Z E M C H 2 0 1 2 I n t e r n a t i o n a l C o n f e r e n c e
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In this unclear and undefined regulatory context, green roofs mounted together with high
insulation levels, that respect the national building standards, risk of becoming less
efficient. Some studies demonstrate that green roofs, although being efficacious
especially in terms of solar heat gains in summer, can find their potential limited if they
are associated to strongly insulated roofs.
Niachou et al. used the results of an experiment, that was carried out on a green roof
mounted on a building in Athens, to validate an analytical model implemented with
TRNSYS (Niachou 2001). Subsequently, they tried to simulate the energy saving
performance of the green roof in terms of different levels of insulation. Results show
excellent benefits of the green roof in the case of existing buildings retrofit, that is, in the
absence of insulation (U value up to 1.99 W/m
2
K) or with moderate insulation (U value
up to 0,8 W/m
2
K), with a reduction in annual energy consumption by up to 48% and 7%,
respectively. Contribution of the green roof to an insulated building (U value up to 0,26
W/m
2
K) turns out to be almost next to nothing (2%).
Simulations carried out by Theodosiou (Theodosiou 2003) on the basis of the analytical
model by Palomo del Barrio (Palomo del Barrio 1998) confirm that increasing the
insulating layer in the roof reduces the capacity of the green layer to draw out the stored
heat during the day, at nightime because it reduces heat flux that passes through the
system.
Wong (Wong, 2003) analysed the performance of a green roof mounted on a building in
Singapore. The U values for the bare roof and the planted roof were 0.58 and 0.45
W/m
2
K, respectively. Authors recorded temperature of the substrate at different levels
and with different types of vegetation. One of the main benefits of having a green roof
was a reduction in substrate temperature, especially with thick vegetation on it leading to
a subsequent reduction in solar heat gains penetrating the building. The author also
recorded the ability of the roof to draw out the accumulated heat during the day, at
nightime providing indoor comfort inside the building.
Parizotto and Lamberts (Parizotto 2011) analysed the thermal performance of a green
roof in both summer and winter by comparing it with other low insulated roofs with
ceramic and metal cladding. The green roof was installed on top of an experimental
single-family house in the city of Florianopolis (Brazil) which has temperate climate.
Results in summer showed that the green roof performed better than other kinds of
roofs. The green roof led to reduced external surface temperatures, better indoor
comfort, reduced thermal flux by up to 97% compared to a metal clad roof, and
increased outgoing heat flux by up to 49%. Moreover, the green roof gave excellent
performance in winter also. Even though the vegetation in the roof reduces solar heat
gains, insulation of culture substrate contributes to reducing the outgoing thermal flux by
up to 52% compared to a metal clad roof.
Zinzi and Agnoli (Zinzi 2011) analysed the performance of cool and green roofs on the
already existing non-insulated (U=1.4 W/m
2
K) and moderately insulated (U=0.6 W/m
2
K)
building roofs with the help of Energy Plus software. They were able to obtain savings in
consumption compared to traditional roof coverings: by up to 13.9% in the case of non-
insulated ones and up to 7.8% in the case of moderately insulated ones.
Jaffal et al. (Jaffal 2012) analysed the impact of a green roof on the energy performance
of a single-family house, by integrating it into a building thermal program. The summer
indoor air temperature and the energy demand were evaluated for different insulation
depths. Results showed that green roofs only exhibit significant effects for uninsulated or
moderately insulated buildings, which makes the use of green roofs more thermally
advantageous for retrofitting than for new building construction.
These results confirm that green roofs are less effective with increased insulation.
By the way, most of the results of the above-mentioned studies derived from analytical
simulations, while more experimental data are necessary. The experiment described in
this paper would give a contribution, by analysing the thermal performance of a green
