How Cool Is Your Paving, Really?
If you’ve ever walked across a dark paved driveway in bare feet in the middle of an Australian summer, you’ll know well that some surfaces get much hotter in the sun than others. Whether you’re choosing a colour for a driveway, a pool surround, or a path through the garden, knowing how it will actually perform underfoot before it’s installed makes that decision a lot easier.
What SRI measures
The measure that explains this is called Solar Reflectance Index, or SRI. It looks at two things: how much of the sun’s energy a surface reflects away, and how efficiently it releases the heat it does absorb back into the air. Put those together and you get a single number that tells you how hot a surface is likely to get under direct sun. The higher the value, the cooler the surface stays. It’s a standardised test method, ASTM E1980, used across the building and construction industry.
Plenty of paving suppliers will describe a colour as “cooler” or “more reflective” simply because it looks lighter, and in a general sense that’s often a fair guide. But the only way to know for certain how a colour performs is to have it tested by an independent lab using a recognised method.
Why it matters
This kind of testing matters well beyond how a surface feels underfoot on a hot afternoon. Paving and hardscape choices affect how much heat a street, courtyard or plaza radiates back into the air, which is why SRI has become a recognised factor in urban heat island planning. It shows up in green building rating tools like LEED and Green Star, and a growing number of local councils reference SRI or similar measures in their planning provisions for new hardscape. The same data that helps a homeowner choose a comfortable colour for a driveway or pool surround is also the kind of information architects, landscape designers and certifiers rely on for larger projects.
How StoneSet adheres to it
StoneSet has engaged CSIRO, Australia’s national science agency, to assess a selection of colours from our permeable paving range, with SRI values calculated in accordance with ASTM E1980. Among the colours included in the available test data, Polar White comes out on top at an SRI of 64, the coolest-performing colour tested. From there the numbers step down through Pearl at 31, Beach at 27, White Beach at 26 and Cudgee Beach at 22, down to Sierra Grey at 17 and Charcoal Pearl at 4 — the darkest and most striking colour tested, which runs warmer underfoot in summer than the lighter tones.
There’s a second layer worth mentioning too. StoneSet’s paving is a permeable, resin-bound system, so water passes through the surface rather than pooling or running off, supporting better on-site stormwater management. That permeability can also work in the surface’s favour on the heat side: when moisture is present within a permeable pavement system, evaporation can provide a temporary cooling effect. The extent of this effect depends on factors including the system build-up, its ability to retain moisture, recent rainfall and local weather conditions. The same product that’s been independently tested for how it handles heat has also been designed to handle water.
Whichever colour you’re drawn to, and whatever it’s going down for, the point of independent testing is simple: you get real numbers to weigh alongside the look you want, rather than a guess.
If you’d like to see more of the data behind these figures, find the technical document here or get in touch with a StoneSet consultant.