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Solar reflectance index (SRI)

SRI combines solar reflectance and thermal emittance into one cool-roof score. Understand how SRI differs from the NCC's solar absorptance limit.

By Chalkline. Last updated 11 June 2026. Verified 11 June 2026.

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Solar reflectance index (SRI) is a composite score defined in ASTM E1980 that ranks how cool a roof surface stays under direct sun. It combines solar reflectance (fraction of solar radiation reflected, 0-1) and thermal emittance (ability to radiate absorbed heat back out, 0-1) on a 0-100 scale, where 0 is a reference black surface and 100 is a reference white surface. A white TPO membrane or light Colorbond colour typically scores 80-100; a dark metal roof sits below 25 (verified 2026-06-11, ASTM E1980).

SRI vs the NCC’s solar absorptance metric

The NCC does not use SRI. The code uses solar absorptance (SA), a simpler 0-1 ratio of absorbed radiation with no emittance component. Get this right before specifying a cool roof on compliance grounds.

NCC DTS limits (verified 2026-06-11, ABCB Housing Provisions Part 13.2; NCC Vol 1 Part J4):

  • Volume Two (Class 1, Part 13.2): climate zones 1-5, roof SA must be 0.64 or less.
  • Volume One (Section J, Part J4): climate zones 1-7, commercial roofs, SA must be 0.45 or less for DTS compliance.

SA and solar reflectance are complementary for opaque surfaces (SA + reflectance = 1.0), so an SA limit of 0.45 equals a minimum reflectance of 0.55. SRI adds emittance on top; two surfaces with equal reflectance can differ in SRI if their emittance differs. The two metrics are related but not interchangeable.

Where SRI is used

SRI appears in Green Star cool-roof credits and product marketing for TPO and cool-coat membranes. Manufacturers publish SRI alongside SA to help specifiers with green-rating submissions. When a spec calls up an SRI value, confirm whether the project is targeting a Green Star credit or NCC DTS: the NCC pathway requires SA, not SRI.

Also known as: SRI, cool-roof index.

Category: Energy efficiency / roofing.

See also

References

Last updated: 2026-06-11. Verified: 2026-06-11.

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