Roof SRI index: understanding, calculating and comparing the performance of a cool roof

What is the SRI of a roof?

The roof SRI — Solar Reflectance Index — is a single number that describes the ability of a surface exposed to the sun to stay cool. It was defined by the American standard ASTM E1980 to compare roofing materials with one another, whatever their colour, composition or texture.

The principle is simple: the temperature the surface would reach under standardised sunshine is calculated and placed between two references. A reference black (reflectance 0.05, emissivity 0.90) is worth 0. A reference white (reflectance 0.80, emissivity 0.90) is worth 100. A roof that heats up as much as the black has an SRI close to 0; a roof that stays as cool as the reference white has an SRI of 100; a roof that stays cooler still exceeds 100.

The SRI is therefore not a direct measurement: it is a calculation based on two quantities measured in the laboratory, solar reflectance and thermal emissivity. Understanding these two components means understanding why two “white” surfaces can have very different SRIs, and why a cool roof coating is not an ordinary white paint.

Solar reflectance and emissivity: the two components of the SRI

Solar reflectance (ASTM E903)

Solar reflectance, or albedo, is the fraction of solar radiation sent back by the surface across the whole solar spectrum — ultraviolet, visible and near infrared, from 300 to 2,500 nm. It is measured with an integrating-sphere spectrophotometer according to standard ASTM E903, then weighted by the reference solar spectrum. A value of 0.95 means that 95 % of the incident energy leaves again; only 5 % is absorbed and turned into heat.

The near infrared accounts for nearly half of solar energy: that is why the colour perceived by the eye is not enough to judge a surface. A pigment can look white and absorb in the infrared, or look light grey and reflect it. Our article white or coloured cool roof details this point.

Thermal emissivity (ASTM C1371)

Thermal emissivity is the ability of a surface to re-emit, as far-infrared radiation, the heat it has absorbed. It ranges from 0 (perfect mirror, which keeps everything) to 1 (ideal black body, which radiates everything). It is measured with a portable emissometer according to standard ASTM C1371.

It is the quantity people forget. Galvanised steel or aluminium foil reflect part of the visible light but emit very poorly (0.1 to 0.3): the absorbed heat stays trapped and the sheet burns. Conversely, a bitumen membrane emits well (0.85 to 0.90) but absorbs almost everything. Only a surface that reflects a lot and emits well stays cool: that is the definition of a cool roof, and that is what the SRI sums up.

How the SRI is calculated (ASTM E1980 method)

Standard ASTM E1980 sets conventional conditions: sunshine of 1,000 W/m², air at 37 °C, sky at 27 °C, and three wind speeds corresponding to low, medium and high convection coefficients. For each surface, the equilibrium temperature at which the heat absorbed equals the heat released by radiation and convection is calculated.

The SRI is then obtained by linear interpolation between the two references:

SRI = 100 × (T black − T surface) / (T black − T white)

where T black and T white are the equilibrium temperatures of the reference black and white under the same conditions. A surface cooler than the reference white obtains a result above 100; this is the case with PrimaTherm®.

Three practical consequences:

• the SRI has no unit and depends neither on the local climate nor on the orientation of your roof: it compares materials, not buildings;
• the result is given for three wind speeds. A good test report states all three values; when they are close, the surface reflects so much that it depends little on convection to stay cool;
• the same reflectance does not give the same SRI depending on emissivity: at 0.80 reflectance, you go from an SRI close to 100 with an emissivity of 0.90 to a markedly lower SRI with an emissivity of 0.30.

The standard is available from ASTM; our article on the SRI solar reflectance index explained gives a plain-language reading of it.

Benchmark values: comparative table of roofs

The orders of magnitude below are deliberately cautious; they depend on the condition, soiling and age of each surface. Only a test report is authoritative for a given product.

Two useful readings. First, the gap between a dark roof (SRI 0 to 10) and PrimaTherm® (120) corresponds to 25 to 35 °C less on the surface: that is the order of magnitude measured on our sites, −29 °C at the handover of the largest building at Airbus Marignane on 19 June 2026. Second, the difference between a conventional white paint (70 to 90) and a certified system (120) is not cosmetic: it comes from reflectance in the infrared and, above all, from durability over time.

The SRI of PrimaTherm®: 120 when new, 119 after ageing

The PrimaTherm® system (manufacturer Primacoating, La Rochelle) was characterised by the Tipee technology platform on samples of the PH107 reflective base coat:

The important point is the last line of the SRI tests: after 4,000 hours of accelerated ageing under UV-B, the SRI is maintained (119 against 120) — conventional service life of 20 years (French CEE sheet BAT-EN-112 methodology). We never write “20-year guarantee”: what is certified is the retention of the SRI. The details of the reports are on the PrimaTherm® certifications page.

Why the SRI matters for your building

The SRI is a laboratory figure; its consequences are very concrete.

• Surface temperature. This is the first thing it predicts: a roof at SRI 120 stays at 25–35 °C where a roof at SRI 10 climbs to 60–70 °C. Less heat on the surface means less heat transmitted to the rooms (up to −6 °C felt depending on insulation) and to the air conditioning (up to −40 % depending on the climate zone).
• Service life of the waterproofing. A membrane that cycles between 20 and 35 °C rather than between 20 and 70 °C cracks much more slowly.
• French tertiary-sector decree. The SRI does not appear in it as such, but the drop in consumption it brings is declared on OPERAT; see French tertiary decree and cool roofs.
• Comparison between products. This is its reason for being: two coatings, two SRIs measured according to the same standard, an objective decision.
• Certification and planning files. LEED, BREEAM, HQE and some local planning rules (PLU) refer to it.

