What are Passive House Windows?
There is no such thing as a passive house window type. The Passive House standard is performance based, not a checklist of specific components to buy. In general, they are characterized by being thermally broken, airtight, and usually triple glazed with the gaps between glass panes filled with argon or krypton gas.
The overall window will boast a low U-value, high R-value, typically maximise solar gain, and minimize heat loss. Indeed, the passive idea is getting the most energy for the least effort or cost, and then maintaining it due to the excellent insulation.
Thus, any window which has a Uw-value of 0.8 W/(m²K) or less is suitable for a passive house. To have a certified passive house, the building’s overall energy use and air leakage are the deciding factors. With regard to passive windows and their thermal performance, the German Passivhaus-Institut notes:
The window frames must be well insulated and fitted with low-e glazing filled with argon or krypton to prevent heat transfer. For most cool-temperate climates, this means a U-value of 0.80 W/(m²K) or less, with G-values around 50%.
Depending on your local climate, budget and component availability, it is also possible to install windows with somewhat higher values and make up for the inefficiency elsewhere in the home.