Test SRI - Solar Reflectance Index

What is the Solar Reflectance Index? Why is it important? How does it contribute to thermal comfort? What does the standard provide for it? How is it measured?

What is the Solar Reflectance Index?


Solar Reflectance Index (SRI) is a parameter that measures the ability of a material to reflect incident solar energy. It is a normalized unit of measurement that combines solar reflectance (SR) and thermal emissivity (ε). 

In other words, SRI indicates the ability of a material to thermally insulate (through substrate protection) when exposed to sunlight. It is expressed on a scale of 1-100, although negative values or higher than 100 are possible.

Materials with higher SRI have a lower increase in their temperature when exposed to solar radiation, in other words, the surface exposed to solar radiation will remain "cool." In fact, we speak of "cool material" or "cool roof" to refer to those materials or roofs with a high SRI.


What materials does the Solar Reflectance Index apply to?


The Solar Reflectance Index is commonly applied to materials used in exterior surfaces, such as roofing, flooring, glass, windows and doors, paints, coatings and other building materials. 

Similarly, the Solar Reflectance Index is determined for all industrial applications that involve exposure to direct radiation and in which there is a need to reduce interactions with the environment or a heat/light source. For example: crude oil piping, piping petrochemical plants, surfaces exposed to equipment working at high temperature (burners, etc.), materials in the solar industry (photovoltaic panels - PV, thermodynamic concentrating panels - CSP Concentrating Solar Power, etc.)...

In general, materials with high Solar Reflectance Index can help reduce the thermal load of buildings or plants and improve thermal comfort and process energy efficiency.


How does the solar reflectance index contribute to thermal comfort in the living environment?


As known, among the main sources of atmospheric CO2 emissions are buildings. The containment of overheating on an urban scale, the so-called "heat island" effect, as well as the reduction of energy consumption for building cooling, are issues that are particularly felt not only at the national level, but also on a European and global scale.


Solar reflectance index (SRI) can contribute to thermal comfort in the home in several ways:


Reduced heat load: materials with a high SRI value, such as reflective roofs and floors, have the ability to reflect more solar radiation instead of absorbing it. This reduces the heat absorbed by the building and, consequently, the internal heat load. A reduced heat load means that the indoor environment requires less energy for cooling, contributing to greater thermal comfort.


Reduction of urban heat island effect: the urban heat island effect occurs when urban areas become significantly warmer than surrounding areas due to heat buildup from man-made surfaces and structures. The use of materials with a high SRI can help reduce the urban heat island effect by reflecting more solar radiation. This helps keep temperatures lower in the urban environment, improving thermal comfort for residents.


Reducing energy requirements for cooling: adopting materials with a high SRI can reduce dependence on mechanical cooling systems, (air conditioning). By using materials that reflect solar energy, the need to cool the room through energy devices is reduced. This not only contributes to greater thermal comfort, but also to greater energy efficiency and reduced operating costs.


Increased durability of building elements: materials with a high SRI tend to reflect more solar energy, thus reducing heat absorption. Overall, the use of materials with a high SRI can promote a more thermally comfortable home environment by reducing heat load, the urban heat island effect, and dependence on mechanical cooling systems.


What does the legislation require?


As early as the early 2000s, the first European directives were issued with the intention of giving guidance to address the energy issue of buildings, both with the aim of reducing energy consumption and improving living comfort.

To date, several mandatory provisions have come into force in Italy, including the Minimum Requirements Decree of June 26, 2015, which to all intents and purposes defines the rules and limits to be respected for the design and redevelopment of buildings with a view to energy savings and thermal comfort. 


And most recently, DM June 23, 2022 No. 256, Official Gazette No. 183, Aug. 6, 2022 - effective Dec. 4, 2022 (the so-called CAM Decree), which establishes the adoption of minimum environmental criteria for buildings (CAM), thus aligning with environmental protection strategies already widespread internationally. 

Regarding SRI, in section 2.2.6 on "Reducing Impact on Microclimate and Air Pollution," the use of materials with a high solar reflectance index is stipulated for impermeable surfaces as follows:

  • exterior surfaces (e.g., pedestrian paths, sidewalks, plazas, bike paths etc.): SRI index ≥ 29
  • materials used for roofs must ensure an SRI index of at least 29, in cases of slopes greater than 15%, and at least 76, for roofs with slopes less than or equal to 15%.

In addition to compulsory measures, there are also measures with a voluntary character, among these are in particular environmental protocols such as LEED® (Leadership in Energy and Environmental Design) and ITACA ((UNI/PdR 13:2015).


How is the Solar Reflectance Index calculated?


The SRI test is performed at Certimac's Chemical-Physical Analysis Laboratory on different types of materials intended for the building envelope or exterior surfaces (roofing, insulation materials, ventilated facades, ceramic tiles, sheathing, plasters, paints, ...) and on materials intended for all those industrial applications that involve exposure to direct radiation and in which interactions with the environment or with a heat/light source must be reduced.


The index is obtained according to a special standard defined by ASTM E1980-11 and ASTM E903-12 by relating two specific values obtained in the laboratory: thermal emissivity (heat that the material emits) and solar reflectance (ability of the material to reflect solar radiation).


Solar reflectance is a measure of the fraction of incident solar radiation that is reflected by an irradiated surface. The value varies between 0 and 1, a totally absorbing and a totally scattering surface, respectively. The closer the reflectance value is to 0 the less the material has the ability to reflect solar radiation.


Thermal emissivity, on the other hand, is the fraction of energy radiated by the material relative to the energy radiated by a black body at the same temperature and takes values between 0 and 1. It depends on factors such as temperature, angle of emission, wavelength and surface finish of the observed body.


If one were to stop at just these two definitions, one could infer that a low solar absorption value and a high emissivity value allow a material not to overheat and to dissipate a lot of energy by radiation. 

However, this is not enough because it is also important to know the material's ability to dissipate energy in the form of radiation. The parameter that combines these aspects is the reflectance index (SRI), and more and more regulations, both mandatory and voluntary, are focusing on this value.


With what instrumentation is the Solar Reflectance Index calculated?


Solar reflectance and thermal emissivity are measured at Certimac using a UV-VIS-NIR spectrophotometer and an emissometer in accordance with ASTM E 903 and ASTM C1371-15 and ASTM E1980-11. From the two experimental values thus obtained, the SRI index is derived.


Extent in the NIR and sphere are critical for measuring reflectance: usually the spectrophotometer operates in the UV-Vis range (between 190 and 800 nanometers) and is used to make measurements on solutions, while the SRI works in the NIR range (Near Infra-Red: 800 to 3000 nanometers) and operates on solid samples.


In addition to SRI testing, Certimac performs analysis and laboratory tests to characterize the physical, chemical, mechanical and thermal behavior of more than 600 types of products and materials for applications in construction, industry, logistics, transportation and energy management. 


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