Reduction Of Heat Flux Green Roof

However this difference was reduced to 5 with a snow layer.

Reduction of heat flux green roof.

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

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. Reduced urban heat island effect a green roof reduces the level of absorbed heat in dense concrete areas. A green roof or rooftop garden is a vegetative layer grown on a rooftop. 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. In a plot study when comparing green and traditional roofs for two weeks in the winter researchers found a 23 reduction in heat flux through the green roof relative to the reference roof in conditions with no snow cover. Also under the described laboratory conditions the uninsulated green roof samples with plants showed an average heat flux reduction of 25 compared to samples without plants. Heat flux reduction depends on the building and roof insulation and moisture in a green roof s soil medium.

Green roofs provide shade remove heat from the air and reduce temperatures of the roof surface and surrounding air. In summer the green roof protects the building from direct solar heat. The average 24 h effect of sedum covered green roof was a 0 2 c increase of air temperature 15 cm above the ground. Typically it can lower the need for air conditioning load to cool a building 7 by 10 to 30 percent.

Using green roofs in cities or other built environments with limited vegetation can moderate the heat island effect particularly during the day. Green roofs reduce the heat flux through the roof and less energy for cooling or heating can lead to significant cost savings.