Civil Engineering Reference
In-Depth Information
Even if the VIP is damaged and the U-value is increased (approximately by a
factor 3), it should be acceptable regarding aspects of building physics.
VIPs are not designed to be in constant exposure to water, UV radiation and
high temperatures.
Because VIPs are evacuated systems, cutting of a VIP after production is not
possible. Planers have to develop an installation diagram and parts list to know
how many panels of which size are needed. The use of standard sizes helps to
decrease costs by avoiding the production of VIP with special dimensions.
Logistic is also of important: site-specific packing of the VIPs, punctual
delivery to avoid a long storage period at the construction site, installation only on
clean and smooth surfaces.
Last but not least, it is important that the companies commissioned to carry out
work related to the installation of VIP have sufficient experience with this tech-
nology and are aware about the special technical features.
4 Microporous Insulation Materials
4.1 Types and Thermal Properties
Microporous insulation materials are porous materials with an effective average
pore size below 1 lm. This fact results in the physical effect that the mean free
path of gas molecules is reduced even at ambient atmospheric conditions and the
effective thermal conductivity of the pore gas is below the thermal conductivity of
still air, i.e. 0.026 Wm
-1
K
-1
(cf. Fig.
5
).
Typical microporous materials used for thermal insulation in building appli-
cations are silica aerogels (Aegerter et al.
2011
; Fricke
1988
), pyrogenic silica, i.e.
fumed silica and precipitated silica and mixtures and blends from those. Since the
pure materials are fragile and brittle, often blends with fibres are used to enhance
the mechanical stability in products. In general, the products are hydrophobic in
order to avoid the absorption of water. Additionally, also infrared opacifiers are
added to reduce the heat transfer by thermal radiation. A picture of silica aerogel
as monolith and granulate is shown in Fig.
22
. Monolithic silica aerogel is a
material which shows one of the lowest thermal conductivity values of the world at
ambient conditions, which could be in the range of 0.010 Wm
-1
K
-1
. In com-
parison with this value, the effective total thermal conductivity increases for silica
aerogel powder to about 0.013 Wm
-1
K
-1
and above 0.02 Wm
-1
K
-1
for silica
aerogel granulate, because more and more large pores between the particles
increase the contribution of the pore gas.
A relative small number of microporous insulation products for building
application are commercially available since years. This number will certainly
increase in future because a lot of research activities are currently under way. An
obvious disadvantage of this group of thermal superinsulation is the high prize
