Green Roofs Emittance And Albedo

Cool roofs are pretty cool but whether or not they save as much energy as a green roof requires as usual a closer look at a a cool roof and b a given green roof design.

Green roofs emittance and albedo.

Although cool roofing is one of the most significant green technologies today it is not a new concept. Mindful that reflectance and albedo can be different a combination of high albedo and high emittance resists solar heat gain most effectively. They can also reduce the urban heat island effect. Cool roofs use roofing materials that are highly reflective also called high albedo roofs of the sun s radiant heat.

Finally keep in mind that the style of your roof will affect the necessary reflectance and emittance values your new roof should meet. Albedo is a unitless measure with possible values ranging from 0 to 1. However emittance also matters because the higher the emittance the more readily the roof surface will radiate long wave ir heat to the sky. An energy calculator for green roofs.

Green roofs may not reflect as much as a cool roof but do have other benefits such as evapotranspiration which cools the plants and the immediate area around the plants aiding in lowering rooftop. Albedo or solar reflectance is a measure of a material s ability to reflect sunlight including the visible infrared and ultraviolet wavelengths on a scale of 0 to 1 an albedo of 0 0 indicates that the surface absorbs all solar radiation and an albedo of 1 0 represents total reflectivity. An albedo of 0 0 indicates that the surface absorbs all solar radiation and an albedo of 1 0 represents total reflectivity. By doing this cool roofs allow for a more comfortable and controlled indoor environment.

This is due to the nature of the roof itself. They may also have a high thermal emittance value. Cool roofs by chris wark energy editor originally posted march 2 2011. A vinyl roof can reflect three quarters of the sun s rays usually far more and emit 70 or more percent of the solar radiation absorbed by the building envelope.

Emittance is a measure of how much of the absorbed radiant energy is given off into the atmosphere keeping the surface cooler as a result. The solar reflectance of green roofs varies depending on the plant types generally 0 3 0 5. Flat or low pitch roofs in particular need to meet a reflectance value of 0 65 and an emittance value of 0 50. Green roof energy series part 6.

Peck is the founder and president of green roofs for. A cool roof prevents heat absorption by reflecting the sun s heat and then emitting its radiation back into the atmosphere. Sustainable roofing glossary albedo. When a roof can deliver high solar reflectance or albedo and thermal emittance values it is known as a cool roof.

Cool roofs are able to reduce heat gain in buildings due to the high solar reflective index.