What the frameworks ask for: LEED, BREEAM, HQE, CEE

• LEED (Heat Island Reduction credit): on low-slope roofs, an initial SRI of at least 78 in the 2009 version; LEED v4 raises the initial threshold to 82 and introduces a 3-year aged value of 64 (39 and 32 for steep slopes). PrimaTherm®, at 120 new and 119 aged, exceeds these thresholds with margin.
• BREEAM and HQE: the credits linked to summer comfort and heat islands reward high-reflectance roofs; the thresholds depend on the version of the framework and the profile of the operation. See our article on cool roofs and HQE, BREEAM and LEED certifications (in French).
• CEE sheet BAT-EN-112 (French energy-savings certificates): it sets performance requirements for the coating when new and after ageing, which PrimaTherm® meets; the eligibility of the operation remains conditional (commercial tertiary building, volume heated and cooled by a heat pump). See cool roof incentives in France 2026.
• PLU and climate plans: some local authorities include the lightness or albedo of roofs in their regulations; see the regulatory context.

How to read an SRI test report

An SRI figure without its report is worth nothing. Here is what we check, and what you should require from any supplier.

1. The laboratory and its identity: name, address, accreditation or recognition (Tipee for PrimaTherm®). An “internal test” is not a test report.
2. The report number and date: ES-PJ000106-015 of 5 April 2022 for the new condition, ES-PJ000911-003 for the aged condition. A report can be cited, checked and attached to the file.
3. The sample: exactly which product (PH107, not “the range”), on which substrate, at what thickness, applied how. An SRI measured on a thick film in the laboratory is only found on your roof if the applicator respects the same thicknesses — hence our thickness checks and our site monitoring sheet.
4. The three quantities and their standards: reflectance (ASTM E903), emissivity (ASTM C1371), SRI (ASTM E1980). If one is missing, the SRI could not have been calculated correctly.
5. The uncertainty: 120.1 ± 2.8 is a measurement; “120” without uncertainty is a claim.
6. The three wind speeds: a complete report gives the SRI in low, medium and high convection.
7. The aged condition: ageing method (ISO 16474-3, 4,000 h UV-B), result after ageing, gap with the new condition. This is the line that distinguishes a durable product from one that shines the first summer.

If a supplier cannot produce these seven elements, the value announced should not enter your specification. Our checklist for choosing a cool roof contractor (in French) covers these points.

SRI and ageing: what the new figure does not say

All surfaces lose reflectance over time: dust, pollution, pollen, micro-organisms, yellowing of the binders under UV. A conventional white paint can lose a significant share of its SRI in two or three seasons; this is what fed the idea that “white does not last”.

Two factors protect an SRI over time. The first is chemical: the PH107TCL topcoat, with PVDF resins, resists UV and limits the adhesion of dirt. It is what explains the 119 after 4,000 h. The second is human: cleaning with clean water or at low pressure every 12 to 15 months, described in our guide on cool roof maintenance (in French). Without a topcoat and without maintenance, even a good product falls away.

SRI, colour and roof type

The SRI depends on the coating, not on the substrate. A PrimaTherm® system applied on steel deck, on bitumen membrane shows the same SRI, provided the film has the planned thickness and the substrate does not migrate through it — hence the blocking primer on bitumen. What changes with the substrate is the preparation, the coverage rates and the indoor gain, not the index.

Colour, on the other hand, changes everything. Light non-white shades (pearl grey, beige) can reach intermediate SRIs thanks to infrared-reflecting pigments, but none matches white. When a PLU imposes a colour, we say so: the SRI will drop, and we quantify by how much from the manufacturer’s data.

Measuring in the field: what you can check yourself

The SRI is calculated in the laboratory, but its effects are measured on your roof.

• Infrared thermal camera: surface temperature, treated zone against control zone, at the same time of day. This is the measurement we take at the handover of every site; on the largest building of our Marignane site, it gave 57.8 °C against 28.6 °C.
• Albedometer or inverted pyranometers: measurement of reflectance in situ, useful for tracking soiling from one year to the next.
• Temperature probes under the roof and air-conditioning consumption readings: these are the data that feed the profitability study.

These field measurements do not replace the test report: they confirm it on your building, in your climate.

Using the SRI in a specification

For a call for tenders or a consultation, three clauses are enough to rule out products without proof:

1. “The coating shall have an SRI ≥ 100 when new, calculated according to ASTM E1980 from measurements according to ASTM E903 and ASTM C1371, attested by an independent laboratory test report attached to the tender.”
2. “The tenderer shall provide the SRI after accelerated ageing (method and duration specified) attested by a test report.”
3. “The applicator shall provide thickness checks and a site monitoring sheet attesting compliance with the manufacturer’s coverage rates, together with surface temperature measurements before/after.”

We meet all three, reports in hand, in every proposal and in the as-built file. To understand how the SRI fits into the subject as a whole, the cool roofing guide covers the physics, the solutions and the limits; for a first estimate on your roof, the free roof study answers within 48 h.