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Rooftop Cooling - White paint?

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Andrea Louise Boa

Rooftop cooling

Scientists at Perdue University have invented the whitest white paint, a Barium Sulfite compound which are said to be able to reflect 98,1% of light and cool rooftops with the power equivalent of 2,8 Air-Conditions pr. 1000 ft2. I have investigated the paints capabilities and compared it to rooftop gardens, to investigate if rooftop cooling solutions are the future of domestic buildings.

BaSO4 Paint vs Rooftop gardens - A look into Rooftop Cooling

DISCLAIMER! This article is targeting Cooling as a Service LEARN subscribers and is only intended to inspire and bring a holistic perspective to cooling of buildings. We cannot offer any guidance in the procurement process of any of these incentives except our own packages.  

In the fight against global warming, many are looking for more sustainable ways of using our resources. Some have new, innovative ideas and some go back to basics. All measures into sustainability are great, especially when it comes to more sustainable cooling, since cooling alone accounts for 10% of the world’s electricity consumption!

Scientists at Purdue University, Indiana have invented the whitest white paint to help reduce the need for air-condition. The uses made me want to investigate the paint and its function related to rooftop gardens, and to investigate possible negative side-effects to this very white paint.  

What is the whitest white paint?

The whitest white paint is primarily made of Barium Sulfate (BaSO4), a white compound used in many different things, such as cosmetics, as a lithopone (a brightening pigment in printer paper and paint), in oil well drilling fluids, glassmaking, creating rubber, and as a radiographic contrast agent to help diagnose gastrointestinal medical conditions.  

In a paper, a team of engineers experimentally demonstrate full-daytime sub ambient cooling performance with BaSO4 nanocomposite paints. The BaSO4-acrylic paint is developed with a 60% volume concentration to enhance the reliability in outdoor applications, achieving a solar reflectance of 98.1% and a sky window emissivity of 0.95 due to an appropriate particle size and broad particle size distribution.  

The BaSO4-acrylic paint shows a standard figure of merit of 0.77, which is among the highest of radiative cooling solutions while providing great reliability, convenient paint form, ease of use, and compatibility with the commercial paint fabrication process1.

BaSO4 is available in the natural mineral barite, and widely used in different industrial fields as radiocontrast agent, paper brightener and main components in cosmetic products. BaSO4 powders cost only $0.44 per kilogram, which is about half the price of TiO2 powders ($1per kilogram). Assuming a 30 ̊ roof angle and 300 μm paint thickness, the cost of the BaSO4 fillers reaches around $100 for a 150 m2 house. With a similar fabrication process to the commercial paints, the BaSO4 paint will reach a comparable price as the commercial white paint.  

In the literature, a comprehensive test was performed with commercial white roofing materials (85% solar reflectance) during one-month period in the summer and concluded the energy saving was 40 to 75Wh/m2/day. The BaSO4cooling paint achieved a higher Solar reflectance of 96.8% to 98.1%, leading to a total energy saving over 80 Wh/m2/day.

The scientists estimate that only 0.5–1% of Earth’s surface (e.g., building roofs) would need to be coated in this paint to reverse global warming to date2, which sounds amazing!  

“If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts3”. explained Xiulin Ruan, a Purdue professor of mechanical engineering.

A standard 2000 Sq feet house with a 3,5-ton AC is using around 3500 watts/hour on average in Cooling mode, making the BaSO4 paint on 1000 sq feet the equivalent of 2,8 AC´s.  

In domestic housing, this is a great start to save money on AC, but it would not be enough to cool down a high-traffic location such as a factory, hotel, or any large commercial building. The cooling power simply isn’t enough. That is why we recommend using cooling measures such as the BaSO4 paint together with a system like our LINKbeta or LEARN package to properly optimize a large commercial cooling system.  

The paint’s whiteness also means that the paint is the coolest on record. Using high-accuracy temperature reading equipment called thermocouples, the researchers demonstrated outdoors that the paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient surroundings at night. It can also cool surfaces 8 degrees Fahrenheit below their surroundings under strong sunlight during noon hours. 

The paint’s solar reflectance is so effective, it even worked in the middle of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the paint still managed to lower the sample temperature by 18 degrees Fahrenheit. 

