Cheap differential thermostat

The STC-1000 is cheap however it does not measure difference in temperature. What I want is a fan which runs when outside is colder than inside, but stops once it gets hot outside. I have used my MH1210A which is like the more well know STC-1000 but with single relay to run fan when under 18ºC outside it works of a kind.

I was looking at air conditioner units, and it seems they need a hole to outside similar to tumble drier, unless one bores a hole then there is a security issue having window open, so if boring a hole then why not simply have a fan blowing in cool air when too hot inside, however it needs to stop when outside warms up.