Environmental Engineering Reference
In-Depth Information
thanks, as an example, to a highly sealed envelope (increasing its thermal
capacity
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) and to the sun shadings or by suitable trees plantings (notations and
numerical parameters for design are reported in article 10 of the guidelines).
The thermal comfort during summer is maximized if the daily solar heat gains
and the internal heat load collected during nigh time inside the buildings are
extracted, using a nightly strategy of natural ventilation (through the windows
opening) or forced (through mechanical ventilation systems).
A well-sealed and heavy structure is the ideal situation for the exploitation of
the summer night cooling of the thermal mass of the building. During night time
the air circulates through the building, using winds or natural air density gradients,
or using a mechanical ventilation system (notations at articles 10 and 13), setting
partly free the building from the heat held by its mass. The greater or smaller
efficiency of the system must however be verified with reference to the local
climatic features of the site where the Municipality resides.
From this short analysis, the sealing features of the building influence the
building energy consumptions both for cooling and for heating; it is thus however
necessary to evaluate the efficiency of a possible reduction of the value of thermal
transmittance of the opaque components of the building considered as a whole.
In climates such as the one where the municipality of Palermo resides, as an
example:
• A possible hyper-insulation of the building components (is a mechanical ven-
tilation system with heat recovery is employed) does not improve substantially
the winter behaviour of the building, since winter shows mild temperatures (on
average the temperature is 13.9 C).
• However an hyper-insulation would penalize the energy performance during
summer, since as the temperature and the internal heating loads rise, such
sealing would not allow the heat to be drained by the ground at a lower tem-
perature. The analysis of the Passivhaus study in the Mediterranean area, shows
how the choice of not insulating the basement is probably the most desirable in
the yearly energy balance.
• A good insulation of the perimeter walls and of the roof is thus needed since it
provides in some cases energy reductions going beyond 50 % as compared to
the not isolated situation.
• The natural ventilation is less efficient due to the limited daily temperature
fluctuation in Palermo (on average only 3 C during summer) and it is thus
necessary some kind of active cooling system to guarantee summer comfort
conditions.
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The thermal capacity is the property that certain materials have to absorb and retain heat in
time, and this is measured in relation to the number of hours of temporal delay between the input
of the heat flux incident on the exposed face (external) and its release on the opposite side
(inside). To be effective, the material mass must be able to ensure a delay in the passage of the
thermal wave such that the heat is released inside during the coolest hours of the day. The delay
of the thermal wave, due to the heat capacity, is called displacement, while the reduction of the
temperature on the inner surface, with respect to the external surface temperature, is called
attenuation factor.
