Don´t assume that the frame U-value is the same for the window sill, head and jam. Check the PHI Component certificate or manufacturers 3rd party test certificate for performance. Generally frames perform worse than glazing so small windows, or those with lots of mullions and transoms should be minized. When picking a window frame, the dimensions should be always considered as well as frame U-value. While it can be tempting to choose the frame with the best U-value, there is oftern trade-off between performance and thickness. The new passive house window certificate from the Passive House Institute (PHI) include and A to C rating which gives an indication of the balance between frames and glazing, because gains are considered as well as losses. Inward and outward opening windows can perform equally well. Inward opening windows allow external shutters, blinds or insect mesh to be fitted, and are usually easier to wrap in insulation, thus helping achieve bettter thermal performance.
And don´t forget to adjust the performance of any rooflights within the PHPP calculation. Note that rooflights are very difficult to shade from overheating in summer.
Excerpt from “How to build a Passivhaus: Rules of thumb”, Passivhaus Trust, April 2015