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THE SRI INDEX APPLIED TO CONCRETE AND PORCELAIN STONEWARE SLABS

TECHNICAL GUIDE

THE SRI INDEX APPLIED TO CONCRETE AND PORCELAIN STONEWARE SLABS

Table of Contents

INTRODUCTION

Maspe has been operating in the outdoor flooring sector since 1976.

Over 40 years of experience in the production of slabs and interlocking pavements, to always better satisfy the needs of our customers. Our company is increasingly attentive to environmental aspects, regarding LEED regulations specifically on solar reflection we have specially developed two new products:

  • Atom Gres Line: Mizar Ice 60x60x2 cm
  • Concrete slabs Line: Bianco Reflex 40x40x3.5 cm and 50x50x3.7 cm

This handbook provides a comprehensive overview of the Solar Reflectance Index (SRI), with particular attention to application on concrete and porcelain stoneware slabs.

The objective is to provide designers and technicians with an updated consultation tool on definitions, calculation methodologies, typical values, recent studies, regulations and practical applications.

DEFINITION AND CALCULATION METHODOLOGY

The Solar Reflectance Index (SRI) is a normalized parameter that measures a surface's ability to reflect and disperse solar energy. It combines two properties: solar reflectance (albedo) and thermal emissivity. The main reference standard is ASTM E1980, integrated by ASTM E903 (spectral solar reflectance) and ASTM C1371 (emissivity). SRI is calculated by comparing the surface's equilibrium temperature with those of standard black (SRI=0) and white (SRI=100) surfaces.

Under standard conditions established by ASTM E1980 standard (solar irradiance 1000 W/m², air temperature 37 °C, sky at 27 °C, with different wind speeds) the equilibrium temperatures of a material and black and white references are calculated; the material's SRI is then calculated by comparing its surface temperature with those of the references through the formula defined in ASTM E1980. Based on this definition, the SRI of many materials typically falls between 0 and 100, but negative values are possible (for very absorbent surfaces) or above 100 (for highly reflective surfaces, 'cooler' than the white reference).

DIFFERENCES BETWEEN SRI, ALBEDO AND EMISSIVITY

Solar reflectance (also called albedo) is the fraction of incident solar radiation that a surface reflects; it varies between 0 (totally absorbent surface) and 1 (completely reflective surface).

A high reflectance value (close to 1) indicates that the material reflects most of the incident solar energy, reducing heat absorption.

Thermal emissivity, on the other hand, is the ability of a surface to re-emit absorbed energy in the form of infrared radiation (heat); it also varies between 0 and 1, and depends on the characteristics of the material and its surface.

A surface with high emissivity quickly dissipates accumulated heat, while with low emissivity it retains heat longer.

SRI combines these two aspects into a single index.

Considering albedo alone is not enough to predict the thermal behavior of a material in the sun: for example, two materials with the same reflectance can heat differently if they have different emissivities.

A high SRI requires both high reflectance (to minimize initial absorption) and high emissivity (to maximize dissipation of absorbed heat). In this way, SRI quantifies the combined effect: 'cool' materials typically have high SR and high ε, staying much cooler under the sun compared to dark or metallic materials with low reflectance or low emissivity.

In summary, while reflectance (albedo) indicates how much sun is immediately bounced away and emissivity how much heat is radiated away, SRI provides a single number that indicates that surface's tendency to heat up in the sun compared to standard references.

TYPICAL SRI VALUES FOR CONCRETE AND PORCELAIN STONEWARE

SRI values of cementitious and ceramic materials can vary in a very wide range, depending on color, surface finish and the presence of pigments or special treatments.

Below are some indicative values drawn from recent studies and certifications:

SPECIFIC VALUES MASPE PRODUCTS: MIZAR ICE AND BIANCO REFLEX

The products Bianco Reflex Maspe and Atom Gres Mizar Ice are characterized by a high capacity to reflect solar irradiation. This characteristic is suitable for combating the heat island phenomenon.

In particular:

  • Atom Gres Mizar Ice: the solar reflection index SRI for this product is greater than 65/100 according to ASTM E1980-11 standard
  • Bianco Reflex Sandblasted: the solar reflection index SRI for this product is greater than 80/100 according to ASTM E1980-11 standard
code dim. cm. kg/sqm pcs/pallet sqm/pallet kg/pallet
51.105/4 40x40x3.5 cm 82 68 10.88 900
51.105/5 50x50x3.7 cm 85 60 15.00 1280

REGULATORY REQUIREMENTS AND BUILDING PROTOCOLS RELATED TO SRI

The adoption of high SRI materials is promoted both by mandatory regulations (laws, decrees) and by voluntary protocols for energy-environmental certification of buildings. Below is a framework of the most relevant requirements in both international and national contexts:

LEED Protocol

LEED Protocol (Leadership in Energy and Environmental Design, U.S. Green Building Council): Provides credits for reducing the heat island effect both on roofs and on external paved surfaces. In LEED v4 (Italy NC 2016) the minimum requirements are articulated in terms of solar reflectance and SRI, also considering aging at 3 years.

In particular for external pavements: they require materials with Solar Reflectance (SR) ≥ 0.28 after 3 years of exposure (or initial SR ≥ 0.33 if 'aged' data is not available). This is equivalent to approximately an SRI of 29-30 for horizontal surfaces. Alternatively or in combination, LEED encourages solutions such as shading of paved areas (vegetation, photovoltaic canopies) or the use of draining surfaces.

Overall, compliance with these requirements contributes to the LEED score in the 'Site Sustainability - Heat Island Reduction' category.

It should be noted that LEED admits both active strategies (shadows, green roofs) and the use of cool materials; SRI is the main tool for quantifying the 'coolness' of a material.

Minimum Environmental Criteria (CAM)

In Italy, since 2022, environmental criteria that include measures against heat islands have been mandatory for public procurement. The D.M. June 23, 2022 n.256 (CAM Construction) in section 2.2.6 requires for external waterproof surfaces the use of materials with high solar reflection index such as:

External pavements, squares, sidewalks, courtyards etc.: SRI ≥ 29

ITACA Protocol (UNI/PdR 13:2015)

In addition to LEED, the Italian ITACA protocol (UNI/PdR 13:2015) and local environmental energy rating systems also consider SRI.

For example, GBC Italia has adapted the heat island credit for use in its own schemes (e.g. LEED Italia, GBC Quartieri, ITACA, etc.), with similar thresholds.

In summary, SRI has now entered both mandatory standards (for public buildings in Italy, and in local building requirements abroad) and voluntary environmental certification systems. This pushes manufacturers and designers to characterize outdoor materials in terms of SRI and to prefer solutions with high SRI to satisfy such criteria.

SOURCES

The information and data reported come from technical standards (ASTM E1980, ASTM E903, etc.), industry guidelines (Cool Roof Rating Council, USGBC LEED), as well as recent studies and publications from Italian laboratories and institutions (Certimac, Centro Ceramico, Ingenio articles, Ceramica.info, Pavimenti Industriali, etc.).

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