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
596
Figure 8: Heat fluxes measured in the roof slabs (roof types 1,2,4,7) with respect to the total radiation
during a week (8-13 August 2010). Positive values represent heat gains, negative values represent
heat losses
It is possible to see how, with respect to the total radiation characterized by peak values
of 800-900 W/m
2
, heat fluxes penetrating the different roofs (measured on the soffit of
the slabs) reach a maximum of 3 W/m
2
because of the very low transmittance of the roof
systems and because of the fact that the building did not have air conditioning. These
values are lower than those seen in previous measurements on less insulated structures
(Parizotto 2011), but they are in agreement with those hypothesized on an analytical
level by other researches (Theodosiou 2003).
In terms of absolute values, the green roof maximum heat flux (MNV_GR) was around
1,5 W/m
2
. However, the potential of passive cooling of the green roof is highlighted by
the fact that throughout the hours of the day the heat fluxes were outgoing (negative
values). The phenomenon was mainly observed after irrigation, because of the
evapotranspirative phenomena that occur in the ground, and it demonstrates the
capacity of the green roof to expel the accumulated heat every day.
On the contrary, the other roofs analysed showed that the heat flux was almost always
incoming and higher. The roof with copper covering (MNV_LR) showed incoming heat
fluxes up to 3 W/m
2
.
The other roofs gave rise to heat fluxes that were not higher than 2 W/m
2
. Also for these
roofs, there were periods during the day when the fluxes became negative. However,
they did not go beyond 0.5 W/m
2
and were also temporarily very limited.
Moreover, it is evident how the flux through the green roof is delayed in time compared
to the fluxes of the other roofs. On the hottest day (12 August), the green roof caused a
thermal delay of incoming heat flux wave (here defined as elapsed time between
maximum external solar radiation and maximum internal heat flux) by 13 hours as
against 4 h 40 min. by the LV6_A ventilated roof, 6 h 30 min. by the MNV_LR non
ventilated roof and 7 h by the MV6_A ventilated roof (Fig. 9).
