Cool roofs work by reflecting sunlight that a dark roof would absorb. That keeps the roof, the rooms beneath it and, if enough roofs are treated, the surrounding streets cooler. Reflective ground can also add a little to the output of bifacial solar panels, which collect light on both faces. But the benefits depend on climate, insulation and upkeep, and in the UK a reflective roof can slightly raise heating needs in winter.
This guide covers how cool roofs work, what UK research has found, the newer radiative cooling paints, and where reflective surfaces genuinely help solar power.
How it works
Two properties decide how hot a roof gets. Solar reflectance, also called albedo, is the share of sunlight a surface reflects rather than absorbs. Thermal emittance is how well the surface sheds the heat it does absorb as infrared radiation.
According to the US Environmental Protection Agency (EPA), conventional roofing reflects only 5–15% of sunlight, absorbing the rest, while the coolest roofing materials reflect more than 65%. In US conditions, traditional roofs can reach 66–85 °C at the peak of summer, and highly reflective, emissive cool roofs stay about 28–33 °C cooler.
Less heat in the roof means less heat in the building. The EPA says cool roofs cut peak indoor temperatures by 1.2–3.3 °C in homes without air conditioning, and reduce peak cooling demand by 11–27% in homes with it. Berkeley Lab’s Heat Island Group found in 2004 that raising roof reflectance from about 20% (dark grey) to about 55% (weathered white) could cut cooling energy use by about 20% in US conditions.
That “weathered white” figure is a reminder that reflectance falls as a roof gets dirty. A white roof does not stay at its original brightness, so cleaning and recoating matter.
Real-world examples
London, summer 2018
A study by researchers at UCL and the University of Exeter, published in Nature Cities in October 2024, modelled London during the record-breaking hot summer of 2018. In a scenario where every roof was a cool roof, average near-surface air temperature fell by about 0.8 °C. The researchers estimated that this could have prevented 249 (32%) of the city’s estimated heat-related deaths in June to August 2018, valued at about £615 million.
In the same model, fitting rooftop solar panels to all roofs cooled London by about 0.3 °C and could have prevented around 96 deaths (12%), while also generating electricity. These are model estimates of what might have happened, not counted lives saved.
London’s two hottest days of 2018
A separate 2024 study by Brousse and colleagues in Geophysical Research Letters looked at London’s two hottest days that summer. It found cool roofs reduced outdoor temperatures by about 1.2 °C on average and up to about 2 °C in places, more than green roofs, rooftop solar or street trees. Air conditioning, by contrast, warmed outdoor central London by up to about 1 °C, because the heat removed from buildings is pumped outside.
The West Midlands
A 2019 study by Macintyre and Heaviside in Environment International modelled city-wide cool roofs in the West Midlands. It found they could offset 18% of the heat deaths linked to the urban heat island over a summer, equal to 7% of all heat-related deaths. City-centre daytime air temperatures fell by 0.5 °C on average and up to about 3 °C. Commercial and industrial roofs delivered more than half of the benefit.
A London office
At the scale of a single building, a 2014 simulation of a naturally ventilated, poorly insulated London office found that a cool roof’s direct effect on overheating indoors was much greater than its effect on outside air. The benefit is likely to be greatest on poorly insulated top floors.
Radiative cooling paints and films
A newer group of materials goes further than a white roof. They reflect almost all sunlight and give off heat as infrared radiation at wavelengths (8–13 µm) that pass through the atmosphere into space. On a clear day, that can cool a surface below the temperature of the surrounding air without using any power.
In 2014, a Stanford University team led by Shanhui Fan reported in Nature a seven-layer coating that reflected 97% of sunlight and cooled to 4.9 °C below air temperature in direct sunlight. It was the first demonstration of this kind of daytime cooling.
In 2021, Xiulin Ruan’s lab at Purdue University developed a barium sulphate paint that reflects 98.1% of sunlight. Guinness World Records lists it as the whitest paint. Purdue says it provides about 10 kW of cooling for a roof of roughly 1,000 sq ft, which works out at about 108 W per square metre. Press reports at the time put its cooling at about 4 °C below its surroundings at noon. Ruan has said its cooling power is “more powerful than the air conditioners used by most houses”, a comparison made with US homes. Purdue says a company partner is working to scale it up.
What the numbers say
- Conventional roofs reflect 5–15% of sunlight; the coolest roofing materials reflect more than 65% (US EPA).
- Cool roofs cut peak indoor temperatures by 1.2–3.3 °C in homes without air conditioning (US EPA).
- In modelling of London’s 2018 summer, cool roofs on every roof lowered average air temperature by about 0.8 °C, against about 0.3 °C for rooftop solar.
