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SRI, heat island and barefoot comfort
How to choose outdoor products based on the colour and thermal performance of the surfaces
Heat island
In urban areas and heavily built-up spaces, the issue of surface overheating has become increasingly important.
Roads, roofs, car parks, courtyards and sun-exposed surfaces tend to absorb a significant proportion of solar energy, storing heat during the day and releasing it slowly into the environment.
This is one of the mechanisms behind the phenomenon known as the urban heat island, i.e. the increase in temperature in built-up areas compared to less urbanised areas or those with more vegetation.
This phenomenon is not limited to perceived comfort outdoors.
It also has a direct impact on the quality of spaces, the liveability of terraces, walkways and hospitality areas, the local microclimate and, in many cases, buildings’ overall efficiency.
Therefore, in contemporary design, the choice of outdoor materials cannot only be guided by traditional aesthetic or performance criteria: it must also take into account how a surface reacts to solar radiation and the context in which these materials are situated.
In this scenario, porcelain stoneware can make an important contribution.
Some surfaces, thanks to their technical characteristics and colour, help to limit heat build-up and improve the usability of outdoor spaces.
This is where concepts such as SRI come into play, but also very practical and immediately noticeable aspects, such as the comfort of walking barefoot on paving exposed to the sun.
sri
An index that helps to interpret surface behaviour
Among the most useful parameters for understanding the thermal performance of a material exposed to the sun, the SRI (Solar Reflectance Index) is now one of the most significant.
This index quantifies a surface’s tendency to heat up under the influence of solar radiation, by correlating two key factors:
- Solar reflectance (SR): the ability to repel incident radiation.
Values range from 0 to 1; the higher the value, the higher the reflectance of the roof. - Thermal emissivity (TE): the ability to disperse accumulated heat.
Values range from 0 to 1; the higher the value, the more heat is dissipated.
The SRI value is calculated in accordance with the ASTM E1980 standard and provides a concise overview of a complex behaviour:
Tb: temperature of standard black (SRI=0)
Tw: temperature of standard white (SRI=100)
Ts: surface equilibrium temperature: i.e. the temperature the material reaches in the sun when the heat absorbed and the heat lost are balanced (calculated using a mathematical formula where SR and TE are the variables).
The higher the SRI value, the more the surface tends to maintain a lower surface temperature compared to materials that absorb and retain more heat.
Conversely, lower values are generally associated with surfaces that heat up more easily and reach higher temperatures.
SRI and certifications supporting the project
For designers, clients and anyone responsible for selecting an outdoor surface, the SRI index is a valuable indicator because it helps link product choice to very specific objectives: moderating the microclimate, keeping surface temperatures down, ensuring user comfort and making outdoor spaces more liveable.
In the case of outdoor paving, this issue is particularly relevant in the context of environmental sustainability protocols.
Walkways, pool edges, outdoor seating areas, terraces, rooftops, private courtyards and commercial spaces are all environments where the material is constantly exposed to sunlight and where the thermal performance of the surface directly affects the user experience.
A product with desirable features can therefore become an integral part of a more thoughtful design that combines aesthetics, performance and perceived well-being.
In particular, the issue of reducing the urban heat island effect is taken into account by building assessment systems, which reward design choices that improve the performance of surfaces exposed to solar radiation.
LEED certification and heat island reduction
When it comes to LEED standards, the issue of mitigating the heat island effect is linked to how well surfaces limit the absorption of solar energy and help to improve the thermal balance of the built environment.
In this regard, the SRI value becomes particularly important for all applications where the material’s surface behaviour is a significant design factor.
Under the Heat Island Reduction section, LEED credits aim to reduce the effects of heat islands on the microclimate and on human and natural habitats, whilst also helping to reduce the energy required for mechanical cooling of indoor spaces.
What criteria must be met to earn LEED credits on the Heat Island?
LEED V5 uses thresholds and performance criteria to determine the extent to which a project can contribute to reducing the heat island effect.
The Heat Island Reduction credit, particularly in Option 1 Nonroof and Roof, evaluates not only the roof, but the overall set of strategies implemented across the entire site.
This option allows you to combine measures applied to nonroof surfaces with measures applied to roof surfaces, such as high-reflectance roofs and vegetated roofs.
