Solar Reflectance Index (SRI)

The Solar Reflectance Index (SRI) is a single number that describes how well a roof surface stays cool in the sun. It combines two measured properties:

• Solar reflectance – the fraction of sunlight the surface reflects.
• Thermal emittance – how efficiently the surface radiates absorbed heat away as infrared energy.

SRI is calculated relative to two reference surfaces: a standard black surface defined as SRI 0 and a standard white surface defined as SRI 100. Very reflective, high-emittance materials can score above 100; the theoretical maximum is around 122. Surfaces hotter than standard black can have negative values.

How it is calculated

In North America, SRI is calculated using ASTM E1980, which sets standard conditions for sun, air temperature and wind (low, medium or high convection) and computes the steady-state surface temperature. The input values of reflectance and emittance usually come from Cool Roof Rating Council ratings. Because results vary with the chosen wind condition, programs normally specify which one applies, commonly medium wind.

Why use a single number

Separate reflectance and emittance values can be hard to compare. A bare metal roof may have high reflectance but low emittance, so it heats up more than its reflectance suggests. SRI accounts for both. For example, stainless steel roofing can reach very high SRI values, while dark asphalt shingles have low SRI.

Where SRI is used

• Green building programs. LEED heat island credits set SRI thresholds for roofs. Under LEED 2009, for example, low-slope roofs needed SRI 78 or higher and steep-slope roofs SRI 29 or higher. Later LEED versions use both initial and three-year aged SRI values.
• Energy codes. Some codes and local ordinances allow SRI as an alternative to separate reflectance and emittance criteria.
• Urban heat island programs. Cities use SRI in cool roof ordinances and incentives.

Limitations

SRI describes the surface only. It says nothing about insulation, ventilation or the roof's overall thermal performance. It also reflects ideal clean surfaces unless aged values are used, and reflectance can drop significantly as roofs get dirty or weather. In cold climates, high SRI roofs can slightly increase heating energy, so cool roof requirements are usually strongest in warm climate zones.