Insulating profiles and spacers made from engineering plastics provide thermal separation that ensures energy-efficient windows, doors and façades.
In no other country in the world is such rapid urbanisation currently taking place as here in China. And even if the construction sector in the ‚Middle Kingdom‘ is for the moment somewhat cautious in its approach, a lot of new living and working space will be needed in the next few years for millions of people moving from the countryside into the towns. The energy efficiency of the resulting buildings has a big part to play here. This is becoming all the more important in light of the Chinese government’s desire that by 2020 more than 50 percent of all new buildings in Chinese towns are to be constructed according to „green“ criteria.
Alongside effective buildings technology, optimum thermal insulation of windows, doors and façades is vital to creating a healthy, environmentally-friendly and resource-efficient building.
High thermal conductivity of aluminium.
With high-rise buildings in particular, architects and planners are keen on using aluminium windows. Aluminium profiles have very high execution accuracy, enable customised design possibilities and are extremely lightweight. In addition, aluminium as a material naturally has high chemical resistance and is mechanically resilient. The disadvantage of aluminium, however, is that it is very good at conducting heat. According to the standard DIN EN ISO 10077-2, aluminium has a thermal conductivity of λ = 160 W/mK. As a result, thermal separation of the aluminium profiles becomes necessary.
How does the thermal separation work?
Heat is energy which, by means of thermal conduction, convection and radiation, always flows in the direction of the lower temperature. The energy transport can be reduced by:
- using materials with low thermal conductivity, e. g. thermal insulating bars made from engineering plastic in the interior of the aluminium frame and Warm Edge spacers in the insulating glass.
- designing the components in such a way that there is minimal circulation of air, for example through flags on the insulating bars or by means of insulating bars with hollow chambers in the interior of the frame.
- using surfaces with low emissivity e. g. a Low-E film on the flags of the insulating bars in the frame.
There are two effective measures for achieving the thermal separation on aluminium windows, doors and façades:
Measure 1: Insulating profiles from engineering plastic in the window frame.
In order to reduce the thermal transmission coefficient of the frame (Uf value), it is necessary to insulate the window frames and thermally separate their outer and inner shells from each other.
For this purpose, insulating bars made from thermally insulating plastic such as insulbar® REG from Ensinger are recommended for thermal separation. This insulating profile is made from glass fibre reinforced polyamide 66. The glass fibre proportion is 25 %. The material used for this insulating bar stands out for its low thermal conductivity value l = 0.30 W/mK and its good mechanical properties – even at higher temperatures.
A non-thermally-separated frame from an aluminium window has a thermal transmission coefficient Uf of 6.8 W/m2K (see Figure 2). If this system is thermally separated with an insulating profile with a width of 42 mm (see Figure 3), then the thermal transmission coefficient can be reduced to 1.3 W/m2K – the energy efficiency of the aluminium frame is therefore improved by over 80 percent.
The calculation is based on the following assumptions: Glass area of 5.1 m2, frame proportion 30 %, temperature difference of calculation 20 K.
This means, therefore, that the more effective the insulation, the smaller the heat losses in the form of thermal conduction (transmission), heat convection and heat radiation (see Figure 4).
Here the individual types of heat transmission can be minimised in a targeted way. For example, a reduced heat flow can be achieved by making the hollow chambers smaller. The reduction can be brought about by means of what are called hollow chamber profiles or by means of profiles with inwardly directed flags. A further, very effective method for minimising the losses through heat radiation is the use of thermal insulating bars with an inwardly directed flag covered with a thin Low-E film. This film ensures a high level of reflection of the heat radiation.
Measure 2: Warm Edge spacer in the glass edge zone
With insulating glass it is vital to minimise thermal bridges in the glass edge zone: Spacers between the panes of glass – as part of the edge bond – are traditionally made from aluminium. They also known by the name „Cold Edge“. By contrast, what are known as „Warm Edge“ spacers have substantially lower thermal conductivity, as a result of which heat losses at the glass edge are significantly reduced. The window edge on the room side remains warmer as a result.
Warm Edge spacers consisting of a bond made from high-performance plastic and stainless steel, for example Thermix® from Ensinger, have a thermal conductivity that is several times lower than that of aluminium. (with Thermix® for example a thermal conductivity that is more than 700 times lower). An ultra-thin diffusion barrier made from stainless steel ensures lasting gas-tightness. The stainless steel used by Thermix®, for example, has a thermal conductivity value that is 10 times lower than aluminium.
By using these spacers in the glass edge bond, the Uw value of the window is improved by 0.1 to 0.2 W/(m2K). But it is not only the energy costs and risks of condensation and hence mould formation that are reduced through the insulation: The higher surface temperature of the window also makes the building more comfortable to live in.
The requirements in China concerning the sustainability and energy efficiency of buildings are exacting. This affects the providers of windows, façades and doors. They need better, thermally optimised products so that less cooling and heating energy escapes „out of the window“. It is vital to consistently minimise thermal bridges on windows, doors and façades. By this means, the consumption of heating and cooling energy and the associated CO2 emissions can be considerably reduced.
Components specially developed for window construction to enable thermal separation, for example insulbar® and Thermix® from Ensinger, consist of highly insulating plastic and have been tried, tested and optimised over decades. With these, the dangers of condensation for the building fabric and the risks to the health of the people living in it can be reduced to a minimum. Investors and project developers are also well advised to pay attention to the optimum insulation of windows. Because ultimately, where the indoor environment quality is right, top prices can be charged.