A Hall sensor on each window (aka open/close sensor), plus temperature sensors in each room and one placed outside.

When the window is closed, but the outside temperature is lower that inside, it sends a notification to open the window. The opposite happens when the outside temperature exceeds the one inside, unless the window is already closed.

This is all written in Python via appdaemon since it's easier than defining the same automation via the UI (although that's also doable nowadays).

Temperatures inside and outside the window, I suppose.