Reduction Of Heat Flux Green Roof

Heat flux reduction depends on the building and roof insulation and moisture in a green roof s soil medium.

Reduction of heat flux green roof.

In summer the green roof protects the building from direct solar heat. Typically it can lower the need for air conditioning load to cool a building 7 by 10 to 30 percent. When water changes from a liquid to a gas energy is needed to overcome the molecular force of attraction between the particles. Shading the outer surface of the building envelope has been shown to be more effective than internal insulation.

Green roofs provide shade remove heat from the air and reduce temperatures of the roof surface and surrounding air. 2 reduced heating and cooling costs provides extra roof insulation. 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. Reduced urban heat island effect a green roof reduces the level of absorbed heat in dense concrete areas.

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. 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 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. 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. Extended life of the roof protects the roof from weather reducing maintenance costs. Using green roofs in cities or other built environments with limited vegetation can moderate the heat island effect particularly during the day. The average 24 h effect of sedum covered green roof was a 0 2 c increase of air temperature 15 cm above the ground.

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