Cool roofs, also known as reflective roofs, as the name suggests, keep your buildings cool, by reflecting back the heat from the sunlight. They help in reducing local air temperatures, as the sun is the major source of energy that keeps the roof hot during the summers. They help the owners in reducing electricity bills, as the need to turn on your air-conditioners ends up being nil.
The working principle of cool roofs lies in the reflection of energy, be it light as well as heat. The outer surface of the cool roofs help in reflecting the light and therefore, keeps the interiors during any climatic conditions. The normal dark roofs absorb nearly 85% of the light and heat energy and makes the interiors as warm as the exterior climatic conditions.
Solar reflectance and thermal reflectance is an important factor that is analyzed when choosing cool roofs. Solar reflectance refers to the amount of sunlight that is reflected by the roofs. Thermal reflectance refers to the efficiency of a surface to cool itself by thermal radiation. Solar Reflectance Index (SRI) is another metric for comparing the “coolness” of roof surfaces. It is calculated from solar reflectance and thermal emittance values. The higher the SRI, the cooler the roof will be in the sun. For example, a clean black roof could have an SRI of 0, while a clean white roof could have an SRI of 100. Dark roofs usually have an SRI less than 20.
Though white is considered as the best colour to reflect heat and light, thereby keeping the interiors cool, not all cool roofs are white in colour. A coloured surface that reflects a major percentage of sunlight is called a cool dark colour.
Types of Cool Roofs
Roofs are generally made of multiple layers. They are coated with special pigments that help in reflecting heat and light. Based on the placement of the roofs and the material used on the roofs, they have been classified as follows:
- Single-ply Membranes
They are pre-fabricated sheets that are fixed on to the roofs with mechanical adhesives.
- Built-Up Roofs
These roofs consist of a base sheet which is covered by fabric reinforcement layers and a dark protective surface layer.
- Modified Bitumen Sheet Membranes
These sheets are synthesized with several layers plastic or rubber with a layer of reinforcing fabric. This is a hybrid in the class of cool roofs.
Precautions and Considerations
Cool roofs have been preferred everywhere while building houses, to keep the interiors cool. But however, there are two sides to every coin. And the same applies to the case of cool roofs as well.
India being a rain-rich country during the monsoons, have the greatest problem while using cool roofs. There can be ponding, or accumulation of water if the roof is not placed steep enough for the water to flow out.
Moreover, due to the cool interiors and the warm exteriors, there could result in condensation of water vapour in the interiors, and in the long run, if left unattended, could cause great damage to the roofs.