Planning of Green Roofs and Green Walls

Andras Bela Olah PhDolah.andras.bela@gmail.com

Lecture 3: Ecological and climatic effects

Climate and the usefulness of green roofsThe green roof is NOT(!) a good solution everywhere.

The original ideology of the invention of green roof is to give back to the nature the area (on the top of the building), which was taken by the builing itself.

But there are certain cases, when there was not taken the area, because originally there was not any vegetation.

These cases: Buildings and settlements in deserts; buildings in high mountain ranges; buildings in Antarctica and in the Arctic.

There are also not so certain cases (i. e. green roofing in Dubai, and in similar semi-desert places). Fom this aspect, the existence of all intensive green roofs can be queried but those, which are irrigated by greywater or which are food producting areas.

In general the denser is the vegetation, the better solution is the green roof.

Just considering the rainforest sites: using enough thick growing medium, the same vegetation can be established that was there before.

Antarctica

Mount Kaliash

Vernacular housing in Yemen

Green roof in Dubai

Green roof in Singapore

Heat insulation and capacitanceThe green roof is a good heat insulator. This is a fact, but the rates can be (and used to be) queried.

The problem and the advantage at the same time is the water.

The green roof is the only building part where the water is welcome.

Water is a bad heat insulator, furthermore in the temperate zone it can cause serious damage due to frost.

All conventional heat insulator material is waterproofed. So that their heat insulation ability can be considered as a constant.

In case of green roofs the heat insulation ability is changing. But as the heat insulation ability decrease due to the appearance of water the heat capacitance increases correspondingly.

Greeen roof is excellent in summer and quite good in winter. Due to the great heat capacitance it is the best solution in regions, where the daily temperature diference is great, while the yearly is low (mediterranean zones).

Daily temperature distribution in winter (graph by János Prekuta)

Daily temperature distribution in summer (graph by János Prekuta)

Climate effectAs it can be seen from the previous graphs, the temparature extremities are highly mitigated by the green roofs both on their surfaces and in the interiors.

Thus, a greater number/area/density of them can have a similar mitigating effect onto the local climate.

The greatest problem of present day settlements besides the air pollution is the urban heat island.

Depending on the size, the number of the inhabitants and the building density the greatest temperature difference of downtowns and the surrounding natural areas can be even 7 °C.

As it can be seen from the graphs the mitigating effect of green roofs can remarkably reduce this difference.

A great number of green roofs in downtowns can have such effect. Nevertheless, altough there are good single examples, there can not be found such a settlement yet.

Urban heat island

The spatial distribution of urban heat island. There can be seen the mitigating effects of green areas by parks. This is the so called Park Cool Island (PCI). The temperature here is not so low like in rural zones, but significantly lower than in the downtown.

Urban heat island

The daily periodicity of urban heat island. Considering the daily distribution of the temperature mitigating effect of geen roofs it can be declared that these vegetation covered rooftops are excellent tools for decreasing the negative effects of the urban heat island.

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Radiation surplus

Radiationshortage

Climate effectAn other very important effect is the mitigation ability of heavy rainfalls.

Because of the global climate change heavy rainfalls occur more and more often. At the same time, the sewage systems of our settlements are not designed for such sudden impacts.

The green roofs, even the extensive green roofs with their thin growing medium layers can remarkably mitigate these rainfalls.

On the one hand they can retain a lot of water in the drainage and the growing medium layers. Naturally the vegetation layer also can retain some.

On the other hand, and from this point of view this is the more important phenomenon, these roof can mitigate slow down and on this way smooth these sudden intensive rainfalls.

This means, that may be the 90% of the rain gets into the sewage system, but not in 15 minutes, rather in 1-2 hours. Thus, the necessary capacitance of the sewage system is only 10-12% (!) that of the other case.

Ecological effectsThe green roofs acts as habitat for wildlife.

The extensive green roofs are excellent for insects, birds, and some annual and perennial plants. These can bear those arid and extreme circumstances, but do not bear the continuous presence of humans.

The intensive green roofs can play similar role. Theoretically, the irrigation provide the possibility of greater biodiversity, but the permanent maintenance, weeding and human presence acts against it.

Even on higher buildings a green roof provides habitat for such species.

Sometimes these are very important, beacuse there exist such species, which absolutely lost their habitat (in case of great cities) and their last asylums are such green roofs.

Some of these green roofs are natural protection areas.

There are so called blue roofs, wich are small ponds on the rooftops.

There exist so called brown roofs, where the plants are native and spontaneously grown (somtimes accelerated by seeding).

Ecological effects (disadvantages)

Workers spray pesticide in Yoyogi park, Tokyo. The park had to be closed because of Dengue fever and infectious mosquitos. A disadvantage comes from the pairing of global climate change and urban heat island.

In case of green roofs the mosquitos live immediately next to the windows.

Blue roof

Oasis 21 Nagoya, Japan. The most extreme example of a blue roof.

Brown roof

A brown roof. Very similar to an exensive green roofs, but with native species.

Brown roof

A brown roof with a bird nest. Sedum can be native even on brown roofs.

Thank you for your attention!