Insulate the cold roof or not? If yes, what thickness?

Hello everyone,

I am currently considering whether we should insulate our cold roof or not (it’s only a storage room).
Our house was built according to KfW55 2016 (completed mid-2018) and therefore has insulation to the cold roof / upper floor ceiling of 240mm.

Under the roof tiles, a vapor-permeable membrane is installed.

Now my thought was to insulate the roof to save heating costs and to avoid extreme temperatures in the storage room.

The rafters are 240mm except for the dormer, which has "only" 140mm.

Now the topics:

Is this even worthwhile?
The roofer told me "if it were mine, I would do it"
The site manager says the costs would not be recovered.

What is your assessment?

Then the insulation itself... in my opinion it only makes sense to insulate the entire roof with the same thickness?! Is that correct?

I do not want to add battens to the dormer and therefore insulating the entire roof at 140mm would make sense??

If yes, in the area of the 240mm rafters, should the air layer be on the room side or the roof side of the insulation? After the insulation, I would then install another vapor-permeable membrane.

Is there anything else to consider?

Thanks in advance.

We are also getting a cold roof. I had read somewhere that it should not be insulated further. It doesn’t make much sense anyway, because it is ventilated – at least in our case.

Forget it!

You won't save on heating costs with this; rather, you'll likely produce higher costs because you'll then have to heat the previously unheated roof space while the insulation to the outside does not improve. Does the heating system even have the necessary "reserves" to cover the additional volume?
Moreover, you will have problems with the vapor barrier, which for this project would somehow have to be connected to the existing one in the area of the upper floor.

In summary: financial effort, working time, high risk of damage, and as a result even higher heating costs.

One might consider such an action if additional living space were being created there, but certainly not just so that a few boxes and old furniture have it cozier.
Ideally, such an expansion should already be planned and appropriately prepared during construction. If that was forgotten, it usually becomes difficult to impossible to carry out the project properly.

You will not save on heating costs with this; rather, you will produce higher ones because you then have to heat the previous cold roof while the insulation to the outside does not improve.

Why does he have to heat it?

Because insulation alone does not keep you warm.