New construction on an existing basement, is insulation worthwhile?

Hello everyone,

we are planning a new building on an existing basement with a granny flat, in which we will replace the old prefabricated house.

The basement is 50% underground and 50% exposed.

Now we are faced with the question of whether we should insulate the basement. We have asked about two people and received three opinions (a classic).

We have to weigh: no insulation vs. insulating the basement.

[*]Insulation: About €9,000 additional costs in house construction (because the house becomes larger as a result) + about €4,000 for the insulation as DIY work.
→ A total of around €13,000
[*]No insulation: Within the family there is a concern that this could lead to mold – a problem that has not occurred in the last almost 50 years.

Architect:
Considers the effort and costs to be disproportionate to the expected effect. Although a positive effect would be present, in his view it is not necessary for the new house and not critical for the basement, since there have been no problems in the last 50 years.

Plumber in the family:
Sees a possible mold risk.

Question:
What do you think?
Is the mold risk real?

Is the risk of mold real?

Where do you see a risk of mold? In the basement or in the house?

The question of insulation is a matter of usage. You don't need to insulate a purely utility basement from the outside.
However, one type of insulation will become mandatory in some form. The basement ceiling. Because for new buildings it also has to meet new building standards – either from above or from below. But the architect will take care of that.

Where do you see a mold risk? In the basement or in the house?

The question of insulation is a question of usage. You do not need to insulate a purely utility basement from the outside.
However, some form of insulation will become mandatory. The basement ceiling. Because for the new building, it must also reach new build standards - either from above or below. But the architect will take care of that.

Thank you for the answer.

So, according to the architect, the new house will be extremely well insulated at the top and downwards, so there is no problem with the house upstairs.
In the basement below, there will still be a granny flat, and that is where people apparently see the risk. The ceiling of the basement is therefore better insulated than before. The thermal bridge would still be the exterior wall, which is partially exposed.

There will still be a granny flat in the basement below, and people apparently see the risk there. The ceiling for the basement is therefore better insulated than before. The thermal bridge would still be the exterior wall, which is partially exposed.

Don't you also have to comply with the current requirements of the Building Energy Act for the new apartment in the basement in the case of a new build? How does your architect plan to avoid that?

I find it exhausting that you dedicate a separate thread to each individual question in the same project.

The basement is 50% underground and 50% exposed.

What does that mean: hillside buried and valley side exposed on a slope, or raised ground floor with basement half above ground level all around?
Generally, pictures would be helpful, also of what is to be removed. We cannot work with the secret knowledge in your head.