What is a passive house?
A passive house enables a constantly comfortable temperature in both winter and summer without using a conventional heating and air conditioning system – the house "heats up" and "cools down" in a purely passive manner. Use of the heat provided by the sun's rays that shine through the windows and of the heat generated by appliances and inhabitants is sufficient to keep the building at pleasant internal temperatures during the colder season. An annual thermal heat value of less than 15 kWh/(m²a) is required for a passive house. This corresponds to a comparable heating oil requirement of around 1.5 l/(m²a). For a European passive house, the total energy requirement including hot water and household electricity is less than 120 kWh/(m²a).
Furthermore, passive houses are characterised by a high degree of thermal comfort. Even if outdoor temperatures are low for an extended period of time, the internal surface temperature of the components must not drop below specified values. In this manner, the "cold radiation" effect (which is caused by otherwise normally large differences in temperature between a colder structural component and warmer ambient air and perceived by the user as a draught) is avoided.
The passive house standard is a low-cost approach to reducing the energy requirement of new builds to a minimum and, at the same time, improving living comfort. The achieved energy saving conserves resources and contributes towards sustainability.
What requirements does this place on the window as a structural component?
Windows for use in passive houses have an especially high heat insulation value. The window's thermal transmission coefficient Uw must be less than or equal to 0.8 W/(m²K). The performance requirements for windows suitable for passive houses are set out in the ift guideline WA-15/2 "Suitability of windows, outer doors and façades for passive houses". But this guideline not only defines the requirements for the thermal losses generated by windows and their structural connections; it also stipulates the minimum surface temperature values for connecting to the structure, glass or panel area. Thermal comfort is thus ensured in the rooms and the formation of condensation in unfavourable locations is avoided. In addition, the usability requirements, which have to be proven by way of tests for air permeability, impermeability to driving rain, wind resistance, mechanical strength and shock resistance on the window, are set out.
Due to their excellent heat insulation and performance properties, windows suitable for use in passive houses are, however, also used when renovating old builds or any kind of new build.