Plate Nº 14 · recorded November 8, 2026
Reported finding
Porous Plastic Layer Keeps Cooling Roof Coatings 5.2°C Colder
A porous polyethylene aerogel from City University of Hong Kong kept cooling coatings 5.2°C colder in outdoor tests by reflecting 96.6% of sunlight while letting heat escape.
By Marcus Bennett3 min read635 words
In brief
- A porous aerogel layer kept cooling coatings up to 5.2°C colder than uncovered coatings in Hong Kong outdoor tests.
- The material reflects up to 96.6% of sunlight and transmits up to 94.5% of targeted infrared light.
- In an Abu Dhabi building model, the layer increased a cooling roof's estimated energy savings by about 55%.
- UV exposure for 75 days barely changed the material's optical performance.
- The study was published in Energy & Environmental Science by researchers at City University of Hong Kong.
A lightweight plastic layer kept an outdoor cooling coating up to 5.2 degrees Celsius colder than the same coating left uncovered, according to researchers at City University of Hong Kong. The layer, a white aerogel made from long-chain polyethylene, blocks most incoming sunlight while letting heat-carrying infrared radiation escape toward the sky.
The finding matters for anyone designing passive cooling systems — coatings that cool buildings without electricity. If the approach scales to full roofs, it could cut daytime heat gain and trim air-conditioning demand in hot climates.
How do these coatings normally work?
So-called radiative cooling coatings do two things at once. They reflect sunlight, and they emit heat as infrared radiation — light at wavelengths that can pass through the atmosphere into space. Because some of that radiation escapes skyward, a coated surface can cool below the temperature of the surrounding air.
The catch is exposure. An open-air coating also absorbs heat from its environment, which erodes the cooling effect. The Hong Kong team wanted a cover that slows incoming heat without trapping the infrared radiation the coating needs to emit.
What did the researchers build?
As reported in the journal Energy & Environmental Science, the team fabricated a white aerogel from a form of polyethylene with very long molecules. The material's structure contains many small pores. Those pores make it an insulator against incoming heat, yet the layer still transmits the infrared wavelengths the coating beneath it relies on to dump heat.
The numbers frame the trade-off:
- The aerogel reflected up to 96.6% of incoming sunlight.
- It transmitted up to 94.5% of infrared light in the wavelength range the researchers targeted.
Together, those two properties let the layer shield the surface while still giving heat a route out — separating two jobs that a single coating usually has to handle alone.
How did it perform outdoors?
The researchers tested the layer over coatings in two demanding climates: Hong Kong and Abu Dhabi.
In Hong Kong, the covered sample reached an additional temperature drop of 5.2 degrees Celsius compared with an uncovered coating. In Abu Dhabi, the covered sample also stayed cooler than the uncovered one over extended outdoor testing.
The team then modeled energy use for a four-story apartment building. In the Abu Dhabi scenario, adding the layer increased the estimated energy savings from a cooling roof by about 55%. That figure describes a boost to the roof's modeled savings — not a 55% reduction in the building's total energy use.
Will the material last in the sun?
Durability is the usual weak point for outdoor optical materials, so the researchers ran an accelerated aging test. They exposed a sample to ultraviolet light for 75 days. Its ability to reflect sunlight and transmit the relevant infrared wavelengths changed little over that period — an early indication, though only an early one, that the layer can withstand extended sun exposure.
The caveats are real. The outdoor results come from samples, not full roofs. The energy figures come from a building model, not measured utility bills. And an actual installation would raise questions the study does not answer: how to attach the layer at roof scale, and how it performs after years of rain, wind, and weathering rather than 75 days of simulated sun.
What changes for designers?
The work gives practitioners a new architectural option. Instead of asking one coating to both reflect sunlight and radiate heat, designers can now split the roles: a protective aerogel layer on top blocks incoming heat while the coating beneath keeps releasing it.
Passive cooling that works reliably at building scale would reduce some of the demand for air conditioning in hot, sunny cities. A 5.2-degree drop from a thin plastic layer suggests the idea has legs.
via knowridge.com (Source)
More from Marcus Bennett
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News editor covering regulation, policy and their impact on general science news at SciBeat.
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