Thermodynamics is hard.
The very concept of the green roof is an abomination, because it's antithetical to the purpose of a roof to begin with - a green roof is literally storing water on top of the building, on purpose. Roofs are sloped and bound by drainage systems to get the water out as thoroughly and quickly as possible. Water is the enemy, and yet with a green roof, we're inviting it into the building on purpose. Sounds cool, looks cool, dumbest thing ever.
But then again, cool roofs are also a problem. One of the ways a building gets rid of unintended and uncontrolled water intrusion is through the silent, invisible power of the stack effect (heat rises). A hot roof facilitates and amplifies this stack effect, cooking-out excess moisture that gets into the interstitial spaces in a building. You can make a building as perfect as possible, but water will get in there, because that's what it does, and because nothing is perfect.
What can sometimes appear like a deficit (roofs get too hot) can actually be an integral part of the overall design (ridding excess moisture).
This is similar to highly efficient (almost magically efficient) ambient heat pumps - they only use the exact amount of energy needed. Traditional air conditioners are over-designed in that they cool the air way more than they need to. But for hot and humid climates, this has the benefit of removing moisture from the air. A modern, magical heat pump that removes the heat but not the moisture, does not make a comfortable indoor environment, and could actually lead to unintended side effects like mold growth.
Cool roofs outperform green roofs in urban climate modeling study
July 2024, phys.org
A three-dimensional urban climate model of Greater London tested the thermal effects of different passive and active urban heat management systems, including painted "cool roofs," rooftop solar panels, green roofs, ground level tree vegetation and air conditioning during the two hottest days of the summer of 2018, and found that if adopted widely throughout London, cool roofs could reduce outdoor temperatures across the city about 1.2 C up to 2 C.Other systems, such as extensive street-level vegetation or solar panels would provide a smaller net cooling effect, only about 0.3 degrees C on average across London, though they offer other environmental benefits. Similarly, while green roofs offer benefits like water drainage and wildlife habitats, their net cooling effect on the city was found to be negligible on average.Though on average the effect of green roofs was negligible, the researchers found that their effect on temperature varied significantly throughout the day. During the warmest times of day, the wide adoption of green roofs could lower urban temperatures by an average of 0.5 degrees C. However, this would be offset overnight as the thermal mass from the roofs would retain daytime heat, releasing when the sun was down and increasing night-time temperatures by about the same amount.
via University College London: Cool roofs could be most effective at reducing outdoor urban temperatures in London compared with other roof top and vegetation interventions: a mesoscale urban climate modelling study, Geophysical Research Letters (2024). DOI: 10.1029/2024GL109634
Research shows how common plastics could passively cool and heat buildings with the seasons
Jun 2024, phys.org
Roofs and walls are not the same. Roofs have a clear view of the sky, where they can radiate their heat upwards. Walls can't radiate upwards, they're blocked by other things, and they absorb a lot of heat from surrounding buildings and pavement. They're also affected by different "kinds" of heat:
Radiant heat moves from buildings to the sky in a narrow portion of the infrared spectrum known as the atmospheric transmission window, so the researchers call this narrowband. At ground level, radiant heat moves across the entire infrared spectrum, and the researchers call that broadband."By coating walls and windows with materials that only radiate or absorb heat in the atmospheric window (like propylene), we can reduce broadband heat gain from the ground in the summer, and loss in the winter, while maintaining the cooling effect of the sky.
via Oak Ridge National Lab, Arizona State University, Princeton and UCLA: Radiative Cooling and Thermoregulation in the Earth's Glow, Cell Reports Physical Science (2024). DOI: 10.1016/j.xcrp.2024.102065.
New fabric makes urban heat islands more bearable
Jun 2024, phys.org
Only hats, shoulder coverings and the tops of shoes - about 3% of clothing - face direct sunlight. The other 97% of are being heated by thermal radiation from the sides and below."Solar is visible light, thermal radiation is infrared, so they have different wavelengths. That means you need to have a material that has two optical properties at the same time."In tests under the Arizona sun, the material kept 2.3 C (4.1 F) cooler than the broadband emitter fabric used for outdoor endurance sports and 8.9 C (16 F) cooler than the commercialized silk commonly used for shirts, dresses and other summer clothing.
via University of Chicago: Ronghui Wu et al, Spectrally engineered textile for radiative cooling against urban heat islands, Science (2024). DOI: 10.1126/science.adl0653
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