, the high temperatures
recorded in the top few centimetres, can affect the normal development
of plant cover, especially in the early years of growth. This
fact may explain the difficulties that Sedum sp has had to extend his
surface, being relegated only to between 20 and 40 % cover. This
affected the richness of the roof, which was invaded by colonizing
species that germinate strongly in spring and early summer, but
then swiftly disappear due to both the lack of rain and the high
surface temperatures, creating holes of vegetation on the plant
layer, and consequently leaving exposed the bare substrate.
For given similar weather condition, the substrate behaves
differently when the plant cover is lower. Fig. 9b shows the temperature
at different substrate depths for a 2010 typical summer
week. At 2 cm and 4 cm the substrate is heated and cooled very
quickly with a rapid fall of temperature after the daily peak,
whereas at 8 cm, similar to the trend in 2012 summer week, the
thermal mass effect is observed.
The 2 cm profile during 2010 summer period clearly reaches
higher values than in 2012 with the daily peak values varying from
46.7 C to 57 C. Also an important difference is seen in the minimum
daily values of temperature that range from 16.5 to 24.4 C
being considerably lower in 2010, compared to the 2012 data. The
average diurnal fluctuation was 34.1 C. The increased presence of
vegetation canopy may induce a limitation in substrate night
cooling whereas serves as good shield for solar radiation during the
day. That can explain the higher peaks and lower minimum temperatures
in low plant cover condition (2010, 10% plant cover),
where an increase in the daily fluctuation of 8.5 C is observed.
Finally, in reference to the measurements done below of the
substrate layer, as expected, the presence of an insulation layer and
a deep cistern filled with air and water heavily strangles the upward
and downward heat fluxes making negligible the influence of
the upper layers toward the indoor environment. From the measurements
it can be observed that the ceiling temperature was
fairly stable during the whole summer week with an average value
of 30.8 C. Also the indoor temperature exhibited small fluctuation
with an average value of 29.9 C.
As stated from other authors [36], if the green roof is above a
well-insulated roof, then the green roof energy balance would be
decoupled from that of the building, and the green roof will have an
impact mainly on the outdoor environment.
3.