This white paint is the result of six years of research building on attempts going back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners. 

The scientists suggest the paint could also cool the Earth's surface. According to the researchers, applying the paint to roads, rooftops and cars would offset some effects of the greenhouse gases they create. 

If the BaSO4 paint doesn’t cause glare, it would be very helpful for night driving in dark areas, leading to less need of road light and increased visibility, potentially leading to a decrease in electricity use in roadside lamps and fewer darkness-related accidents. That would be “cool” too.  

I can see both issues and great things with the use of the BaSO4 paint, but is it a better solution than other sustainable cooling solutions like rooftop gardens?  

Rooftop gardens

There are different types of rooftop gardens, depending on the level of maintenance and or size wanted.  

The two most common kinds of rooftop gardens are4.  

Studies by Environment Canada have shown that the upper floor of a building with a green roof is likely to save 20% of its energy demand by reducing its need for cooling. If a building is five storeys or more, the total summer savings would be in the region of 6% and with a two-storey building, the total summer savings would be between 10 – 12%. The study also suggests that there is a3-10% winter saving on fuel bills due to soil insulating the rooftop.

“The examination of the temperature reduction effect achieved by rooftop gardens showed that the gardens successfully reduced the temperature across a wider area in the morning than in the afternoon. In the morning, about 39% and more of the total area experienced a reduction in temperature compared to the afternoon.  

Roof gardens environmental effects go beyond simply reducing surface temperatures: the plants absorb carbon dioxide and hence help to mitigate the urban heat island effect. Besides, they often support the creation of garden spaces to help maintain a more diverse and resilient biological ecosystem. It has been documented that a 10-cm-deep soil layer can absorb 20–30 L of rainwater in each m2, reducing the pressure on urban storm drains and helping to prevent flooding during extreme rainfall events. Social effects include a more attractive urban environment that enhances inhabitants’ quality life and psychological stability; on the roof of a building with a garden installed, a 20 cm layer of soil has been shown to reduce ambient noise levels by as much as 46dB. The subsequent reduction in the amount of energy required to meet the building’s needs provides a further economic incentive to encourage greater rooftop afforestation”5.  

Another study found that “The plants and the growing medium in the rooftop garden kept the roofing membrane cool in the summer by shading, insulating and evaporative cooling. At one point of measurement, the outdoor temperature peaked at 35 C (95 F) in the afternoon. The membrane on the reference roof absorbed the solar radiation and reached approximately 70 C (158 F) while the membrane on the rooftop garden remained around 25 C (77 F). An exposed membrane absorbs solar radiation during the day and its surface temperature rises. It reradiates the absorbed heat at night and its surface temperature drops. Diurnal (daily) temperature fluctuations create thermal stresses in the membrane, affecting its long-term performance and its ability to protect a building from water infiltration”6. Although the paper doesn’t mention the duration of the long-term performance, the thermal stress does pose a threat and should be investigated when choosing rooftop cooling solutions.  


Rooftop gardens could be more expensive than white paint, but also poses more benefits to our eco system, rainwater absorption, air quality etc.  

Both solutions are great for lowering the need for AC cooling and could be used in different places. The white paint could be used in all the places with low accessibility (where the extensive garden usually is planted), and intensive rooftop gardens could be used where there is high accessibility, such as apartment buildings and public places, where the maintenance level is appropriate for the traffic.  

As previously stated, these measures are not enough to eliminate the need for AC, but a way of lowering the energy consumption. To make sure your Chilled Water System runs as efficiently as possible, we recommend a monitoring solution such as LINKbeta, or a total optimizing package such as our LEAD Solution. We are always here to help if you have any questions.  

Together we might be able to reduce the need for cooling and subsequently, cool humanity without warming the planet.  

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Andrea Louise Boa

Digital marketing assistent

Andrea joined Grundfos in 2021 and started taking care of our newsletters, our SEO strategy and blog posts almost instantly. Andrea has an AP Degree in Marketing Management from Copenhagen Business Academy, but she started working in marketing back in 2016. After her degree, Andrea worked as Head of marketing in a small business selling reclaimed pallet furniture but ended up in Grundfos.

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