White Roofs: All You Need To Know About
Roofing plays a significant role in a building’s energy efficiency. Depending on how it is designed, it can either help or hurt the building’s energy efficiency.
Many people are aware that light colors help to reflect sunlight. That is why, in the summer, we prefer to wear lighter-colored clothing.
What about roofs? Can we use this same principle to keep our roofs cooler?
Yes, that is exactly why white roofs exist. They reflect the sunlight to keep a building cooler, which reduces energy demands for air conditioning.
There is a growing trend in the building industry to install roofing materials that have high reflectivity to reduce the amount of solar energy absorbed.
Newer roofing materials are now either made with highly reflective materials or are coated with a white pigment to reflect sunlight.
In this post, we will discuss what are white roofs, how they work, and some of the benefits and drawbacks of choosing a white roof.
What is a White Roof?
A white roof is a type of roof that reflects the sun’s heat rather than absorbing it.
It is a cool roof strategy since the white color reflects sunlight instead of absorbing it.
White roof surfaces reduce the heat that the building’s interior experiences during hot summer months, decreasing energy demands for cooling.
According to the US Department of Energy, white roofs can reduce a building’s air conditioning needs by up to 15%.
However, a white roof does not necessarily mean a white-colored roof. The term “white roof” refers to roofs that are either white or very light colors and have similar reflectivity to a white roof surface.
How do white roofs work?
Traditional dark-colored roofing materials tend to absorb a lot of solar radiation and transfer heat to the building’s interior.
This is why black asphalt shingles are often the cause of a hot attic, which can lead to higher energy bills.
White roofing materials have high solar reflectivity and thermal emittance that reduce heat absorption.
They help keep roofs cooler during the summer months and decrease building temperatures for better comfort inside buildings.
White roofs achieve the “cool roof” effect thanks to two properties and their combined effects on the roof’s surface temperature. These properties are solar reflectivity (SR) and thermal emittance (TE).
Solar reflectivity (SR) is a material’s ability to reflect sunlight. A roof with high solar reflectivity will have very little absorption of the sun’s energy.
While thermal emittance refers to a roof’s ability to emit absorbed heat via radiation.
This means a roof with high thermal emittance will better cool itself by emitting thermal radiation.
Both solar reflectivity and thermal emittance are measured on a scale of 0 to 1. The greater these values, the cooler the roof is expected to be.
What are the advantages of having a white roof?
There are a number of advantages to using white roofs. Here are some of the main advantages.
Reduces Energy Demand for Air Conditioning
During the summer months, white roofs help keep building interiors cooler. They also reduce the heat that penetrates into a building through roof spaces and attics.
This helps to decrease cooling energy demands for air conditioning in buildings with conventional dark-colored roof materials.
White roofs have been proven to lower cooling costs by up to 15% according to the US Department of Energy.
Reduces Greenhouse Effect and Global Warming
White roofs help reduce building temperatures, which reduces the heat-trapping “greenhouse effect”.
White roof surfaces can also lower atmospheric temperature by reflecting sunlight away from Earth’s surface before it is absorbed as thermal radiation.
In this way, white roofs can reduce global warming and help slow the rate of climate change.
Increases Occupant Comfort
White roofs also help increase the comfort levels inside buildings. They help keep building interiors cool during hot summer months, which makes for more comfortable living and working conditions.
What are the disadvantages of having a white roof?
Although white roofs offer a number of advantages, there are also some disadvantages to having white roofs.
Increases Heating Costs During Winter
White roofs can increase heating costs during the winter months because they reject heat and prevent heat gain from the sun.
Although this can cause a minimal increase in heating costs it is still worth mentioning.
Requires Regular Cleaning
White roofs also require more frequent cleaning than traditional roofing materials.
This is especially important to keep in mind if you live or work near busy roadways because soot and other pollutants accumulate on the surface of white roofs over time.
Therefore, they require regular cleaning to maintain their white appearance and reflective qualities.
Costs More to Install and Maintain
White roof materials are often more expensive than traditional dark-colored roofing.
This is because they are built to have certain optic or aesthetic qualities that traditional roof materials are not.
They require more frequent cleaning and/or coating which might add to the overall installation and maintenance costs.
Is the “white roof” and the “cool roof” the same thing?
Although both “white roof” and “cool roof” terms are often used interchangeably, they are not the same thing.
A white roof is a roof that is either white or very close to white. Because white is a combination of all the colors in the visible spectrum, it reflects all colors of sunlight and rejects the sunlight’s heat.
On the other hand, a cool roof is a type of roof that has high reflectivity. A cool roof can be any color, including white.
A cool-colored surface will naturally reflect a higher percentage of sunlight than a similar-looking conventional-colored surface.
For example, cool-colored red roof shingles will reflect more sunlight than conventional red shingles considering both shingles have a similar tone of red.
Cool roofs achieve this through the use of highly reflective materials or pigments coated on the roof material’s surface or deposited as a thin film.
Therefore, white roofs are a type of cool roof but not all cool roofs are white roofs.
For example, a dark-colored surface may reflect 20% of incoming light, while a cool dark-colored surface reflects 40% and a light-colored surface reflects 75%.
Do white roofs maintain their reflectivity forever?
White roofs lose their reflectivity because of dirt buildup, abrasion, and material degradation.
Therefore, they need to be well maintained in order to ensure they are as reflective as possible.
