There are many important factors to consider when shopping for a new roof. Besides debating a product's resistance to common threats like algae, fire, wind, and impacts, you should consider the material's solar reflectiveness. We're here to break down solar reflectivity (SR) and solar reflectance index (SRI) to give you a better idea of why this feature is essential to your roof's performance.
What is Solar Reflectivity?
Solar reflectivity refers to how well a material reflects solar energy - sending this energy back into the environment. Low or non-reflective materials will absorb the solar energy, which causes the material to heat up.
Roofing materials can have an SR (solar reflectivity) value from 0 up to 1.0. The closer a product is to zero, the more solar energy it will absorb, whereas products closer to 1 will reflect more solar energy than absorb it.
To make this a more simple explanation, the less reflective a roof is, the more heat it absorbs, causing the internal temperature to rise. So, lower Solar Reflectivity equals hotter materials and, therefore, warmer internal temperatures. The more solar energy a roof reflects, the cooler your interior, as there's less solar energy filtering down into the home.
A product's Solar Reflectivity value affects the material's ability to classify it as an Energy Star product. To meet Energy Star standards, a product placed on high or steep slopes (>2:12) must have a minimum Solar Reflectivity value of 0.25 starting and 0.15 or higher three years later. The Solar Reflectivity must be 0.65 or higher for low slope roofs at first, with a 0.50 Solar Reflectivity in three years.
What is the Solar Reflectance Index (SRI)?
Materials will also have an SRI value assigned based on emissivity (releasing absorbed solar energy) and reflectivity. This value comes from ASTM E 1980 calculations and ensures materials comply with LEED regulations. Roofing materials can have an SRI value of 0 to 100.
The LEED requires roofing materials to have an SRI value of 29 or more for steep slope roofs and more than 78 on low sloping roofs. The SRI uses standards of 0 for standard black, with a 0.90 emittance and 0.05 reflectance, and standard white is 100 with a 0.90 emittance and 0.80 reflectiveness.
The lower the SRI, the more heat the material will absorb, whereas the higher the SRI, the cooler the roof will be, meaning lower internal temperatures.
The LEED - Leadership in Energy and Environmental Design - is an environmental certification system created by the U.S. Green Building Council. The USGBC refers to LEED as a green building rating system.
How is an SRI Assigned?
According to the US Department of Energy, the SRI measures a roof's ability to reflect solar heat, demonstrated as a slight rise in temperature. For example, with a 0 SRI, standard black has an increase of 90℉ (50℃) during full sun. While standard white rises in temperature of 14.6℉ (8.1℃).
The maximum temperature increase is then computed by interpolating the black and white values to determine the material's solar reflectance (SR). This chart demonstrates the roof temperature increases for various materials.
Materials | Roof Temperature Rise (in F degrees) |
|---|---|
Smooth bright white | 15 (30 if soiled) |
Rough white | 35 |
Pastel colors (very light) | 15 to 55 |
Intense (non-light) | 79 to 83 |
Medium gray | 52 |
Aluminum-pigmented asphalt | 50 to 65 |
Gravel coated BUR (Built-up Roof) | 61 to 83 |
Asphalt shingles with granulated capsheet | 72 to 90 |
Black | 90 |
SRI receives its value rating based on evaluating multiple factors, including the material's weight, thermal emittance, solar reflecting, and aging. The roof pitch also affects SRI.
Materials with a high SRI value help reduce the urban heat island effect and indirect solar gain inside your building. The urban heat island effect is the phenomenon that urban settings will have higher temperatures (4℃-7℃) than green areas. This issue is due to the replacement of green with hard surfaces that absorb heavy solar radiation.
Why Does SRI Matter for Your Roof?
So, why does SRI affect what type of roof you choose for your home?
If you live in a hot climate with large amounts of direct sunlight, a lower SRI could cost you extra money. This issue happens because as your roof absorbs solar energy instead of rejecting it, that heat filters down into your house. This excess heat can cause your home's internal temperature to rise in hot months, causing your air conditioner to work harder to keep the house cool. This extra run-time reflects in your energy bills.
