Solar reflectance tells you how much sunlight a surface reflects instead of absorbing heat. This makes it an important property when evaluating roofing materials, exterior coatings, and cool roof systems. In this blog, we have provided a complete guide on solar reflectance measurement, ASTM C1549, solar reflectance vs visible reflectance, and its role in roofing performance. This will help you to understand how reflectance affects roof temperature and energy performance.
However, a higher solar reflectance can help to keep the surface cooler and may reduce cooling demand in suitable buildings and climates. But solar reflectance is not simply about choosing a white or light-coloured roof. It covers the visible and invisible parts of sunlight, including near-infrared radiation, which can significantly affect how much solar heat a surface absorbs.
Key Facts You Should Know: Solar Reflectance
- Solar reflectance shows how much sunlight a surface reflects.
- It is measured from 0 to 1 or as a percentage.
- ASTM C1549 is a standard method for measuring solar reflectance.
- Unlike visible reflectance, it covers a wider part of the solar spectrum.
- Near-infrared radiation is an important part of solar energy.
- White roofs usually reflect more sunlight and absorb less heat.
- Cool-coloured coatings can reflect infrared energy while offering darker colors.
- Dirt, ageing, and weathering can lower solar reflectance over time.
- For cool roofs, consider solar reflectance, thermal emittance and SRI.
What Is Solar Reflectance?
Solar reflectance is the ability of a surface to reflect incoming sunlight instead of absorbing it as heat. It is an important property when evaluating roofing materials. Exterior coatings and cool roof systems because they show how much solar energy a surface sends back.
Solar reflectance is measured on a scale from 0 to 1 or expressed as a percentage from 0% to 100%. A higher value means the surface reflects more solar energy and generally absorbs less heat. For example, a roof with a solar reflectance of 0.80 reflects about 80% of the sunlight that reaches its surface. For opaque roofing material, most of the remaining solar energy is absorbed as heat.
Overall, a higher solar reflectance can help to keep cool roofs cooler by reducing heat absorption. However, roof temperature also depends on thermal emittance, insulation, roof material, climate, wind, and building design.
What Affects Solar Reflectance Performance?
Solar reflectance is important, but roof performance also depends on:
- Thermal emittance: How well the roof releases absorbed heat.
- Roof material: Different materials absorb and retain heat differently.
- Insulation: Reduces heat transfer into the building.
- Climate & weather: Sunlight, temperature, and wind affect performance.
- Roof and building design: Construction and ventilation influence overall heat gain.
For better results, solar reflectance should be evaluated with these factors, especially when selecting cool roof systems.
How Is Solar Reflectance Measured?
Solar reflectance is measured by checking how much solar radiation a material reflects as compared with the amount of sunlight that reaches its surface. To measure solar reflectance, two common methods are used:
- ASTM C1549: It uses a portable solar reflectometer to measure the solar reflectance of flat, opaque materials. It can be used for both field and laboratory measurements.
- ASTM E903: It uses a spectrophotometer with an integrating sphere to measure spectral reflectance, absorptance, and transmittance across the solar spectrum.
When comparing roofing materials or cool roof coatings, always check both the solar reflectance value and the test method used to measure it. This ensures that you are making an accurate comparison.
What Is ASTM C1549 and Why Is It Important?
ASTM C1549 is a standard test method that is used to measure the solar reflectance of flat, opaque materials using a portable solar reflectometer. ASTM C1549 is useful for testing roofing materials, roof coatings, and cool roof products in both laboratory and field conditions.
The current standard is ASTM C1549-16 (2022). The test uses a calibrated reflectometer to measure reflected solar energy at selected wavelengths, including 380, 500, 650, and 1220 nm.
Why Is ASTM C1549 Important?
- Measures solar reflectance: It shows how much solar energy a surface reflects.
- Portable testing: Measurements can be taken in the field or laboratory.
- Useful for roofing: Helps to evaluate roofing materials and cool roof coatings.
- Standardized method: Provides a consistent approach for comparing reflectance.
- Different from ASTM E903: C1549 uses a portable solar reflectometer, while E903 uses a spectrophotometer and integrating-sphere equipment for spectral measurements.
