When energy from the sun hits building roofs, it can cause them to reach extremely high temperatures, heating the building. This also puts individual building materials under stress and adds to the urban heat island effect. A green roof can reduce these effects by absorbing the sun's energy and releasing it through evapotranspiration.
This picture shows the cooling effect of a semprivivum plant on the new Butler dorm green roofs. The photos were taken as part of research with PEI to determine the energy savings from the green roofs. On the right is the plant surrounded by its growth medium. On the left is the infrared version of the same plant. Objects which are warmer than their surrounding environment radiate heat as a function of their temperature. The IR camera, which can detect wavelengths up to 14 μm, converts the radiation it sees to a picture much like a digital camera does with light waves, thus revealing the temperatures within its view.