Solar Reflectance Index – HDG Primer

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How Hot Will the Surface Get?

Solar Reflectance Index (SRI) is one consideration in choosing building materials. This measure lets you know how hot the surface is likely to get in the presence of direct sunlight. This is especially relevant for decking surfaces that may get too hot for human comfort. Think pool surrounds and bare feet—human or canine. But barefoot comfort isn’t the only consideration.

Vertical surfaces are also subjected to the sun’s energy. Materials used in a building’s façade can influence the comfort within the structure and also impact the energy used for heating and cooling.

Solar Reflectance Index (SRI)

More technically speaking, the sun’s energy is either reflected or absorbed by materials. When the sun goes away, the materials then give off the previously absorbed energy; this is called thermal emittance.

The Solar Reflectance Index (SRI) combines solar reflectance and thermal emittance into a single value that expresses how hot a material is likely to become when exposed to sunlight. SRI is measured on a scale from 0 to 100, where 0 represents a black surface that absorbs most solar energy and becomes very hot, and 100 represents a bright white surface that reflects most energy and stays much cooler. Many common building materials fall somewhere between those extremes.
A higher SRI means a surface is better able to reject solar heat. A lower SRI means the material is more likely to become hot in direct sunlight.

For specific examples, see the section below with sample materials and their SRI values.

Garden-Courtyard-with-Porcelain-Pavers-HDG Building Materials

Key Principles

Solar Reflectance

Some of the sun’s radiation is immediately reflected away from material surfaces. Generally, light-colored, smooth, and shiny materials reflect more solar energy than darker, rougher, or matte finishes—although advanced coatings and modern materials can sometimes defy expectations. The amount of reflected energy plays a major role in how hot a surface becomes.

Thermal Emissivity

Not all solar energy is reflected; some of it is absorbed by the material. Thermal emissivity describes how efficiently a material can release that absorbed heat back into the environment. Two materials can absorb similar energy, but the one with higher emissivity will cool more effectively once the sun’s intensity changes.

Building Heat Transfer

Solar radiation that isn’t reflected and is absorbed by the building can be transferred inward. Buildings and materials gain or lose heat through conduction (direct contact), convection (air movement), and radiation (energy waves). Together, reflectance, emissivity, and heat transfer characteristics help determine comfort, performance, and energy use.

Solar Reflectance Index Key Principles Illustrated - HDG Building Materials

Relative Temperatures for Porcelain Pavers

Based on empirical testing using an infrared thermometer on porcelain pavers near the 45th parallel north (Portland Oregon, New York, Paris, Venice Italy): The dark greys, blacks, and darkest brown porcelain pavers range from 10-22 percent hotter than standard grey (which is medium-light grey) concrete. Porcelain pavers in mid-range greys, tans, beige colors are about the same or perhaps 5 percent higher than concrete. Porcelain pavers in lighter whites, creams, beige, and the lightest grey colors develop 5-20 percent less heat build-up than concrete.

Kaia Black 60x60 cm Porcelain Paver Outdoor Terrace - HDG Building Materials

Dark Greys, Blacks, Browns

Colors in this range are 10-22 percent hotter than standard grey. In direct sunlight, pavers in darker colors will generally have more heat rise and give your bare feet the feeling of “feet on hot sand.” This may or may not be an issue in your application. See SRI Related Design Considerations.

Concrete Pavers and Buzon Pedestals on Hotel Walkway - HDG Building Materials

Medium-Light Grey Concrete

Standard grey concrete for comparison. The Leadership in Energy and Environmental Design Green Building Rating System for New Construction (LEED-NC™) Reference Guide reports default SRI values for new gray and new white concrete of 35 and 86, respectively.

Rum Beige Porcelain Paver on Raised Pedestal Application

Mid-Range Greys, Tans, Beige

About the same or perhaps 5 percent higher than concrete. Weathering effects SRI on most materials. Default SRI values for weathered gray and weathered white concrete are 19 and 45 down from 35 and 86, respectively for new. Maintenance of material surfaces is key to maintaining the SRI values for the materials at the time they were installed.

