Short answer
A lot, on a sunny day: Lawrence Berkeley National Laboratory's Heat Island Group reports that a clean white roof reflecting 80% of sunlight stays about 31 °C cooler on a summer afternoon than a grey roof reflecting 20%. What matters is solar reflectance across the whole solar spectrum, plus thermal emittance — so 'cool-coloured' dark roofs that reflect invisible near-infrared light can also run much cooler than conventional roofs of the same visible colour.
A roof's temperature in sunshine depends mainly on two radiative properties. Solar reflectance is the fraction of incoming sunlight — ultraviolet, visible and near-infrared — that the surface reflects. Thermal emittance is how efficiently the surface sheds the heat it has absorbed as long-wave infrared. They are independent because they concern different parts of the spectrum: sunlight arrives at roughly 0.3 to 2.5 micrometres, while a warm roof radiates at roughly 4 to 80 micrometres. A shiny bare metal roof can have high solar reflectance but low emittance and still get hot; a white coating on the same roof has both. The Solar Reflectance Index, calculated by the method in ASTM E1980, combines the two into one number.
Only visible light affects colour, but about half of the sun's energy arrives as invisible near-infrared. A conventional dark roof absorbs strongly across ultraviolet, visible and near-infrared. A 'cool-coloured' roof uses pigments that absorb visible light — so it still looks dark — but reflect much of the near-infrared. The Heat Island Group gives the example of a cool-coloured roof reflecting 35% of sunlight staying about 12 °C cooler on a summer afternoon than a conventional roof of the same appearance reflecting 10%. So two roofs that match perfectly by eye can differ greatly in how hot they get, which is a case of the eye's limited spectral window, not of the colour being wrong.
A colour match does not imply a thermal match. Two samples that are metameric — identical in visible colour — can have very different near-infrared reflectance.
Cooler roofs transfer less heat into the building below. The US Environmental Protection Agency reports that in homes without air conditioning, cool roofs can lower peak indoor temperatures by roughly 1.2 to 3.3 °C. In air-conditioned buildings the gain appears as lower cooling energy. Across a city, higher roof reflectance lowers surface temperatures and contributes to reducing the urban heat island. The effects depend on climate, insulation, roof area relative to floor area and how clean the roof stays: dirt and biological growth reduce reflectance over time, which is why rating programmes report aged values as well as initial ones.
Cool roofs are not free benefits everywhere. In cold climates a reflective roof also rejects useful winter sun, although winter sun is low and weak and the net effect is often still small or positive. Highly reflective roofs can send glare towards taller neighbours. And a roof is only part of the heat balance: walls, paving and cars are surfaces too, and research groups now study cool walls and pavements with the same logic. Solar reflectance is one of the few places where building colour has a large, measurable, physical effect, and it is measured with instruments, not judged by eye.
| Roof compared | Solar reflectance | Reference roof | Approximate difference on a summer afternoon |
|---|---|---|---|
| Clean white roof | 80% | Grey roof, 20% | About 31 °C cooler |
| Cool-coloured dark roof | 35% | Conventional roof of the same colour, 10% | About 12 °C cooler |
Why: Soiling and biological growth reduce solar reflectance.
Fix: Compare products on aged, not just initial, reflectance and plan for cleaning.
Why: The two match in visible colour but differ in near-infrared reflectance.
Fix: Specify solar reflectance or SRI, not only a colour, when heat matters.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
About half of the sun's energy arrives as invisible near-infrared light, and cool-coloured dark roofs are designed to reflect this part of the spectrum while still looking dark.
Source: Cool Roofs — Heat Island Group
FactStrong evidence
On a typical summer afternoon, a clean white roof reflecting 80% of sunlight stays about 31 °C cooler than a grey roof reflecting 20%, and a cool-coloured roof reflecting 35% stays about 12 °C cooler than a look-alike conventional roof reflecting 10%.
Caveat: Illustrative figures for typical summer conditions; actual differences depend on climate, wind, insulation and soiling.
Source: Cool Roofs — Heat Island Group
FactStrong evidence
In non-air-conditioned homes, cool roofs can lower maximum indoor temperatures by about 1.2 to 3.3 °C.
Caveat: A range reported by the EPA from the literature; results vary with building type and climate.
FactStrong evidence
Thermal emittance and solar reflectance are independent properties because thermal radiation (about 4–80 µm) does not overlap the solar spectrum (about 0.3–2.5 µm).
Source: Cool Roofs — Heat Island Group
FactModerate evidence
The current ASTM method for calculating the Solar Reflectance Index of horizontal and low-sloped opaque surfaces is ASTM E1980-24.
Source: European Standards online catalogue (EN, ISO, IEC, BSI, ASTM listings)
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.