Climate Protection with Green Walls - Arguments and examples

Heat in urban areas

Green roofs and green walls – these terms refer to green roofs and green walls. To be fair, it must be said that the former has a significantly greater impact on the urban microclimate, particularly in terms of retaining and evaporating rainwater, along with cooling effects. Green facades can have a certain effect when several factors combine, but not on their own; the green areas are usually too small for that.

What is the most effective way to combat the summer heat in our cities? Above all, the evaporation of water. Every fountain significantly cools its surroundings. Any surface in the street that is wet with water has a cooling effect as the water evaporates. Some cities, such as Vienna, enhance this effect by using water misting systems in the street to spray water through nozzles – see photos.

Next come the street trees, which have a cooling effect; indeed, due to their three-dimensional crown shape, they are more effective than flat façade greening. In addition, or as an alternative, a green wall can also take on this function, at least to some extent. But green poles, hanging baskets and green railway embankments also provide cooling, as do green flowerbeds and green seating islands set up in pedestrian areas. Taken together, all these measures can then make our city centres more bearable when the summer heat is at its peak!

Street trees are highly effective at combating the summer heat; here, they are combined with a wall of ivy (right: Hedera helix). Südplatz in Leipzig / Saxony

The finer the water is atomised, the greater the evaporation rate and, consequently, the cooling effect of a fountain in the city. Vienna / Austria

Water features in the city, such as this bubbling fountain, have a particularly cooling effect. Stralsund / Mecklenburg-Western Pomerania

Here, water is atomised through fine nozzles, creating evaporative cooling. Vienna / Austria

A stretch of road with no trees and one lined with trees. Yes, car parks have to be sacrificed for roadside trees – that’s an uncomfortable truth! Vienna / Austria

Fruit trees planted close to the house can significantly cool a building’s façade. Here is a ‘Hardenponts Butterbirne’ in Deisendorf, Baden-Württemberg

There are lampposts standing amongst the trees lining the street, and these have been covered in greenery (Parthenocissus quinquefolia 'Engelmannii'). Vienna / Austria

Even a tram track bed can be transformed into a green space – at least on certain sections. Vienna / Austria

Hanging baskets in the street are watered regularly; the water evaporates, creating a cooling effect! Naumburg / Saxony-Anhalt

The car produces CO2, the apricot trellis (around 1950) recaptures the CO2, and so all is well with the world, right? For those who only drive 200 kilometres per year, this calculation may be correct.

How can greening help the climate?

Climbing plants bind CO2 and particulate matter, increase air moisture, and contribute to lowering temperatures during hot summers. Cooler air means less air movement, means less whirled up dust and dirt ~ in short, greater conditions for life to flourish! Climbing plants have a significant impact on the microclimate, especially if the facade is fully greened.

Climbing plants bind CO2 (assimilate carbon) and form carbohydrates (leaf fibers and wood pulp = sugar). Ultimately, however, the amount is quite minimal (seen in autumn leaves, the increase in stem/trunk thickness, and possibly the fruit). Only the dry mass is measured in kilograms; that means that per house wall, per year, about 1-2 car tank fillings worth of CO2 will have been absorbed by the plants from the air.

A chemical effect of the decomposition of CO2 and the assimilation of carbon (C) is the release of oxygen (O2), which spreads into the ambient air via the stomata on the undersides of the leaves.

When the sun's rays reach the dark green leaves, they are partially converted into heat. But the chemical process of CO2 fixation requires consistent and relatively low temperatures. For that the heat must somehow be dissipated. Water then constantly evaporates from the leaves to keep the temperature stable, which helps humidify the air. The water collected by the roots, in turn, helps to dry the walls and foundations.

Leaves in the garden facade also absorb dust, especially toxic particulate matter. Once 'captured,' these extremely small nano-particles are much less dangerous and are then washed away with the next rain or composted during autumnal leaf-fall.

Green facades will absorb unwanted heat in the summer and help regulate the temperature of the air and the facade, contributing to air and wall cooling. How? 1) the previously described evaporation process consumes the (heat-) energy; 2) through the chemical processes of carbon fixation. The heat created during combustion of carbon to carbon dioxide is needed more or less again when captured and consumed, if the carbon is again involved in photosynthesis... So, the surrounding air cools and 'sinks,' (this is the fresh cool air you feel in a forest). Essentially: warm air is slowed down, which reduces the movement of air at ground level, and by extension the movement of dust and exhaust gases. There will be less dust whirled up, which will then limit smog formation. This phenomenon has become the subject of scientific study.

More and more cities, at least in Germany, are requiring a certain number of square metres of green space for new building projects in order to preserve or improve the urban microclimate. Too often the demand is perceived as annoying and then only half-heartedly fulfilled. One of our colleagues in the building greening sector shared the following scenario that recently ocurred: a large commissioned (not yet implemented) greening project on a new building was aborted by the contractor the day after the acceptance of the new building by the municipal building authority, because the officials suddenly 'forgot' about the greening part of the contract. The cancellation was then officially signed and confirmed by the official as unessential and 'dispensible'-- a 'stroke of luck' for the contractor's wallet indeed!

Places of special effectiveness

Where can green façades have a particularly strong effect on the (micro)climate? Especially where there is no room for street trees, i.e. in narrow alleyways, backyards, sealed areas, etc. You can see examples here!