Glazings suitable for Passive Houses (right hand side) will have comfortable interior surface temperatures even in conditions with heavy frost.

There is no other component in the building sector with as rapid development in quality than in the field of windows. The thermal loss coefficient of windows on the market (U_w-value) has been reduced by a factor 8 during the last 30 years.

At the beginning of the 1970s most windows in Germany had single glazing, the U_w-value was some 5.5 W/(m²K) (approx. R = 1 ft²h°F/Btu in imperial units). The heat losses through 1 m² of such a window added up to some 60 liters of heating oil (0.35 barrel per 10 ft² and year). Such a window causes running costs of some 48 €/m² for final energy for heating each year, calculating with the energy costs of the future in Europe (some $40 per 10 ft² annually).
But not only are the energy losses high. Because of the poor insulation, the external coldness can penetrate through the window directly to the inside surface: If the outdoor air temperature is below -7.5°C (18.5°F), there will be frost on the interior surface, well known as "frost pattern". A poor insulation therefore is related with minimum comfort and increased danger of damages.
Somewhat better is the performance of the so called "thermopane glazings" or double glazings, which have been introduced in new constructions and retrofits in Germany after the first oil price crisis. There is an air gap enclosed between two panes. The heat loss coefficient is reduced to some 2.8 W/(m²K) (R = 2 ft²h°F/Btu in imperial units). The consequence: Almost half of the losses are saved compared to single glazing – and that is half of the annual costs. The interior surface temperature will not be lower than 7.5 °C (45.5°F) even during extraordinary cold periods (-15°C or 5°F). There will not be any frost pattern – but the surface is still uncomfortably cold and will show condensation, because the temperature is far lower than the dew point.

The introduction of razor-thin metal coatings inside the interpane space was the most important success - called "low-e"-layer for the low thermal emissivity. These coatings reduce the thermal radiation between the inner and the outer pane by a factor of 5 to 20. Even more, the gas in the gap is changed from air to noble gasses (Argon or Krypton), which have a far lower heat conductivity. These so called "low-e-glazings" became almost standard construction throughout Germany after the German new "Wärmeschutzverordnung" (Ordinance on Insulation) of the year 1995. It is interesting, that glazings did not become more expensive, in spite of the much increased quality. A commonly used wooden or plastics window using a standard spacer and a standard double pane low-e glazing has a U–value between 1.3 and 1.7 W/(m²K) (R = 3.3 to 4.4 ft²h°F/Btu in imperial units). Thus the heat loss reduced another time by a factor of two compared to the old double pane window. Now the average interior surface temperature will be some 13°C (55.4°F) during heavy frost periods. But still the cold air flow from the window (through convective currents) can be perceived as an uncomfortable cold air layer at the floor, and this problem must be resolved for optimum comfort. (In this regard, thermal comfort is discussed within typical Central European climate. In different climates the interior surface temperatures should be the same to give the same comfort level, but the required U-values will be different).

The breakthrough for energy efficient construction in Germany was the availability of triple-pane low-e glazing. If one adds two gaps between panes with one low-e-coating in each gap and noble gas filling, U-values between 0.5 and 0.8 W/(m²K) result (R = 7.1 to 11.4 ft²h°F/Btu in imperial units). This requires that not only the glazing, but the whole window should have such a quality, including an insulating window frame and insulating spacers. The result is a "thermal comfort window" or a "Passive House window". Using such a window, the annual heat loss is reduced to not more than 8 liters heating oil by each square meter of window area (0.05 barrel oil per 10 ft² of window area) – that is a factor eight compared to the initial value. If one considers the passive solar energy gained through the window in addition, in a Central European climate the losses of a window of this quality are significantly outweighed by gains. It is not by chance that the net heat losses of transparent components of the building envelope are negligible as are the heat losses of opaque parts (walls and roofs). The level of insulation of the opaque envelope (U-values of some 0.15 W/(m²K) is well matched to the quality of the windows suitable for passive houses. Both together allow Passive Houses to be built in the wet-cold winter climate in Central Europe, which will maintain thermal comfort just by heating the fresh air from the ventilation system.

But the advantages of the Passive House window are not only reduced heat losses, but also increased thermal comfort. Even during heavy frost periods, the interior surface temperature will not fall below 17°C (62.6°F). The consequence is that no "cold radiation" can be perceived from such a window nor is an uncomfortable cold air layer possible – if no radiator exists near the window. Of course, all the other conditions for Passive Houses have to be met too, e.g. air tightness and a construction free of thermal bridges. With these conditions the thermal comfort will be independent of the way the heat is transferred to the room. To be able to do so, the improved windows are a key feature.

At the exhibition of the 10th Conference on Passive Houses all the important components of a high quality window can be seen and touched:

• Triple-pane, double-low-e glazings.
• Insulated wooden frames,
• Insulated plastics frames,
• Insulated glas facade constructions,
• Insulating spacers,
• Components for thermal-bridge free and air tight windows.

There is no other component in the building sector with a comparable fast development to improved quality than in field of windows. The thermal loss coefficient of windows on the market (U_w-value) has been reduced by a factor 8 during the last 30 years.