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

Advanced Building Skins
transmission the secondary heat is included. The secondary heat is generated by the absorbed radiation,
which is shifted into the IR range and emitted in any direction (Figure 4). The g-value is not
independent from the Visual Light Transmittance τv. Both values are connected by the equation τv/g <
2.
Figure 4: „g value“, Total Solar Energy Transmittance (TSET) in the solar range 250nm to 2500nm.
The area beneath the graph in Figure 3 and 4 is equal to the appropriate energy content. The visual part
of the solar spectrum contains already 53% of the total solar energy. Even the most effective sun
control coating may only cut off the UV and IR part with together 47% energy content. Such an ideal
coating with 100% visual light transmission and a Colour Rendering Index of 100% results in a ratio
of τv = 100% divided by g = 53%. Hence the result is 1.89 (not considered the secondary heat), thus
the ratio is < 2. This physical fact is called the Spectral Selectivity.
One way to increase this ratio is to cut away the tips of high energy content in the visual range. In this
case the colour rendering index will be less than 100% as some belts were missing in the spectrum. In
other words, the glass will not be neutral and the entering light will have a certain colour. Another way,
influencing the Spectral Selectivity is the use of different inserts. To control the different aspects and
fulfill the whole range of demands, the cavity of the IGU can be used [6]. Not only gas fillings and
coatings but also integral louver systems, capillary honeycomb slabs or even high insulating Silica-
Aerogels allow controlling the physical properties over a wide range.
3 Translucent Insulation Materials
3.1 Function
Light diffusing insulating material, usually called Transparent or Translucent Insulation, is often used
to achieve a low U-value and a certain solar energy gain for the building. The working principle of
such materials is based on the polar bear coat. The material collects and conducts the light,
respectively the solar energy. At the same time a good thermal insulation is achieved.
3.2 Capillary Structures
relative intensity
1
0,8
0,6
0,4
0,2
UV VIS IR
energy content of UV, VIS, IR
~ ~ 3%
3% ~ ~ 53% 53% ~ ~ 44%
44%
Such materials can also be used inside an IGU to expand the features of the façade. When embedded
inside the cavity, inserts like capillary honeycomb slabs improve the thermal performance by
preventing convection. On the other hand the heat flow in the material increases at the same time. By
using a 16mm thick light diffusing capillary insert (Figure 5), the U-value could be reduced from ~3.0
W/(m²K) for an air filled cavity to ~2.0 W/(m²K) without any additional coating or gas filling.
Adequate triple glazing units with an additional gas filled cavity and a low-e coating even reach a U-
Value of ~0.6 W/(m²K). This benefit of an IGU with a Translucent Insulation insert even increases
when used as a roof application. Despite a conventional gas filled unit the U-value of an element with
an adequate insert stays constant, if it is inclined to the vertical (Figure 7). Aside the low U-value of
- 4 -
sensitivity of
human‘s eye
0
300 600 900 1200 1500 1800 2100 2400
wavelength (nm)