Translucent Materials

0.7 W/m2K, and with krypton fi I ng a
U-value of 0.5 W/m K.
Fitting a low-emission coated membrane
avoids the weight and thickness of a third
glass pane (ill. 30).
Evacuating the gap can further reduce
heat conduction by the enclosed gas. But
it does create other problems in respect
of seal tightness and thermal separation.
Early vacuum glazing prototypes achieve
a U-value of 0.6 W/m2K, but it will st I take
some time before full industrial production
is possible.
Allings with thermal insulation properties
Transparent heat insulators make it possi
ble to reduce heat loss and at the same
time gain heat from the incident sun ight
(ills. 27,31).
Transparent and translucent materials like
glass, acrylic glass (PMMA), polycar
bonate (PC) and quartz foam are used, in
layers of varying thickness and structure.
They are protected from the weather and
mechanical damage by being bui tin
between two panes (ills. 28, 30).
They are classified according to four geo
metrical arrangements of the structure:
- Structures parallel with the external
glass like double or triple glazing or
membrane systems reduce heat loss
but cause higher reflection loss.
• Structures that are vertical to the exter
nal glass consist of honeycombs or
capillary elements that divide the gaps
into small air cells.
This arrangement reduces reflection
loss, as the incident radiation is
directed inside by multiple reflection on
the parallel walls.
Honeycomb structures look like multistay
bridges made of transparent poly
carbonate. Capillary structures are
assembled from large numbers of small
glass or plastic tubes.
A 100 mm thck capillary sheet made of
small polycarbonate tubes achieves a
U-value of 0.89 W/m2K and a c-va ue of
0.69 (Oka-Lux®).
Capillary structures made up of
100mm long small glass tubes have
achieved U-values around 1 W/m K.
• Cellular structures result from combin
ing parallel and vertical structures, like
acrylic foam, for example.
• Quasi-homogeneous structures such as
aerogels are microscopically cellular.
The manufacture of aerogel sheets is
very elaborate and thus expensive.
Aerogel granules are much more rea
sonably priced; they are packed loose
into the gap between the panes. A 16
mm granular filling (Basogel®) achieves
a U-value of less than 0.8 W/m2K, a
t-value of 0.41 and a gdiff-value of 0.52
(ill. 33).
Comparing the four thermal insulation
groups shows that insulating glass and
plastic membrane systems are best in
areas where transparency is required for
visibility reasons. They also ensure low
U-values.
Other structures are more or less lightdiffusing
and thus suitable for roof-light
glazing.
In summer, transparent heat insulation
needs sunscreening to prevent the
spaces from heating up.
29
Glass as a building material
Multilayered glass — Insulating glass
C
vn
Four heat
transmission paths:
1. heat radiation 67 %
via Clings
4. peripjieral
ioint area
.33%