Civil Engineering Reference
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Moreover, windows should guarantee natural ventilation, above all in resi-
dential buildings. Finally, they have to also assure adequate acoustic insula-
tion (Oral et al. , 2004). Highly energy-effi cient windows such as nanogel
windows (with silica aerogels in the interspace) could satisfy both these
requirements, due to their high thermal insulation coeffi cient (thermal con-
ductivity of silica aerogel is as low as 0.010 W/mK) and high light transmit-
tance. A state-of-the-art market review of the best performing windows
(AbuBakr et al. , 2008; Jelle et al. , 2012) showed that research has focused
on lower thermal transmittance values of less than 0.5 W/(m 2 K). Vacuum
glazings, smart windows, solar cell glazing, electrochromic windows and
fi nally aerogel windows were considered and investigated as the best
solutions.
Aerogel windows, for which one of the lowest centre of glass U-values
was found (0.30 W/m 2 K), seem to have the greatest potential for improving
the thermal performance, daylight, and solar properties in the windows
sector. Aerogel is a highly porous nanostructured and light material, with
many particular properties that attracted the attention of researchers in
various areas of science and technology, and also for building applications.
The term 'aerogel' was fi rst introduced over 80 years ago, marking gels in
which the liquid was replaced with a gas, without collapsing the solid
network of the gel (Kistler, 1931). Afterwards, the chemical composition of
the material and the applications were progressively diversifi ed, but a great
part of applications was focused on the development of high-performance
thermal insulation materials. The most promising applications in buildings
involved granular translucent aerogels and transparent monolithic silica
aerogels (Baetens et al. , 2011). Transparent monolithic panes were devel-
oped by a Swedish company in the 1990s, but advanced glazing systems with
monolithic aerogel in the interspace are not yet used in mass production
(Duer and Svendsen, 1998; Jensen et al. , 2004; Schultz and Jensen, 2008).
At the same time, granular translucent aerogels with an acceptable trans-
parency were manufactured and, starting from 2005, many daylighting
systems (polycarbonate panels, structural panels for continuous façades,
insulated glasses) with translucent granular aerogel in the interspace
appeared on the market (Rigacci et al. , 2004), offering excellent thermal
performance, high quality of the diffused light, a good solar heat gain, and
good sound insulation characteristics (Reim et al. , 2005). Nowadays, espe-
cially in the last two decades, the production of aerogels is localized in
Europe (Sweden, Germany), USA, Japan, and Russia. This chapter investi-
gates the utilization of silica aerogel in the interspace of highly energy-
effi cient windows. The production process of silica aerogels and the main
physical, mechanical, and thermal properties are discussed fi rst. Then, the
current building applications of aerogels as thermal and acoustic transpar-
ent insulation material (TIM) in windows are discussed in more detail and
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