- On London’s two hottest days of 2018, cool roofs cut outdoor temperatures by about 1.2 °C on average.
- Radiative cooling materials have reached 4.9 °C below air temperature in direct sunlight in the lab (Stanford, 2014).
Reflective surfaces and solar power
A common claim is that reflective surfaces “concentrate” sunlight onto solar panels. Ordinary rooftop panels are not fed by mirrors. Concentrating mirrors are used in solar thermal power stations in very sunny regions, not on UK homes.
The real link is bifacial panels, which collect light reflected onto their rear face from the surface below. A field study at Heriot-Watt University in Edinburgh, by Alam, Gul and Muneer and published in 2023, measured how much extra light reached the back of the panels. Over a year, rear irradiance rose by more than 30% over white pebbles or tiles, more than 20% over concrete and 5–10% over soil. But more light on the rear face is not the same as more electricity, and the study found the chance of an energy gain above 30% was low.
So a white or light-coloured surface under raised bifacial panels can help, but the gain is modest and depends on the site. Combining a cool roof with raised bifacial panels is a reasonable idea in principle, though we found no UK measurements of the two together. For the basics of panels and yields, see our guide to solar energy.
Limitations / honest assessment
The winter heating penalty. A reflective roof absorbs less sunlight in winter too, which can raise heating demand. The EPA says that in the US the summer savings usually outweigh this, and that the penalty is smaller where winter sun is low, days are short and roofs are well insulated. How the balance works out in UK homes, where summer cooling demand is low, depends on the building, and there is no reliable UK figure for it.
Who benefits in the UK. Because cooling demand is low in most UK homes, energy savings are a weaker argument here than in hot countries. The stronger case in the UK is health: cooler streets and top-floor rooms during heatwaves, as the London and West Midlands studies suggest.
Weathering. Dirt and pollution reduce reflectance over time. Without cleaning or recoating, the benefit shrinks.
Radiative cooling and UK weather. Cooling below air temperature works best under clear, dry skies. Cloud and humidity reduce it, so performance in the UK is likely to be lower than in sunnier, drier places. We found no UK field data.
Models, not measurements. The London and West Midlands figures come from computer models of whole-city scenarios, such as every roof being converted. They show the scale of the potential, not what a single street or home would achieve.
The UK angle
Heat is now a real UK problem. The country recorded temperatures above 40 °C for the first time in July 2022, according to the Met Office.
Approved Document O (Overheating) came into force in England on 15 June 2022 and applies to new homes. It focuses on limiting solar gain through glazing and removing excess heat, with stricter expectations in high-risk locations such as central London. Designers can follow a simplified method based on glazing limits and shading, or use dynamic thermal modelling.
The London Plan 2021, in Policy SI 4 on managing heat risk, requires major developments to follow a six-step cooling hierarchy. The first step is to reduce the heat entering a building, with measures that include orientation, shading, “high albedo materials”, insulation and green infrastructure. Active cooling such as air conditioning comes last.
For households, a cool roof is one option among many for keeping a home comfortable. Our guides to saving energy at home and zero-energy buildings cover the wider picture, including insulation, which affects both summer overheating and winter heating.
Frequently asked questions
Do cool roofs save energy in the UK?
Sometimes, but less than in hot countries. They reduce cooling needs in summer, yet summer cooling demand is low in most UK homes, and a reflective roof can raise heating needs in winter. The clearer UK benefits are cooler top floors and cooler streets during heatwaves.
How much cooler do cool roofs make a building?
US EPA figures show cool roofs staying 28–33 °C cooler than traditional roofs at the peak of summer, and cutting peak indoor temperatures by 1.2–3.3 °C in homes without air conditioning. Modelling of London’s 2018 summer found cool roofs on every roof would have lowered average air temperature across the city by about 0.8 °C.
Do reflective surfaces boost solar panel output?
Only for bifacial panels, which collect reflected light on their rear face. A Heriot-Watt University study found rear irradiance rose by more than 30% over white ground, but the actual energy gain was usually much smaller.
What is radiative cooling paint?
It is a coating that reflects almost all sunlight and gives off heat as infrared radiation into space, so it can cool below air temperature without power. Stanford’s 2014 coating reached 4.9 °C below air temperature in direct sunlight, and Purdue’s 2021 paint reflects 98.1% of sunlight.
Are cool roofs required in new UK buildings?
Approved Document O, which applies to new homes in England, focuses on limiting solar gain through glazing and removing excess heat. In London, the London Plan lists high-albedo materials among the first measures major developments should consider for managing heat.
How long does a cool roof stay effective?
Its reflectance falls as it weathers and collects dirt. A white roof can drop to around 55% reflectance once weathered, so regular cleaning and recoating help keep it working.