White Roofs vs. Green Roofs
The roof is one of the most important components of a building. It provides structure and protection to the interior.
It also has a significant impact on energy consumption. Because it absorbs or reflects solar radiation which influences the amount of energy required for heating and cooling.
Roofing technology has taken a great leap forward with the introduction of engineered roofing materials to serve a variety of functions.
Designers and architects now have the ability to choose roofing materials that behave in a particular way when exposed to sunlight.
This means that roofs can be designed to absorb or reflect radiation depending on the requirements of the building.
Although there is no shortage of terms used in roofing terminology, there are two types of roofs that stand out.
They are white roofs and green roofs. Here is how they work and how they are different.
White roofs are light-colored roofs designed to reflect sunlight and reduce the absorption of heat.
They are either made of light-colored concrete or covered with another engineered material. Due to their light color, they reflect a large amount of sunlight and absorb less heat than conventional roofs.
While green roofs are vegetated roofs that not only block the sun and heat from the building but also help manage stormwater runoff.
Green roofs almost completely isolate the sun and heat from the building surfaces, allowing them to be naturally cooled in the summer and warmed in the winter.
This is achieved both by blocking the sun’s path and also the evapotranspiration the process by which water evaporates from plant leaves and cools the surrounding area.
What is a white roof?
A white roof is a high albedo roof that is usually made of light-colored roofing material or covered with another engineered material to increase solar reflectance.
Solar reflectance is a measure of how much solar radiation is reflected by the roof.
White roofs have high reflectance values and help reduce building energy costs because they keep buildings cooler due to their high albedo, or reflective property.
Almost every roof material today is tested for its solar reflectance, thermal emittance, and absorptivity values.
Engineers and building owners can use this data to help determine which roofing material is best for a given building.
Solar reflectance values are represented using SRI (Solar Reflectance Index).
The Solar Reflectance Index (SRI) is the measure of a roof’s ability to reject solar heat, as demonstrated by a small temperature rise.
It is defined as 0 for a standard black surface (reflectance 0.05, emittance 0.90) and 100 for a standard white surface (reflectance 0.80, emittance 0.90).
For example, under direct sunlight, the regular black surface heats up to 90 degrees Fahrenheit (50 degrees Celsius), while the standard white surface heats up to 14.6 degrees Fahrenheit (8.1 deg. C).
After determining the maximum temperature rise of a specific material, the SRI can be calculated by interpolating between the white and black values.
Because white surfaces have a higher SRI, they can reject more solar heat than black surfaces.
As a result, white roofs will serve as an effective passive solar shading measure, allowing for less heat gain for the building.
Although white roofs have good solar reflectance, their ability to reflect the sun decreases over time due to weathering and wear, which can lead to increased heat absorption.
Therefore, they need to be cleaned periodically to remove dust and dirt that can reduce their reflectance.
Also no matter how well the roof material is maintained it will still lose its ability to reflect the sun over time.
Therefore, if you are going to install a white roof, you should research the SRI values of the roofing materials 3 – 20 years after installation.
Because the initial SRI value of roof material can be quite different from its value after 20 years. Most serious roof material manufacturers should be able to provide this data.
What is a Green Roof?
A green roof is a roof made partially or completely of live vegetation and a growing medium, planted over a waterproofing membrane.
A green roof can also serve as insulation for the building, helping to keep the building cooler in summer and warmer in winter.
There are intensive green roofs, which are used for growing a variety of plants that require rich soil layers and irrigation systems.
There are also extensive green roofs, which are used mainly for vegetation that has shallow root systems, such as grasses and wildflowers.
Green roofs can reduce stormwater runoff, which can help reduce the burden on sewer systems and water treatment facilities.
Green roofs also provide insulation, helping to moderate indoor temperatures and reduce the need for air conditioning and heat in summer, while also helping to keep indoor temperatures warmer in winter.
One big advantage of green roofs is the evapotranspiration process it creates. Evapotranspiration is the process by which water evaporates into the atmosphere from plant leaves and stems.
Vegetation naturally cools the air around it by releasing water vapor through transpiration.
This process can help lower surrounding temperatures significantly, decreasing energy use for air conditioning.
Should you choose a White Roof or a Green Roof?
White roofs and green roofs are not direct competitors. They are instead different terms commonly used in the roofing industry that help describe different types of roofs.
|Criteria||White Roofs||Green Roofs|
|Definition||Light-colored roofs that reflect sunlight||Vegetated roofs that block sun and manage water|
|Material||Light-colored concrete or other engineered material||Live vegetation and growing medium planted over waterproofing membrane|
|Solar Reflectance||High||Low (due to plant cover)|
|Energy Efficiency||Reduce building energy costs by reflecting sunlight||Reduce energy costs by blocking sun and providing insulation|
|Stormwater Management||Do not manage stormwater runoff||Manage stormwater runoff by absorbing and retaining water|
|Maintenance||Require periodic cleaning to maintain reflectivity||Require regular maintenance and irrigation|
|Cost||Cheaper than green roofs||More expensive than white roofs|
|Additional Benefits||None||Provide insulation, promote biodiversity, increase aesthetics|
Both white roofs and green roofs are good choices for any type of building to reduce energy costs.
Therefore, depending on your goals, you can choose either a white roof or a green roof to help save energy costs in the long run.
If you simply are looking to reduce energy costs and only want to consider the cost of your roof material, you should choose a white roof.
Because a white roof will require much less maintenance and is cheaper than a green roof since green roofs will require planting, soil, irrigation, and more labor to maintain.
However, green roofs have multifunctional benefits. They can provide insulation, absorb rainwater and reduce storm-water runoff, lower urban air temperatures, mitigate the “heat island” effect of cities (caused by roof surfaces absorbing large amounts of heat), promote biodiversity in urban areas, improve aesthetics for building occupants, and passers-by alike.
Hence, green roofs not only serve to reduce heat gain for a building, but they can also serve as an aesthetically pleasing addition to your roof.
Can you combine white roofs and green roofs?
If you want to have the benefits of both white roofs and green roofs, you can use both a white roof and a green roof on the same building.
For example, you can use a white roof for the majority of your roof and have green roof sections on designated areas of your roof to enhance the aesthetics.
As we discussed in the section above, white roofs and green roofs are not competitors, but rather two types of roofs that serve a different range of goals.
A white roof will only reduce heat gain within a building, whereas an additional green roof component on the roof will reduce stormwater runoff, improve insulation, promote biodiversity, and increase aesthetics for building occupants.