Windows in the passive house – how to size them right

Windows let in daylight and heat from the sun. Their most important effect in a passive house with its high efficiency is the "passive" utilization of solar gains. This way of tapping energy works on the same principle as a glasshouse does.

Glazed southern façade (source: GrAT)

The annual heat input from the sun lies between 10 and 20 kWh/m²a, referring to the heated surface of the building. This means that solar gains in well-planned and oriented buildings are higher than the required residual thermal energy of 15 kWh/m²a.

The solar energy transmittance of the windows should be as high as possible. This is particularly true for the south-facing windows, for which a value of g ≥ 0.5 to 0.6 is the goal.

At the planning stage it is important to analyse very carefully what sizes and positions of window yield the best results.

The following aspects should be taken into account for windows:

Glazing with U_g of at most 0.7 W/m²K
Thermal-bridge-minimized edge seal on glazing, with a thermally optimized spacer made of plastic or stainless steel (with minimal wall thickness, less than 0.2 mm) and a resulting loss coefficient Ψ_g of at most 0.035 W/mK
Frame design with the lowest possible edge-seal coefficient Ψ_F
Deep edge-seal overlap within frame
Overlapping window frame with insulation generously at installation to minimize thermal bridging
The resulting U_w value should be less than 0.8 W/m²K, and less than 0.85 W/m²K when installed.

Passive house windows: https://www.youtube.com/watch?v=Lwyv1YkObTk

Passive house windows for mediterranean climates: https://www.youtube.com/watch?v=g3AgZ0Rp5f8

Triple glazing is available with glass panes 2 mm thick and a total depth of 18 mm. Where passive-house windows with very slim profiles are installed, the glass area is enlarged, which increases solar gains, while U_w values are improved to between 0.5 and 0.6 W/m²K.

In a passive house, certain structural elements, such as external walls or windows, must be implemented in a highly efficient manner.