SPF Testing – Solar Reflectance Index
Climate change, urban densification and a lack of green spaces are causing cities to overheat more and more frequently in summer. Spending time outdoors becomes unpleasant and the energy required to cool buildings increases. One reason for this development is the surfaces of buildings and roads, which absorb the sun's radiation and heat up as a result. This effect can be influenced by using suitable materials.
On the one hand, high reflectance levels – i.e. light-coloured surfaces – are suitable for reducing heating. On the other hand, however, the invisible heat radiation from surfaces also has a major influence on the urban climate. Various Cool Cities programmes use the Solar Reflectance Index (SRI) to characterise the thermal properties of surfaces. The SRI describes both the reflectance and the emissivity of a surface in a single value. For sustainable buildings a good SRI value is requried, meaning that good values also bring financial benefits.
To determine the SRI, a sample of the surface is optically measured in accordance with ASTM E 1980 Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces, and the SRI is calculated from this. The SRI value has only been used in a few European standards to date (e.g. EN 15976:2011 Flexible sheets for waterproofing - Determination of emissivity). However, the value is already being specified voluntarily for various surfaces and colours.
In general, the SRI is a value between 0 and 100. The higher the value, the better a material is able to oppose the warming of the environment. A value of 100 represents a white surface that contributes very little to warming (e.g. a white-painted house), i.e. it is very good. A value of 0 represents a very dark surface that heats up significantly. In theory, values below 0 and above 100 could also be achieved, but the SRI for common building materials are all between 0 and 100. Together with the SRI, the maximum achievable surface temperatures are also determined depending on the prevailing wind speeds.
The SRI is sometimes mixed up with the light reflectance value HBW (0-100). But this only considers the visible part of the spectrum and is therefore only good for judging how a surface looks, not for assessing its heat island potential.
In addition to the SRI values, measurements carried out by the SPF also provide the values for solar absorptance, solar reflectance, thermal emittance, emissivity, albedo, as well as the CIELAB colour values L*, a*, b* and the CIEXYZ colour coordinates X, Y, Z and the brightness value HBW.