There are a few ways to maintain the reflective properties of a white roof:
- Regular cleaning (pressure washing and/or light brushing) is the best way to keep a roof clean. It will help remove any dirt or grime buildup on the surface, which reduces the roof’s reflectivity.
- Application of reflective coating/pigment periodically or painting with a new layer of white paint every few years.
- Using a special surface coating that is more durable and resists dirt buildup.
Building projects can use a combination of the above strategies to achieve a white roof that is clean and reflective.
Although we have mentioned some ways to keep white roofs clean and reflective, it is not perfectly achievable.
The roof’s age, location, weather conditions (especially high humidity), and amount of foot traffic are important considerations that affect its performance and need to be taken into account.
Therefore, it is possible for a white roof to lose its reflectivity even if it was regularly maintained.
If you have a white roof on your building, chances are that it will gradually lose its reflectivity and require some form of maintenance. That is perfectly what is expected to be.
However, the building industry has noticed that different roof materials age much differently than others, even if they initially had similar reflective qualities.
Because each roofing material’s build quality and manufacturing standards are different.
There was a demand for higher-quality roofing materials that could retain their initial reflectivity for extended periods of time.
So building industry come up with 3-year aged SRI/SR values that best represent roof material solar reflectance after three years of weather exposure.
What is the difference between Solar Reflectivity (SR) and Solar Reflectance Index (SRI)?
Both Solar Reflectivity (SR) and Solar Reflectance Index (SRI) represent the reflective qualities of a surface.
SR is measured on a scale of 0 to 1 with 0 referring to no reflectivity and maximum absorbance of sunlight, while 1 means 100% reflectivity.
While SRI is measured on a scale of 0 to 100 where 0 means no reflectivity and maximum absorbance of sunlight, and 100 refers to 100% reflectivity.
If both SR and SRI refer to the reflective qualities of a surface, then why there is more than one index to measure it?
Because the testing methods used to calculate SR and SRI are different.
SR values are calculated according to ASTM C1549 – Standard Test Method for Determination of Solar Reflectance.
While SRI values are calculated according to the ASTM E 1980 – Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces.
Energy Star requires steep slope (>2:12) roofs to have an initial SR value of 0.25 while having at least 0.15 after three years.
It requires low-slope roofs to have an initial SR of 0.65 and 0.50 or higher after three years of use.
Solar Reflectance Index (SRI) is derived using reflectance and emissivity data from ASTM E 1980. For steep slope (>2:12) roofing, the SRI must be 29 or higher, while for low slope roofing, the SRI must be 78 or higher.
|Features||Solar Reflectivity (SR)||Solar Reflectance Index (SRI)|
|Scale||Ranges from 0 to 1, where 0 means no reflectivity and maximum absorbance of sunlight, while 1 means 100% reflectivity.||Ranges from 0 to 100, where 0 means no reflectivity and maximum absorbance of sunlight, while 100 means 100% reflectivity.|
|Definition||SR is the percentage of sunlight reflected by a surface, and is a measure of the surface’s ability to reflect solar radiation.||SRI is a combination of surface’s reflectance and emissivity values, which provides an overall measure of how well a surface reflects solar radiation and emits absorbed heat.|
|Testing Method||SR values are calculated according to ASTM C1549 – Standard Test Method for Determination of Solar Reflectance. The test measures the total solar reflectance of a surface by evaluating the amount of sunlight reflected by the surface over the entire solar spectrum.||SRI values are calculated according to the ASTM E 1980 – Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces. This test measures the total solar reflectance and thermal emittance of a surface to determine its ability to reflect sunlight and release absorbed heat.|
|Requirements||Energy Star requires steep slope (>2:12) roofs to have an initial SR value of 0.25 while having at least 0.15 after three years. It requires low slope roofs to have an initial SR of 0.65 and 0.50 or higher after three years of use.||For Energy Star certification, steep slope (>2:12) roofing must have an SRI of 29 or higher, while low slope roofing must have an SRI of 78 or higher. These requirements ensure that the surfaces reflect enough sunlight and emit enough absorbed heat to reduce energy consumption and improve thermal comfort.|
How did white roofs become popular?
The concept of a white roof (also known as a cool roof) is not new. Researchers at DOE national laboratories in Tennessee and California were assessing the energy-saving effects of “solar radiation control coatings” on test roofs in the mid-1980s.
Although these early tests verified energy savings, they were insufficient to persuade roofing designers and installers to abandon traditional dark-colored roofing systems.
A similar study was begun to investigate the impact of light-colored roofing products on summer urban heat islands.
This study found that solar-reflective roofs, solar-reflective pavements, and plants could reduce urban air temperatures, hence saving energy and improving air quality.
Rolling blackouts in the summer of 2001 prompted California’s public utilities to broaden the scope of its peak load management programs.
They discovered that cool roofing reduces peak electricity demand, lowering expenses and avoiding power interruptions.
White roofs now dominate the commercial roofing market in warm and hot climates, thanks in part to California’s energy code change requiring cool roofing for most commercial buildings with low-sloped roofs.
Today, architects and designers can choose from a pool of over 3,000 ENERGY STAR® compliant cool roofing materials listed by the U.S. EPA.
Building projects can benefit from federal tax credits made available for the use of certain cool materials, such as asphalt shingles and metal products.
Also, many state agencies and utility programs provide incentives for the installation of cool roofs.
Cool roof credits are available in sustainable building programs such as the Leadership in Energy and Environmental Design program of the U.S. Green Building Council (LEED).