After the external wall, the roof – or the ceiling of the heated room – is the largest component through which heating energy is lost. It is often the case that the roof is insulated at great effort and expense, even if the attic is not used as living space and is therefore not heated.

In the attached document, you’ll find a thought experiment illustrating the difference it makes whether the roof or the ceiling is insulated.

You can find a helpful calculation tool here (scroll down a little)

Only the heated room should be insulated.