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Heat gain and paving selection

By Sebastien Miller
It is well known that some surfaces become hotter than others, but now there are quantified results showing how much impact different materials have on the urban heat island effect.
Reports stating that 2016 was officially the hottest year on record reinforce the trend of ongoing and accelerated global warming. In the Middle East, air temperatures have been slowly increasing, reaching a summer high of 50.5°C in the Emirate of Abu Dhabi during 20151 and a scorching 54°C in Kuwait during 20162. Some projections suggest that the rate of warming in the Middle East is increasing 50% faster than the global average and that by 2071, cities could regularly experience days in excess of 45°C, with occasional peaks of 60°C3.

When reading these figures, you must remember that climate modelling is not exact and that there is disagreement among leading scientists about the rate and severity of global warming in the Gulf region. Nevertheless, landscape designers should take every possible step to create more resilient cities, including by making appropriate selections of materials. To this end, I recently undertook investigations into the thermal effects associated with material density in paving. Most designers already know that certain materials are hotter than others but what I hope to provide is quantifiable evidence showing the amount of heat gain associated with the selection of different materials and finishes. 



Figure 1, opposite, shows temperature recordings for each material, colour and finish. These experiments were recorded with an infrared temperature gun at five specific times throughout the day. 

Two sample areas were laid, one in full sun and the other in full shade, with measurements carried out over a four-day period during the beginning of June. This provided average readings, which eliminated unusually high recordings that could skew the data in any particular direction. 


Rate, intensity and duration of heat gain 

While peak temperatures give a snapshot of the worst-case situations, it is more useful to understand the thermal performance (heat gain / heat release) of different materials throughout the course of the day. 



Figure 2 shows the heat gain for different materials throughout the day. The temperatures are noted in degrees Celsius, and represent the average of readings of all materials, regardless of colour or finish. Each material was exposed to full sun in early June (summer). Sunrise was at 5:45am and sunset at 6:52pm. There was no cloud cover: none. Humidity was 35–39% throughout the day. 

Materials such as timber and marble peak before midday, yet remain only 1.2–2.4°C warmer than the recorded air temperature in the late afternoon.  Limestone slowly warms up throughout the morning, peaking by mid-afternoon before it cools to 0.8°C above the evening air temperature. Granite and asphalt absorb significant heat very rapidly, reaching peaks of 12°C higher than the midday air temperature. Not surprisingly, asphalt is the worst contributor, and remains 8°C hotter, even in the early evening when people are most likely to use public spaces in the Middle East. 




If designers were to approach Middle East projects with the aim of increasing ambient heat by an extra 2–12°C, and making the public realm less comfortable for users, we would be ridiculed. Yet by ignoring thermal properties, we do that for almost every project. We look at aesthetics and durability (ensuring projects photograph well), but pay far less attention to user comfort. The graph in figure 3 shows the extent of heat gain and its duration for different materials. This is specific to the Gulf region and is based on the recorded temperatures as noted in figure 2. The air temperature (grey) is provided as a baseline comparison. 



Figure 4 demonstrates the way that different materials respond to the rate of heat absorption, the intensity of heat gain, and the duration of heat release. Analysis of the recorded temperatures demonstrated four key characteristics, which are differentiated by heat absorption, intensity and duration. This is based on the recorded temperature graph as noted in Figure 3. 

Figure 5 shows a housing development in the UAE that uses pre-cast concrete blocks (Heat Gain-3 (HG-3) rating) to minimise the urban heat island effect. Such a material choice reduces public realm temperatures by almost 4°C in the evenings and 7.8°C at peak times. Other benefits of pre-cast concrete blocks are that they create more pedestrian-friendly streets visually. They also reduce vehicle speeds slightly, as higher speeds (60km+) create a noticeable vibration effect and noise for drivers. In contrast, granite (HG-4 rating and shown in Figure 6) can reach temperatures of just over 60°C at mid-day, while concrete (HG-3 rating) can reach temperatures of just under 50°C. In hot climates, such materials should never be considered for seating or planter walls, as these temperatures are physically painful to the human touch. Schemes that do not consider basic human comfort will fail in a warming climate. Gravel aggregates, as seen in 

Figure 7, have recently become popular for Middle Eastern landscapes, for use instead of grassed areas. While this is a brilliant water-saving approach to design, little consideration is given to the urban heat island effect. The materials specified are almost always granite or gabbro on account of their low cost, durability, and availability on the market Unfortunately, granite is rated HG-4 and can contribute to the urban heat island effect by 7°C or more. A more environmental approach would be to use crushed limestone (HG-1), which would only contribute to the UHI effect by 1–2°C.

Conclusion 

In order to reduce the impacts of climate change and build resilience to our cities, designers should look at limiting or avoiding HG-4 materials. While this may not always be possible in the face of performance and cost-related issues, there are some very ordinary projects that have achieved this. Poor material selection has the potential to add 20°C to localised hardscape temperatures and, on a wider scale, it will be certain to contribute to the urban heat island effect. With some scientists estimating that we will have Middle East summer temperatures that regularly exceed 45°C 3, are we faced with living and working in cities that have a real temperature of 55–60°C when combined with the effects of poor material choice? 

Landscape architecture is not just a matter of contributing to better quality and aesthetically pleasing urban environments. It also relates directly to reducing the energy costs and carbon footprints of our cities, while at the same time increasing user comfort. As our planet continues to warm and cities get larger both in area and density, this is an issue that needs to be taken seriously.



Sebastien Miller has 15 years’ experience in landscape architecture, public realm and urban design. He is currently working from the Broadway Malyan studio in Abu Dhabi while studying his MSc. in urban and regional planning.

References
Abu Dhabi, National Center of Meteorology & Seismology;
UN, 2016. Temperature in Kuwait hits 54 celsius. Available from: www.un.org/apps/news/story.asp?NewsID=54559#.WF4X2HfpNAY;
World Bank, 2016. Adaptation to a Changing Climate in the Arab Countries. As referenced in www.thenational.ae/uae/environment/too-hot-to-live-in-the-gulf-dont-be-so-sure;

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