When you choose a high SRI-rated roof, you can see savings in energy costs, enjoy a cooler home, and lower the amount of heat that goes back into the air, reducing the urban heat island effect. You're also reducing the amount of power consumption, also great for the environment.
Cool Roof
The many benefits of high SRI are the main reason most roofers and roofing manufacturers encourage cool roofs.
A cool roof stays cool, despite exposure to full sun. This feature is because these roofs minimize solar absorption while maximizing thermal emittance. In other words, they reflect the heat away from the house rather than absorbing it into the home.
A cool roof can save you money in energy bills and the amount of cooling power your unit uses. But it can also cost you slightly more in heating consumption during winter months, as the roof can't absorb solar heat. Without the solar heat, your heating component will have to work harder to keep the inside warm.
If you live in a climate with mild summers and frigid winters, you might do better with a darker roof with a lower SRI. Lower SRI roofs can absorb more solar heat, warming your home in the winter and reducing the work your heating unit has to do.
Three of the most significant factors that affect whether a roof classifies as a cool roof are the type of material, its color, and the roof slope.
Flat (low pitched) Roof
Roofs with a flat or almost flat pitch have a minimum 1:40 fall, with a 1:80 on finished roofs. The best types of cool roofs for this type include:
- Coated
- BUR (built-up roofing)
- Modified bitumen
Steep-pitched Roof
Steep-pitched roofs pose challenges due to the roof slope, making it crucial that you pick a roofing material that meets these needs. In terms of cool roofs, you'll have limitations.
Asphalt shingles are one of the most popular choices for these roofs due to the material's weight and durability. Numerous options offer high reflectivity, making them cool roofs. You can also find polymer or wood roofing coated with UV-reflective substances.
Clay, concrete tiles, and slate are also common, particularly in warmer climates. You will need to look for options coated with reflective ingredients that reject the sun. These materials' durability against weather conditions is a considerable factor for severe weather-prone locations.
Metal roofs are suitable for both steep and low pitched roofs. In addition, you can find multiple types of metal finished with products that increase thermal emittance and solar reflectiveness. Meeting these criteria allows most metal products to certify as cool roofs.
Types of Cool Roof Coatings
Multiple methods can apply as a cool coating. These coatings are versatile enough to work on asphalt, metal, gravel, or single-ply materials. Types of cool roof coatings include:
- White - opaque, reflective coating of polymeric components mixed with white pigments
- Pigmented - similar materials mixed with darker shaded pigments (blue, green, red) for improved aesthetics
- Aluminum - resin mixed with aluminum leaf flakes to mask material layer composition and improving thermal emittance and solar reflectance
- Elastomeric roof coatings (ERC) - acrylic-based coating applied over an existing roof (low or steep pitched) to protect against extreme weather while drying quickly
Energy Star Approved
Products that receive an Energy Star Label - a program run by the Department of Energy (DOE) and the EPA (Environmental Protection Agency) - meet the minimal criteria for reliability and solar reflectance. The EPA requires roofing materials to meet Graph A or Graph B requirements to get an Energy Star certification.
Graph A
Low-Slope Roof Specifications for Energy Star Qualifications
Energy Efficiency | Performance Specifications |
|---|---|
Initial SR | Equal to or greater than 0.65 |
Maintenance SR | Equal to or greater than 0.50 |
Graph B
Steep-pitch Specifications for Energy Star Qualifications
Energy Efficiency | Performance Specifications |
|---|---|
Initial SR | Equal to or greater than 0.65 |
Maintenance SR | Equal to or greater than 0.50 |
In Closing
Picking a new roof requires understanding your needs and then finding a product that checks all these boxes. Your climate can play a big role in what type of roofing material is right for you, and that includes affecting which solar reflectance index your roof will need. Colder climates do better with low SRI values while warmer climates might benefit from a higher SRI.