Topic 5 Retention performance and thermal behaviour and effectiveness of green roofs

One of the multifunctional benefits of green roofs is the improvement of building thermodynamics.

At the same time, research has shown that the green roof’s thermal behavior and its effectiveness is highly influenced by the surrounding climate.

Green roofs can mitigate for the urban heat island phenomenon while the vegetation layer can influence indoor thermal conditions.

Studies have suggested that different regions, greening styles, plant materials and substrate formulations affect the thermal behavior of green roofs.

For example, the thermal properties of a green roof vary significantly with moisture, which is dependable on the region.

Generally, however, a green roof controls the temperature of the whole building and creates more stable conditions, keeping it cool in the summer and reducing heat loss in the winter.

For optimal thermal comfort and sustainability however, the application of both a vegetated system (green roof) as well as minimal synthetic insulation is advised.

In wintertime, green roofs reduce convective heat loss as heat flux is less than convectional roof.

In extreme weather conditions of sub-zero temperatures, wind and rain, the benefits of green roofs increase, despite the vegetation being dormant. While not as effective as synthetic insulation however, green roofs perform reasonably well.

The winter thermal benefits depend on vegetation type and material properties of layers (thickness, physical structure and thermal conductivity).

The vegetation layer is a better insulator than the substrate layer.

  • The heat flux decreases when both vegetation and substrate are frozen.
  • When snow covers the roof however, it can act as additional insulation layer reducing heat flux.

An occasion where the green roof’s effectiveness is reduced is as ice penetrates, which leads to greater heat losses as the thermal behavior of the soil is affected.

  • The extent of penetration itself is dependent on thickness and vegetation.