Reduction Of Heat Flux Green Roof

When water changes from a liquid to a gas energy is needed to overcome the molecular force of attraction between the particles.

Reduction of heat flux green roof.

Reduced urban heat island effect a green roof reduces the level of absorbed heat in dense concrete areas. Green roofs reduce the heat flux through the roof and less energy for cooling or heating can lead to significant cost savings. Green roofs reduce building energy use by cooling roofs and providing shading thermal mass and insulation. Using green roofs in cities or other built environments with limited vegetation can moderate the heat island effect particularly during the day.

In summer the green roof protects the building from direct solar heat. This suggests that extensive sedum covered green roofs might help decrease air temperatures at night when the urban heat island is strongest but possibly contribute to high daytime temperatures. Green roofs provide shade remove heat from the air and reduce temperatures of the roof surface and surrounding air. Typically it can lower the need for air conditioning load to cool a building 7 by 10 to 30 percent.

Green roofs with silt clay soil required more than twice the amount of soil moisture than green roofs with sand to achieve similar roof heat transfer rates. Extended life of the roof protects the roof from weather reducing maintenance costs. Heat flux reduction depends on the building and roof insulation and moisture in a green roof s soil medium. Green roofs are more effective than conventional and white roofs for cooling buildings because they utilize a heat transfer mechanism known as evaporative cooling which is unavailable to most conventional roofs.

The average 24 h effect of sedum covered green roof was a 0 2 c increase of air temperature 15 cm above the ground. However this difference was reduced to 5 with a snow layer. 2 reduced heating and cooling costs provides extra roof insulation. The best net heat flux gains for vegetated green roofs were 4 7 w m 2 for the sand roof and 7 8 w m 2 for the silt clay roof.

Shading the outer surface of the building envelope has been shown to be more effective than internal insulation. Green roofs can reduce the flow of stormwater from a roof by up to 65 and delay the flow rate by up to three hours.