Area of Nonroof Measures
0.5
+
Area of High-Reflectance Roof
0.75
+
Area of Vegetated Roof
0.75
≥
Total Site Paving Area + Total Roof Area
NON ROOF
These are all the project’s external paved surfaces that are not classified as roofs and that meet the minimum Solar Reflectance (SR) requirements.
MINIMUM SRI FOR COOL ROOF MATERIALS IN LEED V5
Metric
Initial
3 years aged SR
Solar Reflectance
0.33
0.28
HIGHT-REFLECTANCE ROOF
These are all the surfaces that are classified as roofs and that meet the minimum SRI requirements.
MINIMUM SOLAR REFLECTANCE FOR HARDSCAPE IN LEED V5
Slope
Initial SRI
3 years aged SR
Low sloped roof
≤ 2:12
82
64
Steep-sloped roof
> 2:12
39
32
VEGETATED ROOF
These are all the surfaces that are classified as roofs on which greenery has been installed.
The ultimate aim of the certification is to achieve an overall balance in which the weighted sum of the strategies is at least equal to the sum of the floor and roof areas considered.
The choice of material can therefore make all the difference in achieving the LEED Heat Island Reduction credit.
In this context, porcelain stoneware is a valuable asset in sustainable design and in meeting the environmental requirements set out in the LEED protocol, thanks to its durability over time — which allows the three-year performance values to be applied immediately — its resistance to weathering, and the wide availability of products in light colours with suitable Solar Reflectance SR and SRI ratings.
SRI e Riflettanza dei prodotti Kronos
Comfort when walking barefoot
When designing outdoor flooring, comfort is not measured solely by the surface’s aesthetic appeal or durability, but also by the sensation it provides on direct contact.
You only need to walk barefoot on outdoor flooring on a summer’s day to realise just how much a surface’s thermal performance can affect comfort.
Not all materials react in the same way to solar radiation.
Metals, stones, wood and ceramic surfaces can reach very different temperatures, and, more importantly, they can transfer heat to the body in different ways.
The difference depends on several factors: the material’s ability to absorb solar energy, its mass, its thermal conductivity, its surface finish and, of course, its colour.
The issue, therefore, concerns not only the colour of the surface, but also its material nature.
UNI EN ISO 13732-1: the standard governing hot surfaces and the risk of burns.
The text of the Standard and the behaviour of Kronos products.
UNI EN ISO 13732-1 is the standard that covers skin contact with hot surfaces and is used to assess when a surface temperature can become dangerous for humans. It provides threshold values and assessment methods for various types of materials for the risk of burns from contact with hot solid surfaces.
TEMPERATURE THRESHOLD °C
Material
for 1 min
for 10 min
for 8 hours
Uncoated metal
51
48
43
Coated metal
51
48
43
Ceramics, glass, stone
56
48
43
Plastics
60
48
43
Wood
60
48
43
In this comparison, porcelain stoneware stands out as a technical material that offers a balance and the right compromise between performance, safety, durability and control over surface behaviour.
HERE ARE THE RESULTS FOR OUR PRODUCTS
COLLECTION
COLOUR
BAREFOOT COMFORT
TEST
PIERRE VIVE
Loire
Compliant
Orval
Compliant
Morvan
—
Brionne
—
MATERIA
Gesso
Compliant
Cemento
Compliant
Sandalo
—
Seta
Compliant
Cenere
Compliant
Tortora
—
NATIVA
Lux
Compliant
Aurum
Compliant
Tibur
—
Lapillo
—
PIASENTINA STONE
Flamed
Compliant
ESSENCE
Pure
Compliant
Ambre
Compliant
Musk
—
TERRA CREA
Calce
Compliant
Corda
Compliant
Limo
Compliant
Pomice
—
Mattone
Compliant
REVERSE
Opal
Compliant
Dune
Compliant
Taupe
—
Nuit
—
PRIMA MATERIA
Cemento
Compliant
Cenere
Compliant
Sandalo
Compliant
LES BOIS
Slavonia
Compliant
Bocote
—
Cobolo
—
Mogano
—
EVOLUTION
Evo Greyge
Compliant
Evo Gris Fonce
Compliant
Evo Noir
—
CARRIERE
Bruges
Compliant
Gent
Compliant
Namur
—
WOODSIDE
Oak
—
Nut
—
Namur
—
ROCKS
Porfido
—
Silver Black
—
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