ᐅ Roof: Should insulation be replaced when possible?

Happy holiday,

we are planning a renovation in autumn. During the planning phase, we wondered if it makes sense (work, time, money) to insulate the roof anew while we are at it (the panels on the second floor will be removed anyway).

Currently, the entire roof structure is insulated with fiberglass wool. This is partly deteriorating, so the roof tiles are visible from the inside.

Moreover, daylight is visible in various places, for example where the chimneys are located. Drafts can be felt in some spots.

Questions:

• Does new insulation significantly save on heating costs?
• The second floor gets VERY hot in summer, making it almost unbearable to stay there. Will new insulation improve this?

We are considering insulation between rafters plus insulation under rafters plus a vapor barrier foil:

• Which materials offer the best value for money?

We are inclined towards mineral wool (formaldehyde-free):

• What is the cost per square meter?

Finally:

• Should the entire roof or only the attic space be insulated (there are sloped ceilings on the second floor)?

If full insulation is done:

• Is it easy to apply the insulation materials and foil from the floor (attic) or how does this work?

I hope I didn’t overwhelm anyone with my first post. Best regards,
Paulemann