"Sucking cold air into your house rather than paying for air conditioning," the post that confuses and enrages Americans every year!
Prerequisites: no nearby wildfires, temperatures lower at night, one fan.
Bonus nice-to-have: indoor/outdoor thermometer/hygrometer. Incense (or vapor or a friend who vapes) to visualize airflow.
Method: at night, open two windows; one in the lowest part of the house (unless you have a basement colder than the outside, in which case find the coldest window that's not colder than the outside), one in the highest. Single-storey places, open a window in the coolest room and another in the hottest. Houses with cats, check window screens, replace screen fabric where necessary.
Orient fan in hottest room, one to two metres away from the open window, with airflow facing OUTWARDS. Put the fan on full blast. As the hot air is forcibly exhausted through the open hot-side window, cool night air is drawn in from the open cold-side window.
It might make more intuitive sense to place the fan facing inward in the cold side window, but try it both ways and you'll see how much easier it is to blow hot air out than blow cool air in. The hot air already wants to leave, it wants to expand into the cold night, adding the exhaust fan encourages it.
The optimal placement of the exhaust fan will be different from room to room; it'll always be facing the window and blowing outwards, but you can make a big difference to efficiency by moving it closer to the window or further away. Experiment, using incense or vapor to make airflow visible.
In the morning, close both windows and turn off the fan.
Advanced: check overnight air temp forecast on your weather app, set timer to pull cold air in until just before sunrise or whenever air is coldest.
Edit: this never occurred to me because I don't have an upstairs bathroom; if you have a bathroom upstairs with an extractor fan, experiment to see if that works well enough in place of the fan I've been talking about this whole time
Binder
thepoliticalcat