Windows for highly energy-efficient buildings

A window for a highly efficient building must be designed as part of the complete envelope. Uw, solar g-value, airtightness, orientation, summer protection and the wall junction must be assessed together. Minergie-P and Passive House are separate standards, so always use the version applicable to the project.

Written and reviewed by the BO Fenster technical team

Minergie-P and Passive House are not synonyms

Minergie-P is a Swiss building standard covering the envelope, airtightness, summer comfort and overall energy, among other criteria. Passive House has its own framework. A figure from an older module or another standard should not be presented as a universal current requirement.

Values to assess

Uw applies to the complete window, Ug to the glazing and Uf to the frame. The g-value describes transmitted solar energy. Each must be assessed using the project dimensions, orientations and summer conditions.

Window criteria for highly efficient buildings
Criterion	Purpose	Check
Uw	Complete-window heat transfer	Calculated size and configuration
g	Solar gains	Orientation and overheating risk
Airtightness	Limits air leakage	Product, joints and wall connection
Installation	Envelope continuity	Thermal bridges and sealing layers

Double or triple glazing?

Triple glazing is common in highly efficient buildings because it can provide a low Ug value. The final choice still depends on the energy calculation, climate, orientation, weight, dimensions and desired solar gains.

Installation is part of envelope performance

Product Uw does not automatically describe the wall junction. Window position, reveals, fixings, perimeter insulation and the air- and weather-sealing layers must be coordinated. An airtightness test assesses the full building envelope.

Orientation and summer comfort

Solar gains can help during the heating season but large glazed areas also increase overheating risk. The g-value, external shading, obstructions, night ventilation and project climate data should be assessed together.

Documents to request with a quotation

Request Uw for the important sizes, glazing Ug and g, spacer type, relevant airtightness classes and a wall-junction principle. Every performance claim should identify its dimensions, glazing and technical declaration.

Uw calculation and reference dimensions
Glazing build-up with Ug and g
Frame and spacer details
Installation and sealing concept
Compatibility with the project energy assessment

Quick answers

Does Minergie-P always require Uw 0.8?

Use the current rules and project assessment. An older module requirement should not be generalised to every current certification.

Is triple glazing mandatory?

It is common, but the selection should follow the energy calculation, orientation, climate and chosen standard.

Is Uw enough to choose the window?

No. Solar g-value, acoustics, airtightness, summer comfort, dimensions and installation must also be checked.

Why does window size affect Uw?

A small window has proportionally more frame. Because Ug and Uf differ, that proportion changes the complete-window Uw.