ASTM E1980: Solar Reflectance Index (SRI) Testing — Method & Calculation Guide
What Is ASTM E1980?
ASTM E1980 — Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces — defines a procedure for calculating the Solar Reflectance Index (SRI) from measured values of solar reflectance and thermal emittance. SRI is a single numerical parameter that characterises a roofing or paving surface’s ability to reject solar heat — combining both reflectance and emittance into one composite indicator.
SRI is widely used in green building certification (LEED, Energy Star), cool roof specifications, and urban heat island mitigation programmes to rank the thermal performance of roofing and paving materials.
Understanding Solar Reflectance and Thermal Emittance
Solar Reflectance (Albedo)
Solar reflectance (SR) is the fraction of incident solar radiation (UV, visible, and near-infrared, 0.3–2.5 µm) that a surface reflects. A perfectly reflective surface has SR = 1.0; a perfectly absorbing black surface has SR = 0. Higher solar reflectance means less solar energy absorbed — lower surface temperature.
Solar reflectance is measured per ASTM E903 (spectrophotometric method) or ASTM C1549 (near-ambient temperature pyranometer method) — both provide spectrally integrated solar reflectance values.
Thermal Emittance
Thermal emittance (ε) is the fraction of long-wave infrared radiation (typically 3–50 µm) that a surface emits relative to a blackbody at the same temperature. High-emittance surfaces (ε close to 1.0) radiate heat efficiently; low-emittance surfaces (metallic, ε ≈ 0.05–0.2) retain absorbed heat. Thermal emittance is measured per ASTM C1371 or ASTM E408.
The SRI Calculation (ASTM E1980)
SRI is calculated from SR and ε using a defined energy balance model that accounts for the combined effects of solar absorption and infrared emission on steady-state surface temperature under defined standard environmental conditions:
Standard conditions (per ASTM E1980):
- Solar insolation: 1000 W/m²
- Air temperature: 35°C
- Wind speed: 1 m/s (low-wind standard) or 5 m/s (high-wind standard)
The SRI calculation determines the surface temperature predicted by the energy balance model for the given SR and ε values, then normalises this temperature relative to the temperatures of a standard black surface (SRI = 0) and a standard white surface (SRI = 100):
SRI = [T_black − T_surface] / [T_black − T_white] × 100
SRI values above 100 are possible for very highly reflective, high-emittance surfaces.
Typical SRI Values for Common Surfaces
| Surface | Solar Reflectance | Emittance | SRI |
| Standard black (reference) | 0.05 | 0.90 | 0 |
| Standard white (reference) | 0.80 | 0.90 | 100 |
| White EPDM membrane | 0.78 | 0.90 | 96 |
| Gray gravel | 0.14 | 0.90 | 10 |
| Red clay tile | 0.33 | 0.90 | 36 |
| Bare aluminium | 0.76 | 0.05 | 56 |
| Cool roof white coating | 0.82 | 0.92 | 102 |
Regulatory and Green Building Applications
LEED v4 Sustainable Sites credit (Heat Island Reduction): Requires SRI ≥ 82 for low-slope roofing (≤2:12 pitch) and SRI ≥ 39 for steep-slope roofing. Energy Star Cool Roof requirements: Minimum SR of 0.65 and emittance ≥ 0.90 for low-slope roofing (equivalent to SRI ≥ 78). California Title 24 Energy Code: Prescriptive cool roof requirements specifying minimum SR and SRI values for residential and commercial buildings.
Why Choose Infinita Lab for SRI and Cool Roof Testing?
Infinita Lab provides solar reflectance testing (ASTM E903, ASTM C1549), thermal emittance testing (ASTM C1371), and SRI calculation per ASTM E1980 through our nationwide accredited building materials testing laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
Why does ASTM E1980 include thermal emittance in the SRI calculation rather than just solar reflectance? A surface that is highly solar-reflective but has low thermal emittance (like bare aluminium) retains heat because it cannot efficiently radiate the small absorbed solar fraction as infrared. Including emittance in SRI accurately accounts for both the reduced solar absorption (reflectance) and the surface's ability to shed absorbed heat (emittance) — giving a more complete picture of actual surface temperature than reflectance alone.
What is the difference between initial SRI and aged SRI for cool roofing products? Initial SRI is measured on new, clean product samples. Aged SRI is measured after 3 years of outdoor exposure (ASTM D7897 or Energy Star protocols) — accounting for dust, soiling, biological growth, and photodegradation that reduce solar reflectance over time. Energy Star and CRRC (Cool Roof Rating Council) report both initial and aged SRI values. Aged SRI is more relevant for long-term building energy performance.
How does solar reflectance affect building energy consumption? Higher SRI roofing reduces heat gain through the roof — lowering cooling loads in air-conditioned buildings by 10–40% in hot climates, reducing peak electricity demand, and lowering roof temperatures by 20–50°C compared to standard dark roofing. For unconditioned buildings, cool roofs directly reduce indoor temperatures and heat stress.
Can the SRI of a surface be increased by applying a coating? Yes. White or light-coloured cool roof coatings with high SR (0.75–0.85) and high emittance (0.85–0.92) can dramatically increase SRI of existing dark roofing from near 0 to 80–100+. These coatings are evaluated for SRI per ASTM E1980 before and after application to verify the performance improvement.
Is SRI a material property or a surface property? SRI is a surface property — it depends on the visible and near-IR optical characteristics of the surface layer (colour, texture, coating) and the surface's infrared emittance. The same material can have very different SRI depending on surface treatment, colour, and cleanliness. Measuring SRI characterises the surface as it will perform in service.
