Civil Engineering Reference
In-Depth Information
Fig. 5 Thermal conductivity
of porous insulation materials
as function of gas pressure for
an external load of 1 bar and
at 20 C. The typical range of
the average pore size of the
investigated materials is also
given. For microporous
materials, e.g. precipitated or
fumed silica, the average pore
size is below 1 lm
glass fibres
precipitated
silic
a
PS foam
PU foam
fumed silica
40
p
ext
= 1 bar
T = 20°C
30
20
40 -
70
µm
0.3 - 1 µm
10
0
0.001
0.01
0.1
1
10
100
1000
gas pressure p
gas
[ mbar ]
independent regime at low gas pressures indicate the sum of the solid thermal and
radiative conductivity, k
s
? k
r
.
From Fig.
5
, also two obvious directions for the realisation of superinsulation
could be recognised. Firstly, the possibility to evacuate porous insulations would
lead to effective thermal insulation systems with thermal conductivity values about
10 times lower as they are known for conventional insulation materials. This effect
leads to the development of vacuum insulation panels for building applications.
Secondly, the reduction in pore size below 1 lm would have the effect that even at
ambient conditions thermal insulation products would have thermal conductivity
values below those of still air. This effect was well known since the invention of
silica aerogels in 1932 by Kistler (
1935
). However, only in the last years, new
research activities were launched to develop such microporous insulation materials
for building applications.
3 Vacuum Insulation Panels
3.1 Construction and Physical Properties
The possibility to use vacuum insulation panels (VIP) for thermal insulation of
buildings was first investigated by ZAE Bayern in the years 1998-2003 (Schwab
et al.
2004
). The challenge at that time was to develop a flat more or less vacuum
tight panel with a lifetime durability of more than 50 years. Vacuum insulation
was well known for technical insulations, e.g. storage of cryogenic liquids and for
the vacuum flask for more than 100 years. However, the storage devices had a
cylindrical shape to withstand the atmospheric pressure of 1 bar, which corre-
sponds to 10 t/m
2
. In a vacuum flask, no insulation material is used. The hollow
space between two concentric glass cylinders is evacuated to such a low gas
pressure that heat conduction via the gas phase is negligible. A detailed discussion
