At 4 cm (Fig. 11 left c) the effect of spatial influence is also
visible. Gravel temperature in plot 2 rises to the value of 52.8 C at
17:00; the substrate temperature in plot 3 reaches the peak at the
same time but with a value of 46.7 C, substantially lower than
gravel. Again substrate in plot 1 behaves differently. During night
time and in the early morning the temperature is significantly
higher than both plot 2 and 3. Owing to the different exposure to
sun radiation the peak, although with similar value of plot 3, is
attained at 18:40. The process of charging the growing media
starts later in plot 1 so that consequently the discharge period is
delayed, and the temperature during the night assumes higher
value.
In addition, as previously mentioned in the floristic composition
section, large differences in floristic composition between plot 1
and 2 were observed. Differences on plants species could result on
Fig. 10. Substrate temperatures and volumetric water content. (a) Winter 2012. (b)
Winter 2010.
P. Bevilacqua et al. / Building and Environment 92 (2015) 305e316 313
different thermal behaviour in the roof, although it is not an easy
effect to asses due to the mixture plant composition, and therefore
it was not confirmed in this study. Future studies should address
this issue more thoroughly, since the contribution of different plant
species may actually be different (transpiration effect, shade provided,
growth typology, etc.). According to the results obtained, the
spatial effect was related to solar radiation exposition, which is the
key factor that directly influences not only the substrate temperatures
performance but also the floristic composition. Spatial effect
is not despicable and it must be taken into account for future
studies and designs of extensive green roofs.
3.2.2.2. Winter analysis. In winter (04/01/2012) the evolution of
the daily temperature above the roof follow the same pattern than
summer (Fig. 11 right a and b), but the differences between gravel
and green area are more accentuated, either at 30 cm and 5 cm, due
to evapotranspiration. The temperature measured in plot 1 shows
the typical time delay of the peak, due probably to the different
exposure.
In terms of substrate behaviour, due to the plants species
recession, the level of substrate exposure to solar radiation and
environmental conditions was higher during winter period.
Because of the lower plant cover in winter, the substrate was
extremely heated during the morning, reaching nearly the same
level than gravel area. The differences between plot 1 and 3 in the
daily evolution of the substrate temperature show the great impact
that the spatial factor may have on the green roof thermal performance
(Fig. 11 right c).