Renew roof insulation or not?

Hello dear forum community, the following situation:

House 1982, insulation in the roof Rockwool 18 CM. In winter without heating (0 degrees outside temperature) around 18 degrees inside.
In summer up to 40 degrees under the roof.

We want to buy this house and have now heard two opinions: 1.) New insulation, 2.) 18 CM Rockwool is more than enough.
If the insulation were bad or defective, we wouldn't have these good temperatures in winter. However, the temperatures in summer worry me. What could be the reason for this? And would new insulation be worthwhile?

Regards
Ahri

For summer heat protection, only massive components with a high heat storage capacity are suitable. New insulation would probably be of little use. Probably the best solution is an air conditioning unit or additional roof windows (shaded) to quickly get the heat out at night.