Civil Engineering Reference
In-Depth Information
Table 3 Overview of insulation materials: thermal conductivities for conventional and super-
insulation materials (adapted from Koebel et al.
2012
)
Insulation material and composition
Ambient thermal conductivity
k (W/m K)
Mineral wool (inorganic oxides)
0.034-0.045
Glass wool (silicon dioxide)
0.031-0.043
Foam glass (silicon dioxide)
0.038-0.050
Expanded polystyrene (EPS) polymer foam
0.029-0.055
Extruded polystyrene (XPS) polymer foam
0.029-0.048
Phenolic resin foam (polymer foam)
0.021-0.025
Polyurethane foam (polymer foam)
0.020-0.029
Silica aerogels (SiO2 based aerogel)
0.012-0.020
Monolithic aerogel
0.004-0.010
Organic aerogels (aerogels derived from organic compounds)
0.013-0.020
0.003-0.011
a
Vacuum insulation panels (VIP, silica core sealed and
evacuated in laminate foil)
0.0001-0.0005
b
0.003-0.008
c
a
Vacuum insulation panels age with time. As the pressure inside the evacuated element rises
because of envelope permeability, so does the thermal conductivity of the core materials.
Equivalent conductivity values of 30-year-aged VIPs depend strongly on the product and
materials used and are typically in the order of 0.007-0.012 W/m K
b
The equivalent thermal conductivity of vacuum glazings k was determined from typical
U-values in the order of 0.5-0.3 W/m
2
K and a thickness of the evacuated gap in the
0.3-1.0 mm range, without taking into account the thickness of the glass pane
c
k-values determined from typical U-values of 0.3-0.5 W/m
2
Vacuum glazing (VG, double-glazing unit with evacuated space
and support pillars)
K and a total thickness in the
10-16 mm range, considering glass panes
The very low thermal conductivity is due to the pore structure in the porous
materials; the total heat transfer depends on the following:
• conduction through the solid material;
• conduction through the pore medium;
• convective transport by the pore medium;
• radiative transport from solid surfaces through the pore fluid; and
• radiative transport from the solid through the solid network or bulk.
In the aerogels, the extremely low thermal conductivity is due to the combi-
nation of low density and small pores (Baetens et al.
2011
): the small pores limit
conductive and convective gas transport, while the solid network offers limited
pathways for conduction due to the low density. In particular, convection by the
pore fluid (i.e. air) is reduced significantly with micrometre pore sizes, so that only
conduction and radiation remain; for these reasons, k can be further reduced by
decreasing the maximum pore size, by filling the aerogel with a low-conductive
gas or by applying a vacuum: with a pressure of 50 mbar; the thermal conductivity
can diminish down to 0.008 W/m K.
