Frames and energy balance
Generally frames perform worse than glazing so small windows, or those with lots of mullions and transoms should be minimised.
When picking a window frame, the dimensions should always be considered as well as frame U-value. While it can be tempting to choose the frame with the best U-value, there is often a trade-off between performance and thickness. A slimmer yet worse performing frame can sometimes prove the better option, as the amount of glazing (which has a better U-value) is optimised and solar gains are increased.
The new Passivhaus window Certificates from the Passivhaus Institut include an A to C rating which gives an indication of the balance between frames and glazing, because gains are considered as well as losses. However, the best approach is to model the various options in PHPP as the ‘best’ option can vary between projects.
Inward and outward opening windows can perform equally well, although outward opening windows are more common in the UK market. Inward opening windows allow external shutters, blinds or insect mesh to be fitted, and are usually easier to wrap in insulation, thus helping achieve better thermal performance.