I hope the question is not to stupid, but ...

For diagnostic the thermic isolation quality of rooftops and windows it only makes sence when there is a high difference between tbe temperatur out- and insite the house?

Yes, you need a large temperature difference so that any leakage is as visible as possible.
In other words, perform the inspection on a very cold day while the house's heating is turned on.
Then any leakage will show up as clearly as you will find it.

Thank you. So wintertime would bring the best solutions for that.