Based on the findings, the following conclusions could be
drawn:
(1) Without the benefit of building thermal insulation (BTI),
denoting the poor insulation state, the cooling load imposed
by Control (bare) roof is considerable in the whole day.
Installing green roofs brings notable reduction in cooling
load and energy saving. Despite inherent differences in biomass
structure and photosynthesis–transpiration physiology,
Sedum and Peanut green roofs have comparable effect
on energy saving.
(2) Without BTI, green roofs can serve as surrogate thermal
insulation to restrict heat entry into indoor space.
(3) With BTI, cooling load at Control is suppressed. The two
green roofs behave differently. In conjunction with BTI,
Sedum roof lowers energy consumption. However, BTI coupled
with Peanut roof maintains energy consumption at a
level similar to the omission of BTI.
(4) Regular supplementary water supply due to irrigation has
raised and sustained moisture content of the thicker Peanut
substrate which has a higher moisture-holding capacity.
Some moisture has migrated to the BTI. Overall, the
enhanced thermal capacity and conductivity of the Peanut
roof and underlying BTI have raised the cooling load.
(5) The ability of green roof to bring cooling benefit in hot summer
periods may be nullified by the synergistic development
of green-roof heat-sink effect and building heat-sink effect.
(6) Green roof choice and design could be optimized to avoid
establishment of the heat-sink effects. The choice of plant
species and associated substrate composition and thickness
has critical bearing on thermal-energy performance. The following
attributes are preferred: common C3 photosynthesis–
transpiration physiology, sufficient vigor to establish a
full cover in one growing season, maintenance of full cover
with minimal maintenance, relatively thin substrate, and
relatively low water requirement.
(7) Water content in the green roof and building roof is critical
to the enhancement of thermal capacity and conductivity,
and hence passage of heat to indoor space. Controlled experiments
can be conducted to ascertain the minimum irrigation
requirement of the chosen species for the climatic
zone and microclimatic conditions necessary to support reasonably
healthy plant growth.
(8) The direct measurement of air-conditioning energy use in the
indoor space below green roofs can mitigate the uncertainties
in estimates based indirectly on temperature measurements.
The empirical experimental approach has furnished direct
and systematic evidence and avoided making assumptions
about the heat transfer pathways between roof and indoor.