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

Advanced Building Skins
does not consider the actual behaviour. Therefore, the actual insulation capacity of standard IGUs can
be much less than that expected of the preliminary design when used as a roof glazing. The reason for
this behaviour is explained in Figure 2.
Figure 2: Convection inside an IGU in the case of façade application (left), convection inside an IGU in the case
of roof application (right).
Inside a vertical glazing the thermal convection forms a long, slow loop as warm air rises along the
interior side. The convection is not significantly high. In the case of a horizontal glazing, the warm air
meets the colder outer side more quickly when rising. The result is the formation of many small rapid
circuits and an accelerated airflow. Thus the thermal energy is transferred faster through the cavity and
the insulating capacity degrades while the U-value rises. Therefore, the performance of a gas-filled
standard IGU drops significantly, when used in a roof application. The real U-value can be more than
50% higher than the U-value measured or calculated according to the appropriate code. If this is not
considered carefully the energy consumption of the whole building may unexpectedly rise due to the
increased heat loss.
2.2 Radiation Properties
Cold Warm
outside inside Cold outside
Beside the insulating capacity the radiation properties like the Visual Light Transmission and the Total
Solar Energy Transmission characterise the functionality of an IGU. The Visual Light Transmittance
τv is defined as the amount of visible light (λ = 380nm – 780nm) passing through the IGU, less the
reflected and absorbed fraction as illustrated in Figure 4. The ultraviolet part (UV) and the infrared
part (IR) is not been considered. The Visual Light Transmittance usually is given in % of the original
value and can be determined according to the standard DIN EN 410 [5]. τv specifies how much light
comes into the building, in other words it indicates how bright or dark the glazing appears.
relative intensity
1
0,8
0,6
0,4
0,2
380
Figure 3: Visual Light Transmission, visible range from 380nm to 780nm.
The Total Solar Energy Transmittance (TSET) is also known as „g value“ or Solar Heat Gain
Coefficient (SHGC). This value considers the whole solar range from 250nm up to 2500nm, including
the ultraviolet (UV), visual (VIS) and infrared part (IR) of the solar spectrum [5]. Beside the direct
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Warm inside
Solar radiation (spectral range)
Sensitivity of human‘s eye
Visible range
780 nm
0
300 600 900 1200 1500 1800 2100 2400
UV VIS IR
wavelength (nm)