Quarry White 60x60 cm Porcelain Paver in Rooftop Deck - HDG Building Materials

Whites, Creams, Light Greys

“Cool Colors” develop 5-20 percent less heat build-up than concrete. Generally, light-colored materials have higher solar reflectance than dark-colored materials. However, color isn’t a 100 percent reliable indicator of reflectance. Sunlight, or solar radiation encompasses ultraviolet, visible and infrared radiation. Visible light makes up about 40 percent of the energy in the solar spectrum.

Solar Reflectance Index – Design Considerations

Use darker colors in shady areas or northern climates
Cool materials are increasingly beneficial as the ambient temperature rises
Will people be barefoot as is typical for a pool surround?
High SRI materials used in cold climates can increase heating demand because of low energy absorption
SRI can change over time as material surfaces age, weather, and discolor (maintenance is key)
Consider what’s nearby. Will there be trees in the landscape, water, or other objects?
Is the decking application conducive to elevated pedestals and an air gap? Similar to a rain screen siding application the air gap can be beneficial in reducing demand for cooling and heating.
Is energy consumption a consideration for your project? A pedestrian courtyard set at ground level doesn’t impact a buildings energy profile as much as a courtyard designed at rooftop.
Natural stone used on a building’s envelope can decrease urban heat island effect
Are LEED credits for heat island reduction or other credits important to achieving your design goals?

Understanding Reflectivity of Various Roofing Materials Helps Understand Solar Reflective Index (SRI)

Albedo, is latin for “whiteness.” It is a measure of the reflection of solar radiation out of the total solar radiation received and ranges from 0 to 1. The albedo for fresh snow is 0.9 and for charcoal about 0.04. The albedo of several types of roofs is depicted here courtesy of the US EPA.

Urban Heat Island Effect

An Urban heat island (UHI) is a metropolitan area that experiences warmer air temperatures compared to its surrounding rural areas. The higher temperatures are due to stored heat by low reflectance materials used in roads, roofs, decking, and other structures and objects in the built landscape.

Using cooler materials with high “albedo” in paving, decking, roads, and roofs can mitigate these effects. See also Green Roofs – Your City – Get Ready.

Solar Reflectance Index is related to Urban Heat Island Effect - HDG Building Materials

Solar Reflectance Index (SRI) Values for Various Building Materials

Bamboo

SRI Range: 35 – 48

HDG Dasso.XTR Classic Espresso: Solar Reflectance Index of 35 (New), 48 (Weathered One Year).

Aljoya House - Turtle Shell Limestone Wall Cladding

Natural Stone

SRI Value: 54

Standard Gray Limestone has a Solar Reflectance Index of 54.

High numbers mean cooler temperature underfoot so this is an excellent choice for the decking around swimming pools. Bare human or canine feet will stay cool even on hot summer days.

Concrete Pavers

SRI Range: 5 – 85

Solar Reflectance Index varies considerably across concrete paver products.

In the NW Series of Concrete Pavers, Graphite Charcoal, Platinum Grey, and Latte Brown have SRI values of 5, 30, and 34, respectively.

The 4C Series in color Flint 765 has SRI of 14 compared to an SRI of 85 for Shotblast Series in color White 15. See more representative values for Shotblast, Granite, Fine, and 4C series in a range of colors in this document.

Thermory®

SRI Range: 39.56 – 39.81

The Solar Reflectance Index for a sample of Thermory wood was tested to be 39.68 under medium wind conditions. Under low or high wind it was, 39.81 and 39.56 respectively.

Note: for non-metal surfaces, SRI is insensitive to changes in convection coefficient (rate of heat transfer from the surface to air induced by the air movement).

Resysta®

SRI Value: 29

Resysta standard color stain – FVG-C26 (Rust): Solar Reflectance Index of 29. This is a mid-range color, neither on the light nor dark side. Please contact us to access the test report.

Kebony®

SRI Range: 27 – 49.3

Kebony Radiata Clear: Solar Reflectance Index of 49.3
Kebony SYP Clear: Solar Reflectance Index of 27.0

Help and Resources

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