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Quality thermal protection for windows has advanced more rapidly than for any other building component. The overall heat transfer coefficient (Uw-value) of windows available on the market has been reduced by a factor of 8 in the last 40 years!
⇒ Assuming average energy prices in Germany, each square metre of such a window “costs” € 42 in heating energy each year.
Not only the energy losses are increased, however. Due to the poor insulaton, the cold can directly reach the interior surfaces. The temperature there is often below 0°C, and is visible as frosting (see the frost patterns in the picture). Poor thermal protection is associated with decreased comfort and increased risk of damage .
Significant progress is possible by a scientific development made by Prof. Glaser in Germany and Prof. Rosenfeld in California: by applying an extremely thin metallic coating (low-e coating) to one of the glass surfaces in the space between the panes, the heat radiation between these panes can be greatly reduced; and most of the heat is transferred by heat radiation (longwave infrared).
The breakthrough for energy efficient construction in Germany was triple glazing using two low-e-layers.
⇒ The result is a “comfort window”, also called “Passive House window” which reduces the annual heat losses to less than the equivalent of 7 litres of heating oil per square metre – less than one eighth of the initial amount.
If one also takes into account that the (cost-free) passive solar energy entering through the Passive House window almost compensates for the heat losses in winter, the net losses through windows of this quality are negligible; in a lot of climates it will be a net-energy-gain window. Besides that, the additional investment for the better quality is compensated for by the energy savings due to the low-e triple glazing alone.
The windows' potential for solar gains just matches the thermal protection level of the insulated envelope (with U-values of about 0.15 W/(m²K)). It is only by combining these two qualities that Passive Houses can be realised in cold climates such as typical for Central Europe - or in even colder ones, even those near to the polar circle. The result is a house with negligible heat losses, which stays comfortably warm and can be heated with a very low energy demand.
The Passive House window excels in that it not only has lower heat losses, but also provides improved comfort. Even with cold outdoor temperatures like -15°C, the interior surface temperature does not fall below 17°C.
In these circumstances the thermal comfort in the room is ensured independently of the type of heating supply. That this was made possible, is due in considerable part to the the improved windows.
Here are the important components of a window suitable for Passive Houses in cold moderate climates :
|Thermal image of a Passive House window from|
the inside. All surfaces are pleasantly warm:
block frames, casement frames, and glazing.
In this picture, even at the edge of the glass the
temperature does not fall below 15 °C.
(Photograph: PHI; object: Passive House
in Darmstadt Kranichstein; the heaters are
located on the interior walls.)
|For comparison, an insulated glass window in an|
old building: the average surface temperatures
are less than 14 °C here. The installation also
shows conspicuous thermal bridging especially
at the concrete lintel.
|Double-glazing (in a newly installed
french window here) has somewhat
higher surface temperatures (16 °C on
average). In this image the extremely
poor insulation of the conventional
window frame is particularly noticeable.
Such high heat losses and low surface
temperatures are now a thing of the past:
Passive House frames allow a significant
improvement in quality.
Passive House windows are high quality products which are now produced and supplied by more than 200 manufacturers. The energy savings achieved by these windows do not amount to just a few percent, in fact they amount to more than half of the heat lost through windows. They not only save energy but also money, and also contribute to climate protection. Passive House windows are a good example of efficient technology which has been developed in Europe and which creates jobs in the region, and at the same time relieves the burden on the energy market - especially dependencies on monopolistic energy suppliers.
|The development of constantly improved glazing: from single-galzing (on the far left) to the Passive-House-suitable glazing (far right). Only this has pleasantly warm interior surfaces even in winter. Lower energy losses and improved comfort go hand in hand.|