What Is the Difference Between Solar Reflectance and Visible Reflectance?
Solar reflectance and visible reflectance both describe how much radiation a surface reflects, but they measure different parts of the spectrum. Visible reflectance focuses on the light that the human eye can see and mainly affects how we perceive a surface’s solar properties, whereas solar reflectance covers a much wider part of the solar spectrum, including visible and near-infrared radiation.
The visible color of a roof does not always indicate its solar reflectance. For accurate cool-roof evaluation, solar reflectance should be considered rather than color alone.
Solar Reflectance vs Visible Reflectance
| Feature | Solar Reflectance | Visible Reflectance |
| Measures | Solar energy reflected | Visible light reflected |
| Spectrum | Broader solar spectrum | Visible light only |
| Includes near-infrared? | Yes | No |
| Main importance | Heat and roof performance | Appearance and colour |
| Used for | Cool roofs, coatings, roofing materials | Paint colour and visual appearance |
| Can a dark surface have a high value? | Yes | Generally lower |
How Does Solar Reflectance Relate to Infrared Radiation?
Near-infrared radiation is an invisible part of sunlight that carries a large amount of solar energy. It cannot be seen by the human eye, but it can significantly affect how much heat a roof absorbs.
- Dark roofs often absorb more near-infrared radiation, increasing heat gain.
- Cool-colored coatings are designed to reflect more near-infrared energy while keeping a darker appearance.
- A roof does not have to be white to have high solar reflectance.
- Specially designed pigments can help dark-colored roofs reflect more solar energy.
Why Do White Roofs Generally Reflect More Solar Energy?
White roofs generally reflect more solar energy because their surfaces reflect a large amount of visible sunlight and often have high solar reflectance. Basically, white roofs are naturally effective at reflecting sunlight, but cool-colored roofs can also achieve good solar reflectance with the right materials and pigments.
- High solar reflectance: White roof materials can typically reflect around 60% to 90% of incoming solar radiation energy, depending on the material.
- Less heat absorption: When more sunlight is reflected, less energy is absorbed as heat.
- Lower roof temperature: Higher reflectance can help to keep the roof surface cooler under direct sunlight.
- Not only white: Modern cool roof coatings and materials can also reflect significant near-infrared radiation while offering darker colors.
How Do Ageing and Dirt Affect Solar Reflectance?
Solar reflectance can decrease over time as a roof is exposed to dirt, dust, and pollution, weathering, UV radiation, and biological growth. These factors can make the surface absorb more sunlight and reduce its reflective performance.
- Dirt and dust can block the surface and reduce reflectance.
- Weathering and UV exposure may change or degrade the roof surface.
- A new roof may have higher reflectance than it does after years of use.
- Proper cleaning, when recommended by the manufacturer, can help to maintain reflectance.
- ASTM C1549 can be used to measure changes in solar reflectance.
How to Choose a Solar Reflective Roofing Material?
When choosing a solar-reflective roofing material or coating, do not look at colour alone. Choose roofing materials based on tested performance, durability, and long-term reflectance, not just their colour or appearance. Check these important factors:
- Solar reflectance: Check the reported reflectance value.
- Test method: Verify how the value was measured, such as ASTM C1549.
- Thermal emittance: Check how well the roof releases absorbed heat.
- SRI: Consider the Solar Reflectance Index (SRI) where applicable.
- Aged performance: Look at how reflectance changes with dirt, weathering, and ageing.
- Roof compatibility: Make sure the material suits the existing roof surface.
- Climate: Consider your local weather and building requirements.
- Manufacturer data: Review reliable technical specifications and test reports.
Conclusion
Solar reflectance is an important factor when choosing roofing materials and cool roof coatings. A higher reflectance means more sunlight is reflected and less solar energy is absorbed as heat. ASTM C1549 helps to measure solar reflectance, while factors such as thermal emittance, SRI, ageing, and dirt resistance also influence long-term performance.
Whether you’re evaluating a new roof or looking for a heat-reflective roof coating, understanding solar reflectance can help you to make a better choice.
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