is known as the heat island effect. An urban heat island (UHI) is significantly warmer than its surrounding rural areas due to excessive human activities. Higher temperatures in urban heat islands directly translate to increased energy usage, mostly due to a greater demand for air conditioning. And when more energy is being spent, the power plants burn more fossil fuels, which in turn increase both pollution and energy costs.
Fundamentals of solar reflectance Index (SRI)
Defined by the Cool Roof Rating Council (CRRC), SRI is calculated as ‘the ratio of the reflected flux to the incident flux.’ Fundamentally, it is the ability of a material to reflect the visible, infrared and ultraviolet wave lengths of the sun, reducing heat transfer to the building. As such, a material’s contribution to a heat island decreases with increasing SRI. SRI is measured on a scale from 0-100, with the higher value representing a Cool roof. Zero refers to a temperature as hot as a black surface, while 100 refers to a temperature as cool as a white surface. So, a cool roof is a solar reflective roofing system that delivers higher solar reflectance and higher thermal emittance than standard designed roofing products.
Benefits of Cool Roofs
Cool roofs reduce significantly the heat island effect, a phenomenon in which heat-absorbing buildings increase the outside air temperature in urban areas by a few degrees centigrade. And they avert heat absorption by reflecting the sun’s heat and then emitting its radiation back into the atmosphere, resulting in comfortable and controlled indoor environments. The decrease in energy demand results in reduced burning of fossil fuels and decreased emissions. And with fewer pollutants being emitted into the atmosphere, the overall air quality is enhanced. The cool roof system that reflects solar radiation and emits thermal radiation, thus saving energy costs can earn LEED points. The key components of a cool roof system are a thermal insulating layer, a waterproofing membrane and a solar reflective coating.
Thermal insulating layer
In cool roof systems, the principal feature of a thermal insulation material is its ability to reduce the heat exchange between a surface and the environment, or between one surface and another surface. This is known as having a low value for thermal conductivity. R-Value, K-Value and U-Value are the three significant parameters of all thermal insulating materials. K-value indicates the capability of a material to conduct heat. Materials with low thermal conductivities do not easily allow heat energy to dissipate. R-value refers to a material’s ability to resist heat transfer at a certain thickness. While K-value deals solely with just the material, R-value refers to the material and its thickness. The higher the R-value, the more efficient is the insulation. U-Value assesses the rate of heat loss through a given thickness of a building element (roof, wall or floor).
There are a few thermal insulating materials used widely in cool roof installations such as extruded polystyrene foam, Fibre glass board, XPS board and PU foam. See the Table 1 to see the comparable properties of different thermal insulating materials and one can conclude that PU foam has the highest R Value and hence I would consider PU foam as the preferred thermal insulating layer of a cool roof system.
Spray polyurethane foam, frequently referred as SPF, is sprayed as a liquid which instantly foams 20 times to its original mass. SPF is environmentally friendly, contains no formaldehyde or ozone depleting chemicals and assists in providing good indoor air quality, and has the highest R-Value among the known thermal insulating materials. When the resin and hardener are mixed in the spray nozzle, an instant chemical reaction starts, and the liquid expands to form a solid foam surface that adheres well to the roofs. There are no seams or joints, which are the known sources of the leaks in traditional roofs. This seamless closed cell surface is wind resistant, water proof and resilient to foot traffic. Its high insulation value of R 6.5 to R 7 lessens heat penetration that lifts the buildings’ overall energy efficiency. Within minutes, SPF can be walked on as it achieves 90 percent of its full strength in about four hours. Polyurethane foam roofing claims numerous advantages over other roofing materials, mainly due to its preeminence in insulation and thermal efficiency.
The most significant part of a cool roof system is the seamless waterproofing membrane which prevents water ingress into the roofs. These liquid applied coatings which cure to form rubber-like elastomeric membranes, are capable of stretching and returning to its original shape without damage. There are two types of liquid applied PU membranes in the market: A single component (1K) system and a two component (2K) system. Both 1K and 2K liquid applied membranes perform very well, having exceptional physical and mechanical properties, with a tensile strength of 4-8 MPa and an elongation of 300-600%. The inherent toughness and resilience enables them to bridge structural or shrinkage cracks that may develop in the concrete in due coarse of time. They are almost 100% solids by volume (1K: 80-90 % solids, 2K: 100% solids) and able to build very thick films. These membranes are applied by conventional methods like brush, roller or single component airless spray, yielding around 450-500 microns per coat with a re-coat window of 4 hours (For 1K, the cure depends on relative humidity). They have ex-cep-tional crack bridging abilities to tackle the structural movements in the concrete and offer complete water-tightness with minimal maintenance and are applied without the use of bitumen, open flames or solvents.
Plural spray applied Polyurethane membranes
Liquid applied membranes described above, have certain limitations in film thickness as the single coat yields only 450-500 microns and hence multiple coats are desired to have a film thickness that matches with the preformed membranes, which led to the development of rapid setting liquid PU membranes, applied by plural component airless spray to achieve higher film thickness of 1.5- 2mm at one shot. These coatings, on spraying by plural component airless equipment like Graco or WIWA, cure instantly and achieve trafficable condition within an hour. Protecting the substrate from bulk water and root penetration, these membranes create a monolithic barrier across the roofs, sidewalls, and sloping surfaces. The pot life, once mixed of resin and hardener (normally in the ratio of 1:1 or 2:1), lasts for a few seconds only and the film sets rapidly, imparting excellent mechanical properties like tensile strength (8-10 MPa) and elongation (250-350%).
Plural spray applied Polyurea membranes
The top most line in these rapid setting liquid applied membranes is polyurea with its specific advantages. Even at very low temperatures, the polyurea coating will cure without producing any coating defects, usually associated with polyurethane coatings like bubbles, foams and moisture-caused blisters. Polyurea coatings have distinct advantages over PU membranes such as fast cure, moisture insensitivity, hydrolysis resistance, greater tensile strength, higher abrasion resistance, and rapid return to service. The most obvious advantage of polyurea is its superb tear strength, because of which the cured film cannot be punctured or torn under any sort of pressure.
Solar reflective coatings
The third component of a cool roof system is the solar reflective, aliphatic, polyurethane coating with a very high SRI, which on curing imparts a flexible, UV resistant film having excellent waterproofing characteristics. It contains hollow glass microspheres in order to achieve remarkable energy efficiency through superior insulating properties. Solar reflective coatings such as Terraflex SR work by refracting, reflecting and dissipating radiant heat, thus reducing heat build-up and heat transfer through building walls, ceilings and roofs. It is applied over the cured layer of polyurea or polyurethane membrane and is in white colour.
The role of cool roofs or solar reflective roofs is very significant in sustainable building design, which can deliver high solar reflectance and thermal emittance. A durable cool roof system, which is thermal insulating as well as waterproofing, is absolutely critical to ensure the long-term durability of the roofs as well as to keep the energy costs low. A PU based cool roof system, consisting of a PU foam, a polyurea waterproofing membrane and an aliphatic polyurethane solar reflective coating, will deliver the most desired performance in terms of waterproofing, thermal insulation and solar reflectance. The PU cool roof system that reflects solar radiation and emits thermal radiation, can earn LEED points. And lastly, they require very little to no maintenance and all the components of PU cool roof systems are manufactured, employing green technology which doesn’t contain any toxic or ozone depleting substance.
For Further Details
Cipy Polyurethanes Pvt. Ltd
T-127, MIDC, Bhosari, Pune – 411026. Maharashtra,
Ph: 020 20 66316400 / 20 66316430,