What is a passive house?
A passive house is characterised by the fact that no additional, "active" heating energy has to be supplied. The house is so well insulated that all of its heating energy is provided by solar radiation, the body heat given off by its occupants and surplus thermal energy from electrical appliances such as the cooker, other electrical devices or lighting. This requires a ventilation system that extracts the residual heat from the air expelled to the outside and thereby warms the incoming fresh air. In addition, the building envelope must have very good insulating properties.
The passive house standard defined by the legislator stipulates that, in a passive house, the annual heating energy demand including hot water preparation must not exceed 15 kWh per m² of living space (kilowatt hours per square metre of living space). A ventilation system also consumes energy, of course, and most passive houses have supplementary heating for the domestic hot water tank in case the solar radiation on the collector array is not sufficient to provide hot water. Such consumers must not cause the above-mentioned value to be exceeded.
The effect of passive house windows in the building envelope
For a building envelope made up of walls, floors and ceilings to be suitable for a passive house, it may have a heat transfer coefficient (U-value) of no more than 0.15 W/(m²K). This value expresses, in watts per square metre and kelvin of temperature difference, how much heat is lost through one square metre of surface to the colder side. Window surfaces cannot, by their very nature, be insulated quite as well, since they are openings that provide contact with the outside world. For the passive house standard it is stipulated that they must reach or fall below 0.8 W/(m²K). As only physicists and building technicians can make much sense of this physical value, the heating energy equivalent is given here: over an entire year, only as much heat escapes from a house through one square metre of window surface with a U-value of 0.8 W/(m²K) as can be generated by seven litres of heating oil. Since a window facing south, and to a lesser extent one facing east or west, simultaneously helps to heat the house through captured solar warmth, such a passive house window ultimately has a positive energy balance for a building.
The glazing of windows to passive house standard
For a window to be able to achieve passive house standard, a number of design features have to be met. First of all, triple thermal insulation glazing is required, which achieves a Ug-value (the g stands for the English word "glazing" and denotes the U-value for the glazing) well below the targeted 0.8 W/(m²K). This is necessary because the Uf-values (f for the English word "frame") do not reach 0.8 W/(m²K) even with the best materials. In order for the overall value for the window (Uw-value, w for the English word "window") to reach the required 0.8 W/(m²K), the glazing has to be correspondingly better. The Ug-value states the heat transfer coefficient for the glazing including the edge seal. The edge seal of the glazing consists of spacers that hold the individual panes. These used to be made predominantly of aluminium, which, however, is not suitable for the passive house standard because of its good thermal conductivity. Modern spacers, which make a "warm edge" pane assembly possible, are made from composite plastic or from stainless steel encased in plastic. The term "warm edge" indicates that the temperature on the inside of the glass surface at the edge is only minimally lower than in the centre. This is necessary for a good U-value of the entire pane assembly, but it also means that such panes ensure a comfortable indoor climate. A large temperature difference between the centre and the edge of the pane leads to air movement in the room through convection, which is perceived as unpleasant draughts. This is ruled out by a passive house window with a "warm edge". It also means that the perceived temperature is higher than the actual one, which in turn leads to energy savings.
Frame profiles for passive house windows
What properties must frame profiles have in order to be suitable for passive house windows? First there is the insulation value of the material itself to consider. As a material, timber already has a very good U-value. If it is installed in the right way, a window to passive house standard can be built with it. Metal conducts heat well, so windows with metal frames are only suitable for passive houses in combination with plastic components that provide the appropriate thermal insulation. Window profiles made of uPVC must be designed in such a way that several self-contained air chambers ensure a good Uf-value.
Regardless of the material used, the window profiles must have three sealing planes at the transition between the window frame and the window sash. This also creates two enclosed air chambers at this point, which ensure a very good Uf-value when the window is closed.
The features of a passive house window at a glance:
- Uw-value of 0.8 W/(m²K) or better
- Triple glazing
- "Warm edge" spacers
- Suitable frame profiles for a very good Uf-value
- Three sealing planes with two enclosed air chambers between the window frame and the window sash
- Coated panes
- Inert gas filling of the cavities between the panes
The energy balance of passive house windows
The panes used in thermal insulation glazing are coated on one side with metal, which ensures that the long-wave solar radiation can penetrate but the short-wave heat radiation from the room cannot escape back outside. In this way the windows act even more strongly as a heat trap for the incoming solar warmth than is already the case with ordinary glass. In addition, the cavities between the panes are filled with an inert gas, usually argon, which increases the insulating performance still further.
Windows that are designed and manufactured in the manner described have a positive energy balance for a house, provided they face south. This means that, over the course of a year, they capture more heat from solar energy than they lose through their surface. This also makes it clear why, in a passive house, window surfaces facing south are planned to be as large as possible, whereas those facing north are planned to be as small as possible.
