Green roofs for noise reduction : literature review and new insights

Green roofs are able to reduce the sound exposure near or inside a building by mitigating diffracting sound waves over (parts of) roofs and by reducing sound transmission through the roof system. A literature study of reported absorption curves of green roof substrates show a large variety. In addition, well-controlled impedance tube measurements showed the strong effect of substrate moisture content. However, a related real-life experiment near a building's edge proved that in practice this not necessarily leads to significant reduction in shielding performance. A re-analysis of actually measured spectral insertion losses of green roofs showed that road traffic noise insertion losses are expected in between 2.3 to 5.5 dBA. Since flat rigid roofs already provide good shielding, the insertion loss of a flat roof equipped with a green roof is rather limited, as consistently shown by both measurements and simulations, all ending up in the range 2-3 dBA. The new idea of shapeable and growable foams as substrate is introduced to enhance the green roof's noise reduction also in the case of a flat roof. In addition, recent finding indicate that there is no need for competition between green roofs and solar panels. Green roofs significantly enhance solar panel performance due to their cooling effect, while specific plant species can easily colonize shaded zones on roofs. In addition, the presence of such acoustically rigid panels was not found to deteriorate the green roof's noise reducing performance by means of numerical simulations. Literature on the acoustic insulation properties of green roofs is scarce. The existing literature stresses its potential due to their relatively large surface mass density, low stiffness and pronounced damping properties. Especially their performance in the low-frequency range is worth mentioning.