In this first of the "COOLER Cities" article series, we investigate the surface temperatures around us using our thermographic camera.
It may come as a surprise that our naked eyes can only perceive what is known as the ‘visible light’, naming every hue of the colour spectrum. Yet, there’s more than meets the eye. With the aid of a thermal imaging device, we unlock the secrets of the ‘invisible light’, known as infrared radiation, which we then translate into visible displays.
Testing our device on a few roof samples - black, grey and white-coloured roof panels, revealed a fascinating discovery: colours, or rather albedo values, affect surface temperatures. ‘Albedo’ is an expression of the surface’s ability to reflect sunlight on a 0 - 1 scale, with a value of 1.0 for perfectly white surfaces bouncing off all the light and absorbing none. Consequently, when reflecting all the sunlight the surface remains cool and does not heat up the air near those surfaces, making the light-coloured roofs more energy efficient (less hot) than their darker counterparts. From Image 1 below, our thermographic eyes discovered a vast difference of nearly 30°C between dark (albedo of 0.05) and white (albedo of 0.93) roof panels.
The image above very clearly shows the direct correlation between the albedo of materials and their temperature under the hot sun. The lower the albedo, the higher the temperature. A computer simulation of different roof surface colours shows the same trend:
Our simulation shows that a very light coloured roof (93% solar reflectance = 0.93 albedo) has a lower surface temperature than the ambient air temperature, even during sunny days. The reason being that the roof absorbs less heat than is radiated out to the cool sky. The above graph shows an average annual sky temperature of about 18 degrees Celsius, but our own thermographic photos of the sky show that the sky temperature can be significantly cooler, particularly on clear nights:
But this is not the whole picture. Because the albedo only addresses how much of the solar radiation is reflected or absorbed, but not how much heat can escape from the surface, once its become hot. For this purpose, we have to introduce the emissivity factor, which also varies between 0 - 1, with a value of 1.0 for perfect heat radiation away from the surface as opposed to not radiating any heat at all, which in the building sector is known as a radiant heat barrier and typically comes in the shape low emissivity (low-e) window glazing or as aluminium-foil installed in the roof assembly to block heat entry.
Once the two surface properties of albedo and emissivity combine, it is possible to calculate the Solar Reflection Index (SRI). The reference surface with a 100% solar reflection index, is a white surface (0.8 albedo) with an emissivity of 0.9. Likewise, the reference surface with a 0% solar reflection index, is a black surface (0.05 albedo), also with an emissivity of 0.9. In fact, most materials have an emissivity around 0.9.
When applying the SRI calculation to the two roofs that reflect 80% of the solar radiation, we get a surprising result, as most people would think a shiny metal roof would be cooler than a white painted roof. But it is not, solely due to the higher emissivity of the white painted metal roof:
The above image shows a remarkable lower solar reflection index of the shiny metal roof (SRI 61%) compared to a white painted metal roof (SRI 100%). Just because something looks shiny, does not mean that it is good at releasing heat. This is in fact the age-old principle that was just to keep coffee pots warm:
Interestingly, it is possible to get SRI values that exceed 100%, namely for materials that have higher albedo and/or higher emissivity than the standard white reference materials. An example is included in the table below, namely of a reflective roof material that reflect more than 90% of the solar radiation and also maintains a high emissivity value:
The above SRI graph shows that "cool" building materials like white plaster and SkyCool roof films can obtain SRI values beyond 100%. At the other end of the spectrum, an aluminium sheet roof that only reflects 55% of the solar radiation and has low-e properties (0.1 emissivity), ends up with the worst SRI performance of -14, i.e. lower than a black painted surface. Once again, it's the emissivity value that makes all the difference; refer to the SRI input parameters below:
“My fake green grass feels really hot! Why?
The grass isn’t always greener on the synthetic side, as fake grass fails to emulate the true essence of natural grass. Looking into the intricacies of nature’s design reveals a contrast; it not only lacks the authenticity of real grass but also plays a counterproductive role. While the real green grass had a measured temperature of 31°C on a scorching sunny day, the fake grass registers 20°C hotter, spiking up to 55°C.
Our thermographic investigation showed that is not only the physical surface properties of colour/albedo/emissivity that matters, but that the biological processes of plants, such as evapotranspiration, also play a crucial role in keeping down temperatures. The cooling effect of trees and plants will be the topic for our next article in the COOLER Cities article series, so stay tuned!
Contributions by: Simon Laporte and Gregers Reimann (IEN Consultants)
SRI Calculation tool:
SRI calculator by LBNL: https://www.usgbc.org/resources/lawrence-berkeley-national-laboratory-sri-calculator The excel calculator is free of charge
References:
Engineering Toolbox
E-Source magazine