Windows are energy guzzlers? That used to be the case. Modern windows with a high level of thermal insulation today supply more energy than they lose — provided they are planned and installed properly. They therefore have a special role in optimal building planning with regard to energy, for example for passive houses. With a high degree of thermal insulation, thanks to the passive use of solar irradiation through the windows and the heat emitted to the interior by residents and technical devices, passive houses do not require an additional heating system. According to the certification criteria of the Passive House Institute in Darmstadt, Germany, a passive house must demonstrate a heating demand of maximum 15 kilowatt hours per square meter per year. There are also specific limits to be observed for primary energy demand, air tightness and minimum efficiency. Precise and extensive planning is required to reach these values. In addition to the particularly good thermal insulation, passive houses avoid thermal bridges and have a high level of air tightness. To fully utilise the positive effects, certain things must be taken into consideration when selecting, planning and installing the windows.
Considering the glass and the frame
In order to meet the high requirements for heat protection, windows in passive houses must be triple glazed and the frame must also have a high level of thermal insulation. As approximately 30% to 40% of the window opening is taken up by the frame, the quality of the frame is very decisive. The required qualities can be achieved with timber frames, UPVC profiles or transom/mullion constructions. To guarantee the usual comfort for the user, the inner surface temperatures for maximum room-height (2,50 m) windows must exceed 17°C. For passive house windows, the thermal transmission coefficient Uw must not exceed 0,8 W/(m²K). To ensure sufficient solar energy gains via the window areas, a passive house window should have a g-value (total energy transmission value) of approximately 0,5.
Considering summer and winter
The arrangement and orientation of passive house windows must be planned sensibly to ensure the required solar gains. This allows the windows to let more solar energy into the building in winter than the heat that they release to the outside. The best position for larger glass areas is a south-facing orientation. As the sun is higher in summer, the position of the sun does not allow so much sunlight into the inside of the building and a large proportion of the irradiation is reflected — in summer therefore, the solar irradiation is limited for south-facing windows and no further sun protection is usually required. By contrast, as a result of the low sun positions of summer mornings and evenings, windows that face east or west can cause overheating more easily and may require corresponding sun protection.
Window areas with slants of less than 75° in relation to the horizontal position should not exceed 15% of the useful area behind the windows or must be fitted with a temporary sun protection with a reduction factor of at least 75%. For south-facing windows, the limit is 25% of the useful area behind the window.
As an approximate rule of thumb, you can assume that east-facing and west-facing windows receive 60% and north-facing windows 40% of the usable solar irradiation of a south-facing window. For passive houses, a maximum deviation from the south-facing direction of 10° should be observed. However, the local sun positions over the course of the year must always be considered and incorporated in the planning. Local factors such as shade provided by other buildings or trees also have an impact here.
Deciding on metal or prefabricated components
In addition to selecting suitable, high-quality products for passive house construction, correct installation of the windows by an expert is also important. In passive houses, windows have to be installed in the insulating level of the outer components, with particular care being given to an airtight connection to the component and to avoiding thermal bridges. An additional over-insulation of the frame significantly reduces the heat losses. The decision must be made whether to install the windows before or after applying the thermal insulation. If they are installed before the thermal insulation is applied, metal brackets or metal plates are a common form of attachment. However, these must be checked for any problems with potential thermal bridges. An alternative is prefabricated window reveal elements with attachment options for window installation. These reveal elements are made from insulating material and feature a stop for the window, which further insulates the window frame.