Civil Engineering Reference
In-Depth Information
Buildings can also contribute to the definition of the microclimate surround-
ing the construction itself. Until now, most buildings have contributed to the urban
heat island effect (UHI), raising the ambient temperatures as a consequence of
heat absorption on external surfaces and its release to the outdoor air. Two exam-
ples of how buildings can minimise their negative contribution are cool roofs and
green roofs.
Cool roofs and paving materials deliver high solar reflectance (the ability
to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing
heat transfer to the building) and high thermal emission (the ability to release a
large percentage of absorbed, or non-reflected, solar energy) which reduces the
heat build-up on external surfaces contributing to UHI (Akbari and Konopacki
2005
).
Green roofs are vegetated roof covers with growing media and plants taking
the place of bare membrane, gravel ballast, shingles or tiles. A vegetated surface
remains at a lower temperature and may include a water retention system to add to
the benefit by evaporation from the wet soil (Barrio
1998
; Fiori et al.
2013
).
These examples show how buildings can become elements that improve the
surrounding environment, contributing to the reduction of air pollution, the mitiga-
tion of ambient temperatures and the production of energy.
TiO
2
materials and coatings
Titanium dioxide is the most widely used white pigment because of its brightness and
very high refractive index. Titanium dioxide is a photocatalyst under ultraviolet light.
Recently it has been found that titanium dioxide, when spiked with nitrogen ions or doped
with metal oxide like tungsten trioxide, is also a photocatalyst under visible and UV light.
Titanium dioxide is added to paints, cements, windows, tiles, or other products for its
sterilizing, deodorizing and antifouling properties.
Cool roofs
The outer layer surface temperature remains close to the air temperature compared to a
standard, dark cladding. The benefits lie in a reduced heat flow in the building with con-
sequent savings on cooling and a reduction of the urban heat island effect. With lower
temperatures both indoors and outdoors, user comfort also increases.
In the temperate climates that are typical of most of Europe, the cooling benefits of a
highly reflective roof surface far outweigh the potential winter month heating benefits of a
less reflective, or black, roof surface.
Cool roofs are categorised as:
•
inherently cool roofs
: these roofs are covered with light-coloured paint or plastic
membranes (vinyl) that achieve very high reflectance values;
•
coated roofs
: technologies are available that make standard cladding products more
reflective. Products include shingles, tiles, tinted paints, membranes and metal roofing;
green roofs
: these are a specific type of cool roof and they are treated separately.
Cool roof solutions are currently rated in the US under programs such as Energy Star,
Cool Roof Rating Council, Green Globes and LEED.
The benefits of a cool roof system are more significant in buildings that have large roof
to floor area ratios. This makes them less appealing to high-rise buildings, although there
are still beneficial effects on user comfort.
•
