advanced building skins 14 | 15 June 2012 - lamp.tugraz.at - Graz ...

2 Physical and Solar Properties
2.1 Insulating Capacity
Advanced Building Skins
Insulating Glass Units (IGU) consist of minimum two glass panes, separated by a spacer as illustrated
in Figure 1. The spacer is filled with desiccant to absorb rests of moisture, captured inside of the cavity
during production. The cavity can be filled by air or by a special gas like Argon or Krypton to improve
the thermal insulation capacity. A primary seal of permanent plastic Butyl has the function of a vapour
barrier. The secondary seal of Silicone or Polysulfide has a structural purpose to bond the glass-glass
sandwich together statically. The insulating capacity achieved by the IGU is described by a physical
parameter called thermal transmittance, respectively U-value.
Figure 1: Make-up of a typical Insulating Glass Units (left) and the heat transfer from warm inside to cold outside
through a double-pane IGU (right).
The U-value can be defined by the amount of heat Q which is transmitted through an area A within the
time period t along a temperature gradient ∆T (1). The U-value can be calculated according to the
standard DIN EN 673 [2]:
Q
U
A T
t
W
, [ U ]
m²
K
The insulating capacity is influenced by the internal heat transfer hi, the heat transfer in the cavity ht,
which can be divided into ~17% heat conduction, ~17% convection and ~66% radiation and the
external heat transfer he (2).
1 1 1 1
U h h h
i
t
e
- 2 -
Outside
Amount of heat Q
Te Ti
Temperature difference ∆T
Inside
It becomes obvious that the radiation in the cavity plays a dominant role which determines the total
heat transfer. Therefore, the development of the coating technology was a main step. By reducing the
long-wave Infrared emissivity of the glass surface from εglass = 89% to εcoating = 2% the U-value could
be reduced significant by blocking the heat radiation.
Before 1960 monolithic glass with a U-values of ~6.0 W/(m²K) has been standard. By introducing
double glazing IGUs the U-value could cut in half. Today a U-value of ~1.0 W/(m²K) can be achieved
with low-e coatings and gas fillings. Triple glazing achieve an U-value of ~0.5 W/(m²K), depending
on the type of gas used, the glass thickness, the cavity and the inclination installed.
Generally, the dependence of the U-value on the inclination angle is not taken into consideration in
daily praxis. The U-value is defined and measured in vertical position according to the appropriate
standard DIN EN 674 [3]. Furthermore the DIN EN 1279 [4] refers to the U-value of DIN EN 674 and
(1)
(2)