Civil Engineering Reference
In-Depth Information
Degradation through time of the thermal resistance occurs mainly due
to air and moisture intake through the panel envelope, and depends on the
environmental conditions and the foil resistance for moisture and air trans-
port. The intake results in an increase of the inner gas pressure
δ
p
g
(
Γ
,
κ
),
moisture pressure
δ
p
wv
(
Γ
,
κ
) and water content
δ
uw
(
Γ
,
κ
), resulting in an
increasing thermal conductivity
δ
k
(
p, u
) through time strongly depending
on the panel dimensions
for air and moisture of the
envelope material, and the environmental properties in the domain of
application. Knowledge of the long-term thermal performance of vacuum
insulation panels is still limited: predictions are based on calculations or
short-term on-site measurements. General values for the thermal conduc-
tivity are 0.007 and 0.010 W/(mK) for large vacuum insulation panels
after respectively 25 and 100 years, whereas higher values of 0.009 and
0.015 W/(mK) are depicted for small panels as shown in Table 9.1. The
main differences between large and small panels is due to the envelope-to-
volume and edge-to-volume ratio.
Thermal bridging at the panel edges occurs due to the metalized panel
envelope. The edges reduce the effective overall thermal resistance of the
insulation panel with a linear thermal transmittance coeffi cient
Γ
, the permeability
κ
e
(
k
e
,
k
),
depending on the equivalent thermal conductivity
k
e
of the panel envelope
and the center-of-panel thermal conductivity. In case of undamaged panels,
the linear thermal transmittance measures 0.01 W/(mK) for metalized fi lms
and 0.04 W/(mK) for metal foils. Resulting from both degradation through
time as well as thermal bridging of the panel edges, an equivalent thermal
conductivity
k
eq
is generally used of 0.008 W/(mK), twice the center-of-
panel thermal conductivity under pristine conditions.
Limitations of installation are due to the panel envelope which serves to
maintain the inner vacuum of the panel, as this envelope may not be
damaged. Vacuum insulation panels cannot be cut on-site into the required
form, and much attention goes into the careful placing of the panels during
construction and protection of the panels against mechanical damage
during service life. When the vacuum is not maintained due to damage of
the panel envelope, the thermal conductivity
k
increases to that of the
core material under standard pressure conditions, i.e., 0.020 W/(mK) for a
fumed silica core.
ψ
Areas of application
Vacuum insulation panels have already been introduced to the market in
large-scale production, but manufacturing is still mainly hand-labor. In
recent years, several building applications for VIPs have been proposed
and/or tested, and a large-scale study has been carried out on the possibili-
ties of vacuum insulation panels in insulated building envelopes. The main
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