SRI Explained: What Solar Reflectance Index Means for Your Roof
If you're comparing cool-roof coatings, one number cuts through the marketing: the Solar Reflectance Index, or SRI. It rolls two properties into a single figure for how cool a surface stays in the sun — the higher, the better.
Reflectance and emittance — combined
SRI blends two things. Solar reflectance is the fraction of the sun's energy a surface bounces back rather than absorbing. Thermal emittance is how readily the surface sheds the heat it does pick up. A good cool roof scores high on both: it reflects most of the sun, and releases what it absorbs instead of holding it. SRI expresses that as one number so you can compare surfaces at a glance.
The scale
SRI is anchored so that a standard black surface sits at 0 and a standard white surface at 100 — but modern reflective coatings can push beyond 100. Tired dark felt or asphalt sits near the bottom; a fresh, high-performance reflective white sits at the top. The bigger the gap between your existing roof and the coating, the bigger the temperature drop you'll feel.
What an SRI of 112 means in practice
An SRI of 112 puts a surface firmly in cool-roof territory — it's reflecting the large majority of incoming solar energy and emitting heat efficiently, so it runs far cooler than the dark bituminous roof it covers. In real terms that means lower surface temperatures, cooler rooms beneath, reduced cooling demand, and much less of the daily thermal cycling that ages a membrane.
SRI and ageing: keeping the number up
Any roof weathers and collects dirt, and soiling drops reflectance over time — which is why the finish matters. A washable finish coat lets an occasional clean restore most of the original performance, rather than the reflectivity quietly greying away. When you compare products, ask not just about the headline SRI but how well it's held after ageing and cleaning.
Why it matters for a retrofit
On an ageing flat roof, SRI is the clearest single measure of the summer benefit you'll get from coating it white. It's the difference between a roof that bakes and one that keeps its cool.
Our The GOAT Solar Retrofit carries an SRI of 112 and is a waterborne, 0 g/L VOC coating built to go straight over existing bituminous roofs. Want to know the likely benefit on your building? Tell us about your roof.
Did you know? Passive Purple is best known as an airtight, vapour-control membrane — but it also carries a Solar Reflectance Index of around 52. That’s a genuinely strong number for a coloured coating whose main job is airtightness, not cooling. So on a build where Passive Purple is already sealing the envelope, it’s quietly bouncing back a fair share of solar heat at the same time. Explore Passive Purple →
Related reading
New or newly-laid roof? Choose The GOAT in solar reflective white — see Solar Reflective Roofing. Existing bituminous roof? The GOAT Solar Retrofit goes straight over the top.
- The GOAT: 344% stretch, crack-bridging & a cool-roof white (SRI ~104 — the test data)
- How to cool an ageing bituminous roof — without ripping it off
- Cool roofs and Part L: where a reflective roof fits UK building regs
- Waterborne vs solvent-based reflective roof coatings
- White roofs work — the only catch was keeping them white