Environmental Engineering Reference
In-Depth Information
material and mechanical properties of coatings, paints, plastics, etc., plants will also
have an effect on durability aspects. This is a beneficial side effect, which will have
a cost-effective effect on maintenance costs of buildings. The denser and thicker the
plant layer on the green façade, the more beneficial these effects are. As for
example, life expectancy of bare roofs is in general 15 years, whereas for green
roofs this is up to 40 years.
Finally, due to the adsorption of heat during the day, the bare roof will reradiate
the adsorbed heat during night contributing to the urban heat island effect(Eu-
morfopoulou and Aravantinos
1998
). In an urban heat island effect situation, even
night air temperatures are warmer because of built surfaces adsorb heat and radiate
it back during the evening hours (Getter and Rowe
2006
). Covering a building
with vegetation prevents solar radiation from reaching the building skin (shading
effect of leaves), and in the winter, the internal heat is prevented from escaping. By
constructing green façades and green roofs, great quantities of solar radiation will
be adsorbed for the growth of plants and their biological functions. Between
façade and a dense vertical green layer (for both rooted in the subsoil as rooted in
artificial soil-based systems) a stagnant air layer exist. Stagnant air has an insu-
lating effect; green façades can therefore serve as an ''extra insulation'' of the
building façade (Minke and Witter
1982
; Krusche et al.
1982
; Ottelé
2011
). Also
direct sunlight on the façade is blocked by the vegetation. This blocking of the
sunlight ensures that the temperature will be less high inside a house. In winter, the
system works the other way round, and heat radiation of the exterior walls is
insulated by evergreen vegetation. In addition a dense foliage will reduce the wind
speed along the façade, and thus also helps to prevent that the walls will cool.
The insulation value of vertical greened surfaces can be increased basically by
different mechanisms (Peck et al.
1999
; Rath and Kießl
1989
; Pérez et al.
2011
):
• by covering the building with vegetation, the summer heat is prevented from
reaching the building skin (shadow), and in the winter, the internal heat is
prevented from escaping, reflected, or absorbed.
• thermal
insulation
provided
by
vegetation,
substrates,
and
configuration
(mostly related to living wall concepts).
• by trapping an air layer within the plant foliage, since wind decreases the
energy efficiency of a building by 50 %, a plant layer will act as a buffer that
keeps wind from moving along a building surface.
• cooling of air due to evapotranspiration of plants and substrates (if used).
Green façades and roofs will cool local air temperatures in two different ways.
As explained, first of all, walls behind greened surfaces absorb less heat energy
from the sun (traditional façade and roof surfaces will heat up the air around them).
This effect is clearly visible in Fig.
6.5
a, b where uncovered parts of the façade are
heated up (color red) and the parts covered with leaves considerable lower (color
blue and green). Secondly, green façades and roofs will cool the heated air through
evaporation of water (Wong et al.
2009
) (for evaporation of 1 kg water, 2.5 MJ of
energy is necessary); this process is also known as evapo-transpiration. Besides,
hard surfaces encourage the runoff of rainwater into the sewage system. In urban
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