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46
ZEB
annual report 2014
Windows are key elements in buildings that
have a goal of reducing their energy demands
and carbon footprint. Previous studies shows
that a large part of the net energy demand
of an office building is related to window
heat loss and cooling demands induced by
solar irradiance. It is therefore crucial that
the combined effects of solar heat gains and
thermal losses are optimized Large window
areas will in general be beneficial in terms
of increasing daylight availability, whereas
smaller window areas are desirable in terms of
heat loss reductions.
Windows which have good insulation
capacities and, at the same time, let a
suitable portion of the solar heat gains into the
buildings are important to ensure low energy
demands. At the same time it is important
to keep in mind that the windows are the
elements that provide communication between
the interior and exterior of the buildings
and thus must provide sufficient daylight
to the interior. The combination of high
thermal insulation levels and a high daylight
transmission is traditionally difficult to achieve.
We have therefore looked at solutions
for improvement of the thermal insulation
properties of the glazing units.
There are several parameters which can be
improved in order to improve the insulation
properties of a glazing unit and hence improve
the entire window. The three factors most
prone to improvement are the following: the
number of glass panes in the glazing unit,
the emissivity of the glazing surfaces and the
thermal conductivity of the gas fillings in the
cavities.
It has been found that increasing the number
of glass panes in the insulating glazing
units (IGU) is, by far, the most efficient way
of improving thermal resistance. However,
adding more glazing layers to the glazing unit
will increase the weight substantially, and
both mounting and operating the windows will
become cumbersome. A solution to this is to
use glazing units with thin, intermittent layers.
Here, the structural stability is maintained by
using tradition glass layers, typically 4-6 mm
thick, in the outer panes. The intermittent
layers can then be replaced by polymer layers
or glass layers with thicknesses down to 0.1
mm. By doing this replacement, the weight of
a multilayer glazing unit can be kept close to
that of a double- or triple pane glazing unit.
It must be kept in mind that improving the
thermal insulation of a glazing unit only is one
piece of the puzzle. Improving the thermal
resistance will in general deteriorate other
properties, such as the solar energy and
visible light transmittance of the glazing unit.
In addition to the thermal performance, optical
properties and aesthetics, ageing properties
and robustness must be maintained at
adequate levels. One of the findings in this
work was that the thermal stresses to the
LIGHTWEIGHT WINDOWS
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LETTE VINDUER
Life is nothing without a view
Steinar Grynning (NTNU)